Atmospheric Ozone

ATMOSPHERIC OZONE Arlette Vassy Laboratoire de Physique de I'Atmosphhe. Facult6 des Sciences de Paris. Paris. France

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.......................................................... 2. Measurement of Characteristic Parameters ................................ 2.1. Different Aspects and Points of View ................................. 2.2. Absolute Measurements ............................................. 2.3. Chemical Measurements Requiring a Calibration ....................... 2.3.1. Electrometric Method ........................................ 2.3.2. Radioactive Clathrates ........................................ 2.3.3. Rubber Cracking ............................................. 2.3.4. Colorimetry ................................................. 2.3.5. Fluorescence and Luminescence ................................ 2.3.6. Catalytic Analyzer ........................................... 2.3.7. Brewer's Equipment .......................................... 2.4. Spectroscopic Methods .............................................. 2.4.1. Ground Level Concentration ................................... 2.4.2. Total Amount or Reduced Thickness ........................... 2.5. Absorption Coefficients ............................................. 2.5.1. Ultraviolet .................................................. 1 Introduction

116 117 117 118 120 120 120 121 121 121 121 122 122 123 124 125 126 129 130 130 130

2.5.2. Visible ...................................................... 2.5.3. Infrared ..................................................... 2.6. Units ............................................................. 2.6.1. Total Ozone ................................................. 2.6.2. Ozone Concentration for a Given Pressure Level. Particularly Surface 131 Ozone ...................................................... 131 2.6.3. Vertical Distribution of Ozone .................................

3 . Concentration a t Ground Level .......................................... 3.1. Measurements and Results .......................................... 3.2. Exchange Phenomena ..............................................

....................................................

3.4. Ozone and Atmospheric Pollution .................................... 3.5. Origin and Interest of Tropospheric Ozone ............................

132 132 133 134 134 136

136 4 . Reduced Thickness and Temperature ..................................... 136 4.1. Introduction ...................................................... 4.2. Ozone Distribution Irregularities at the Earth's Surface . . . . . . . . . . . . . . . . . 138 4.3. Diurnal Variation .................................................. 143 144 4.4. Ozone and Terrestrial Magnetism .................................... 145 4.5. Relations with Solar Activity ........................................ 4.6. Relations with Meteorology ......................................... 146 147 4.7. Average Ozone Temperature ........................................ 11s

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6. Vertical Distribution ................................................... 6.1. Direct Methods .................................................... 6.2. Indirect Methods .................................................. 6.3. Methods Applicable to the Mesosphere. ............................... 5.4. AFew Results ..................................................... 6. Ozone in the Upper Atmosphere ......................................... 7. Origin of Atmospheric Ozone.. .......................................... References ............................................................

148 148 150 153

159 162 165 168

1 . INTRODUCTION The problem of atmospheric ozone has aroused much curiosity. The widespread interest in this constituent is quite disproportionate to its very small amount. Although ozone is only 3 parts in 10,000,000of our atmosphere, its very existence or, more exactly, its absorption of solar ultraviolet light makes possible the presence of life, as we know it, on earth. We will not consider here the biological problems connected with ozone but will restrict this review to geophysical considerations. The importance of ozone in atmospheric physics was disclosed only gradually; its importance stems mainly from its remarkable optical properties, which have many ramifications. First, ozone offers to meteorologists a convenient method for studying air masses in the stratosphere; it helps in measuring some of their physical properties, such as temperature and pressure, and it may be used as a natural tracer for their motions. Presently, other tracers are also available, i.e., artificial radioactive products. Both tracers give similar and consistent results. Ozone is also of interest in dynamic meteorology. This rare gas plays an important part in the heat budget of the atmosphere because it absorbs both terrestrial infrared and solar ultraviolet radiations. In this respect it acts as a reservoir, the ultraviolet energy being stored and delivered a t a slower rate after its conversion into infrared energy. We know that ozone is responsible for the relatively high temperatures in the mesosphere and also for the stabilization of these temperatures. An increase in temperature results in a decrease of the amount of ozone and of its heating capacity. Thus, in meteorology, ozone is not only a research tool but also a factor in atmospheric equilibrium, and its importance has been acknowledged by developing a network of ozone stations. Atmospheric ozone may be considered from several points of view, according to the various problems in which it plays a role. Attention can be given to the total amount of ozone in the atmosphere above an observation site, or to its mean temperature; these features characterize stratospheric air masses. For local climatology, measurements of the

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ground level (or surface) concentration are important. Finally, we are able to measure the variation of concentration with height or vertical distribution of ozone, which is a more elaborate aspect of atmospheric ozone. Advances in this topic do not take place regularly, but rather in sudden increments. Owing to the diverse interests, new aspects become available a t times when the situation seems stabilized. For example: About 1930 ozone studies were limited to the measurement of total amount and seemed to merit not more than limited attention from some geophysicists,Soonafter, Dobson andhisco-workers undertookstudiesof the relation between ozone and air masses, and physicists began to get the first direct measurements of vertical distribution and of mean temperature. The interest was for many years directed chiefly to the stratosphere, between 10 and 30 km. Now, thanks to the numerous results collected during the IGY (International Geophysical Year) and to space research, we know that ozone data are capable of yielding information on still higher levels and that ozone plays a part in photochemical reactions in the upper atmosphere. Therefore, it seems profitable to revise the basic data and to provide theoretical computations with accurate values. Likewise, as presented a t the I.U.G.G. Assembly in Berkeley (1963), many dynamic changes in the upper atmosphere are related to changes in ozone, e.g., breakdown of the polar vortex, Berlin effect, relationship between stratosphere and mesosphere, and reversal of equatorial winds. Problems in dynamic meteorology are not necessarily restricted to energy problems, especially when instability is suspected, and ozone may perhaps be the “trigger” mechanism which will start large changes in the flow patterns. Moreover, we must not forget that ozone is one of the most effective reactors to solar radiation in the terrestrial atmosphere. Thus i t may be a possible link between upper and lower levels. Our purpose is to present a general picture of atmospheric ozone with special attention to these new aspects of the question. Many problems are not yet solved, and explanations are still missing. We can only give an outline of our present knowledge and its recent developments. Such an outline is not entirely satisfactory because the problem has not reached complete solution and the results are not all consistent with the theories presently in vogue. But we think it worthwhile to show how ozone contributes to the more recent progress in geophysical research. 2. MEASUREMENT OF CHARACTERISTIC PARAMETERS

2.1. Different Aspects and Points of View Two different groups of methods are used for routine measurements of tot,al amount or of local concentration: the chemical methods, including

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chemiluminescence, and the optical methods or, more exactly, spectroscopic methods, which require as a basic knowledge an accurate value of the absorption coefficients. Broadly speaking, in chemical methods the amount of ozone to be measured is contained in a sample volume collected a t a definite location, the aim of the instrument designer being to have the sample volume as small as possible. Optical methods measure the amount of ozone along the beam path, between light source and receiver. Owing to this basic difference, these methods are capable of different applications. In the troposphere and lower stratosphere, the choice between these two types of procedure may be optional, each having its own advantages and limitations. On the other hand, when the measurement concerns regions that the observer or the instrument cannot reach, the optical method alone is available. This happens to be the case for the mesosphere which was, for a long time, above the ceiling of exploring carriers, and which remains barred to instruments operating withliquidsolutions. Furthermore, when ameasureof the total ozone content of the atmosphere is wanted, the result is acquired directly with optical methods, but only after summation with chemical methods. Since a chemical determination is basically involved in optical measurements, we shall first consider chemical methods, with special attention to those that do not require a previous calibration.

2.2 Absolute Meavurernents The main drawback of chemical measurements is the destruction of the substance, which is not significant in the case of atmospheric ozone; theif advantage is to yield an absolute value of the ozone content of a sample, provided the reactions are specific. The main difficulty in the atmosphere is the fact that ozone concentrations are low, from lo-' to lo-', and with these high dilutions other oxidizing substances can compete for the reagent; this requires serious attention. We will examine exclusively the methods valid for atmospheric concentrations. Another limitation is the minimum volume of atmosphere needcd by the instrument for a correct indication; this volume must be small enough to allow frequent measurements so that the final result is a nearly continuous curve, in cases of surface instrurncnts as well as ozone radiosondes (in this case the ascending rate is 300 mctert.l/min).Hence we are using the techniques of microanalysis. Different instruments were developed and are used routinely a t many stations. For all of them, the reagent potassium iodide is used according to the following reaction: 2 K I -t 0

3

-t HZ0

+0 2

+ 2KOH + 12

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The technique used follows from the electrometric analysis elaborated on by Paneth and his co-workers [l]. Ehmert [2] and his co-workers, after 12 years of constant improvements, designed an instrument which was adopted in many places of observation. The reaction cell, made of special resistant glass, is charged with 3 om3 of a neutral 2 % solution of KI and a minute quantity of sodium thiosulfate (1 cm3 of 0.01 N solution for 750 om3 of the KI solution). About 5 to 10 liters of air are bubbled through the solution, iodine is released by ozone, and the following reaction takes place: 2 Nazi3203

+ 12 + 2 NaI + NaaS406

The remaining thiosulfate is measured by the following potentiometric method: Four electrodes dip into the solution, 2 of which are used for the electrolysis of KI which is present in large excess. The iodine combines first with the remaining thiosulfate and, when thiosulfate is exhausted, polarizes the 2 other electrodes, allowing an electric current to flow. A voltage of about 0.18 volts is maintained between the latter two electrodes. The same process is applied to a sample of the standard solution. The thiosulfate absorbed by iodine is measured in terms of the quantity of electricity required for the electrolysis. The accuracy is gm of iodine, so that the main error lies in the volume measurement. The method has the disadvantage of not being automatic. V. H. Regener 131 has overcome this difficulty. A small amount (8 cm3) of the 2 yo potassium iodide solution, added with sodium thiosulfate to give a concentration of 5 x lO-ON, is admitted into the reaction cell. Air is drawn through the cell. Two electrodes dip into the liquid, and as soon as the charge of sodium thiosulfate is exhausted, a current appears between the electrodes. After amplification, the current is used to actuate relays which evacuate the solution and inject a new charge. The measurement consists in recording the number of pump strokes necessary to use the charge of thiosulfate. There is no attempt to exceed an accuracy of better than 5%. The method is entirely automatic. Recently [4] the instrument was improved by operating on a stream of air and by using a second cell in which the same amount of air is admitted after heating a t 300°C to destroy ozone; the difference is a measure of ozone, avoiding the influence of other oxidants. Coming back to Ehmert's four-electrode system, the author uses a controlling device which maintains a constant current through the sensing electrodes. Very similar to this last device is the equipment designed by A. Vassy [ 5 ] , which has been thoroughly modified since the first model appeared in 1952. The reacting solution is a 5 x 10-ON solution of thiosulfate with a large excess of KI. The solution is buffered with NaHpPO,, and when the thiosulfate is exhausted, the current delivered after polarization of the two

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electrodes deflects a galvanometer which acts as an optical relay, and initiates a succession of operations: evacuation of the used-up solution, rinsing of the cell, admission of a charge of fresh solution, recording of the volume of air which corresponds to the preceding charge by a printing counter, and zero resetting of the counter. The instrument also is entirely automatic and is in commercial production; it has been operating satisfactorily a t remote stations (Kerguelen Islands and Antarctic). The accuracy is estimated to 3 yo. Britaev [a] has designed an instrument on the same basic lines, but he preferred to measure potassium iodide by conductivity. He pointed out that alternating current is more desirable with better frequencies between 1000 and 2000 cps. Several pieces of equipment using potassium iodide were also commercially designed in the United States. Among the absolute measurements, we must give special attention to a method which is not very sensitive but may be of some interest: If ozone is completely dissociated in oxygen by heating to about 200°C, the reaction produces an increase in pressure (after the initial temperature has been restored) according to: 203-t 302

This increase gives a numerical value of the ozone which has been destroyed.

2.3. Chemical Measurements Requiring a Calibration

No previous calibration is needed for the above given methods, the ozone amount being determined by a known quantity of the reagents (or the increase in pressure in the last case). We will consider now some other processes which cannot be used without a calibration. 2.3.1. Electrometric Method. I n Pring and Westrip’s [ 7 ] electronzetric equipment, the basic reaction is the oxidation of hydrobromic acid by ozone with liberation of bromine; the oxidation-reduction potential of the reacting system is a measure corresponding, under suitable conditions, to the volume of ozone used. The ca,libration is performed by comparison with the potassium iodide method. 2.3.2. Radioactive Clathrates. Hommel et al. [8] proposed to measure ozone concentration by pumping the air laden with ozone, after having removed water vapor, through a radioactive bed made of quinol clathrate containing KrS5atoms. The following reaction releases Kr radioactive atoms which are counted with a ratemeter: [C&(OH)a]3

KFJ5

+

0 3 -t

3 CsHiOa

+ 3 Ha0 + Krss

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I n spite of the fact that the reaction could give an absolute measurement, the instrument is calibrated by comparison with the potassium iodide method. The sensitivity is high, by volume, and the response is linear; but the method is not specific, since it is also sensitive to strong oxidants such as NO, and C10,.

2.3.3. Rubber Cracking. It was proposed on several occasions that rubber cracking be used as a measure of ozone concentration in a gaseous atmosphere. Haagen-Smit [9] developed a quantitative method. The air is passed over special rubber strips a t a rate of approximately 1 liter/min. The time of initial cracking gives, after calibration, the amount of ozone. The accuracy is estimated a t 10 yo. 2.3.4. Colorimetry. Long chain molecules, such as phenolphthaleine (C,,H,,O,) or N-phenyl-2-naphthylamine, are broken on reaction with ozone. The reactions are associated with changes in the color of the substance. By comparison with standard samples, the amount of ozone can be readily determined. Nitrogen oxides must be previously eliminated.

2.3.5. Fluorescence and Luminescence. Ozonometry by fluorescent solutions has been known for a long time. Fluorescein was recognized as perfectly specific [lo] and yet was not used on a large scale. The advantage of reactions exciting luminous emission lies in the facility of the measurement by a photocell and of its transmission by radio. Bernanose ( 1 1) has investigated several aspects of this method and shown its limitations. I n spite of numerous difficulties, Regener [12] did not hesitate to make use of luminol in his new radiosonde, in consideration of its simplicity and elegance. The stream of ozonized air is pumped and passes for 15 sec along a disk which is coated with a mixture of luminol and silica gel and which faces a photomultiplier. The photoelectric current is amplified and supplied to the transmitter. The volume is about 100 cm3 of air. A calibration is done immediately before launching. Nitrogen oxides (NO,), which could react with the luminol, are neglected as their concentrations have to be 500 times greater than ozone concentration to excite the same luminous intensity. 2.3.6. Catalytic Anulyzer.,,Olmer [13] designed an instrument based on the catalytic decomposition of ozone. This decomposition is promoted by the circulation of the gas on a thermistor coated with hopcalite (a mixture of metallic oxides). A second thermistor inserted in a bridge circuit is used to measure the increase in temperature observed during the catalytic reaction. This method needs special care for a concentration as low as lo-', but seems

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insensitive to other oxidants provided that their concentration is less than however, peroxides may affect the measurements.

2.3.7. Brewer’$ Equ.ipment [14]. The instrument designed by Brewer for his radiosonde is based on the potassium iodide reaction, but a calibration is required before flight to be sure that no contaminants destroy ozone during the measurements. We have seen in Section 2.2 that the release of iodine, under proper conditions, can yield an electrometric current; if this iodine is not drawn away, it returns to the cathode and the current is no longer available. In the first device, the transmogrifier, the solution flows from the cathode to the anode. Now a second device is used, the bubbler. An anode of silver or mercury is employed, the iodine combines with the electrode, giving an insoluble product. The air is bubbled through the electrolytic cell, which contains a 0.1 or 0.2 % III solution, buffered with neutral phosphate. The accuracy is estimated to 6 yo. 2.4. Spectroscopic Methods The absorption spectrum of ozone covers a wide spectral range with absorption coefficients extremely variable and reaching values as high as 150 cm-’. This is very suitable for spectroscopic measurements, as the spectral range may easily be chosen according to the ozone amount. Of course, the ozone is not destroyed and no calibration is required, provided that the absorption coefficient is known with sufficient accuracy and for the proper conditions of the measurement. We will consider later the problem of coefficients which is not yet completely adequate. Nevertheless, such spectroscopic methods have been widely employed for a long time. Fortunately, former measurements can usually be corrected and adjusted when new coefficients become available. Spectroscopic methods are useful under two different circumstances: ground level concentration and total amount in the atmosphere. In both cases, the measurement consists in the determination of the optical density D of the ozone present along a given optical path; the apparatus consists of a light source and a spectrometer (or a receiver filters). The amount of interposed ozone will be expressed by a thickness (reduced to normal pressure and temperature conditions), x = D / a , u being the absorption coefficient for the wavelength considered and for the same conditions as during the measurement. When atmospheric ozone is concerned, D cannot be measured by elimination of the absorbing material. The method consists in varying the optical path in the most suitable way. Another possibility is to measure two densities D , and D, for the same optical path, but for two wavelengths with different coefficients. I n this case, the optical characteristics of the source and the receiver must be known.

+

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123

2.4.1. Ground Level Concentration. The first experiment on ozone absorption a t ground level, performed by Lord Rayleigh in 1918, was only qualitative. The quantitative method was proposed by Fabry and Buisson in 1929, and the first measurements were made by Buisson et al. [15], using a mercury arc and a slitless spectrograph. Absorption wa8 measured using the difference between two distances along the same beam, 589 and 2506 meters, in the Hartley band. The method and computations, perfectly explained with useful details, have remained a pioneer work and a model for subsequent experiments. This method is accurate and rigorous and, moreover, provides interesting information on the atmosphere in addition to the ozone absorption. On the other hand, it must be handled by physicists, more precisely, by specialists of photometry; the analysis is long and tedious. Several attempts have been made to simplify it for the purpose of routine measurements. Gotz, Schein, and Stoll first replaced the photographic emulsion by a photon counter which yields immediately the intensity of the incident light. Later, in 1954, Stair et at!. [16] designed an instrument in which the light source is modulated a t 510 cps by a rotating disk; the receiver is a photomultiplier; in front of it, a second disk bearing 3 optical filters selects 3 spectral ranges, one a t 2537 A, the second a t 3655 A, and the third between 3655 and 4358 A. In addition the equipment includes a tuned amplifier and a recorder. The measurements are made every minute, over a distance of 470 meters. The computations are made easier by use of numerical tables. This instrument was subject to the following restrictions: By daylight, in spite of the modulation, the photomultiplier is overloaded by solar-diffused light and the response is disturbed; the instrument is sensitive to the other atmospheric gases absorbing in the ultraviolet, a difficulty which is easy to overcome with prism instruments. The instrument was improved by Renzetti [17] following V. H. Regener’s suggestions. Filters were replaced by a prism, and a scanning recorder was placed behind. Using a mercury arc lamp, measurements are made for wavelengths of 2650, 2800, and 3130 A. The ratio of the intensities gives the ozone concentration. The distances used are 90 and 108 meters. The choice of several definite wavelengths is of great help in detecting spurious absorption by aerosols. Before concluding the discussion on spectroscopic apparatus for ground level concentrations, one should recall that on many occasions simultaneous measiirements have been made both with optical and with chemical equipment. The results prove satisfactory when the location is not contaminated by industrial effluents and when there is enough wind to pass fresh air continuously in the chemical instrument.

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2.4.2. Total Amount or Reduced Thickness. Again, pioneer work was done by Fabry and Buisson. For measurement of total amounts the Huggins bands are suitable. I n order to obtain a variable ozone thickness, or more exactly a variable quantity of atmosphere, interposed between the extraterrestrial light source and the receiver, only one solution was available, which took advantage of the variation of the sun’s zenith distance with time. This method, called the Bouguer-Langley method, is now well known; it is precise and not liable to objections, except when the atmosphere is not homogenous, or when it varies during the course of the measurements. This method has proved very useful. Several modifications ensued, especially the so-called “short” methods. These methods have some limitations and must be checked from time to time by the original “long” method. Fabry and Buisson and the French School use spectrographs, with or without a front slit, and with photographic plates; the light source is either, the sun, the moon, the stars, or the blue zenith sky. Dobson, however, preferred using a photoelectric receiver in his spectrophotometer. Visual instruments, operating in the Chappuis bands, were also designed; they require special attention to the water vapor absorption bands which more or less overlap the ozone bands (see Fig. 1). 40

Oa

-

5 I

Loo0

0.05

0.04

L

c

..E

5 0 c

500

0.03

0.02

.c

I 0.01

n $

4

0

4000

5000

6000

7000

8000

FIG.1. Absorption bands in the visible: 1, ozone Chappuis bands; 2, water vapor bands (relative scale).

For routine measurements, simpler instruments were developed by replacing the prism with optical filters; the spectral range has to be as narrow as possible. The first instrument was designed by Stair and Hand [18] and was used as a part of a radiosonde. The ratio of intensities received through the different filters, in the range 3000-3300 A, allows the ozone amount to be computed, provided the spectral distribution of energy of the source is

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known, as well as the spectral sensitivity of the receiver and the transmission factor of filters. Absorption by molecular scattering must be taken into account, and absorption by aerosols should also be considered, but is it generally unknown. We have also designed [19] very simple and small-sized filter equipment; two models are available, one of which is a recording instrument. For routine measurements, in order to obtain results consistent with a world-wide network, these instruments are calibrated by comparison with a spectrograph (Fabry’s method) and with a Dobson spectrophotometer. Both calibrations give identical results, owing to the fact that both instruments have comparable dispersions. The Japan Meteorological Agency has developed a more elaborate filter instrument which seems very attractive. It makes use of four filters that operate in two groups in order to measure the difference in the transmissivities of two filters. The modulated current yielded by the alternation of the filters is equivalent t o the transmissivity of a narrow band filter. Two such narrow bands are finally used. Obviously, good accuracy is needed. The instrument is intended to be launched as an ozone radiosonde. We must draw attention to the fact that, besides the total amount, the methods using a spectrograph or spectrometer are capable of giving, at the same time, the mean temperature of atmospheric ozone. The method originated with E. Vassy in 1935 and has been extensively used since that time. Most often, optical measurements concerning ozone are made in the ultraviolet region; but the infrared spectrum offers interesting possibilities, especially the strong absorption band a t 9.6 p. This band was observed in emission by Devaux as early as 1934. Owing to its unusual properties, J. Strong studied it in the laboratory and proposed its use for a rough estimate of the mean altitude of atmospheric ozone. Other scientists developed infrared methods for the measurement of ozone temperature and vertical distribution. These problems will be considered in Section 4.

2.5. Absorption Coeficients It is easily understood that all optical measurements have the same basic requirement, an accurate and exact knowledge of the absorption coefficients and of their variation when absorption does not follow the usual laws. Needless to say, measurements are not absolute if there is any doubt concerning the chemical measurement involved in the coefficients. In some cases, such as a study of variations, the problem is not very serious. The different observers agree on the choice of a series of coefficients, so that all measurements are consistent. Since 1957, Vigroux’s coefficient [20] have been recommended by the International Ozone Commission.

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But other fields of research are more involved, for example, the knowledge of the ozone content of the mesophere and the inferences that can be drawn about the photochemistry of the upper atmosphere. Instrumentation problems can become quite important; these instrumental effects have to be distinguished from the error of the coefficient itself. This seems to be the case for Dobson’s spectrophotometer. We will not elaborate upon these special cases which require the experimental determination of a proper correction factor. For these reasons, it seems worthwhile to consider the present state of knowledge in the field of ozone coefficients and their applications to the atmosphere. The usual exponential absorption law, D = ux, x being the thickness of the absorbing material, is rigorously valid only for a monochromatic radiation. It is still usable for a narrow range in which the absorption curve is not too complex, that is, without fine structure. However, a correction may be needed to take account of the slit width or, more exactly, of the resolving power of the instrument. Such is the case for the ozone Hartley band. The law, D = ux, does not hold when a group of lines is concerned. With some acceptable restrictions, the absorption varies according to D = a&, a being a constant. But in other cases, a or 01 is not specific of the absorbing material alone; a or a may depend on the temperature, but Beer’s law remains valid, as for the Chappuis or Huggins bands. Also, the pressure (partial pressure of the gas, or total pressure of the mixture) may introduce alterations in the rtbsorption coefficient, as for the ozone infrared band a t 9.6 p. Finally, the presence of foreign molecules (nonabsorbing) may alter the absorption coefficient, generally owing to association with the absorbing molecule. We will examine in detail the present state of our knowledge for the different absorbing regions. The numerical values will refer to the decimal absorption coefficient.

2.5.1. Ultraviolet. Between, roughly, 2000 and 3000 A lies the Hartley band; the absorption coefficient reaches very high values, the maximum being situated a t 2563 A. On the long-wave side of the Hartley band are the Huggins bands, down t o 3470 A. It has been definitely recognized that ultraviolet absorption is independent of pressure and foreign gases, but for some wavelengths it depends on temperature. The absorption coefficients have often been measured, but only the measurements of Ny and Choong [21], A. Vassy [22], Vigroux [20], and Inn and Tanaka [23] were determined from a light source with continuous spectrum. Figure 2 shows that, roughly, the four curves could be made consistent by multiplying them by a suitable constant factor. The situation could be considered as favorable were we in a

I50

I

1% FIG.2. Hartley band: 1, Ny and Choong; 2, IM and Tanaka; 3, Vigroux; 4, A. Vesey; 5, Inn, Taneke, and Watanabe.

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position to obtain error-free values for the coefficients of a few selected wavelengths, as was formerly done by Fabry and Buisson. Such measurements have beer recently undertaken by Hearn [24,26] who used a physical method (complete dissociation) for the measurement of the ozone amount. Hearn's results have directed attention to the urgent need of a revision of Vigroux's data. In agreement with Hearn's coefficients are those of Inn and Tanaka (except for 3341 A in the Huggins bands), and also the recent measurements of Ehmert and Schropl [26]. However, Vigroux's data, as well as those of Ny and Choong, are more detailed and more accurate in wavelength, having been obtained with a larger dispersion (see Fig. 3).

0

3300

3200

3100

MA, FIG.3. Huggins bands: 1, Ny and Choong; 2, Inn, Tanaka, and Vigroux.

31 00

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129

The discrepancies among these results cannot be explained by the different photometric techniques. Our personal opinion is that the differences stem from the measurement of the amounts of ozone, owing to deficiencies either in the overlapping of the different spectral ranges or in the method of measurement. I n this respect, Hea,rn’s method is very attractive, in spite of its lower accuracy (2%). His data could be used as reference values for correcting previous measurements. On the short-wave side of the Hartley band, values have been obtained by Inn et al. [27] up to 1000 A. This spectral range is particularly valuable for high atmospheric levels; the coefficient reaches values higher than 100 cm-’ between 1100 and 1400 A (Fig. 2, curve 5). Some minor problems are still unsolved for the Huggins bands, mainly concerning the temperature effect. For a long time, it has been known that some wavelengths are affected by temperature variations, but the quantitative data are not yet consistent. E. Vassy [28] observed no variation for the maxima in these bands, but Vigroux [20] announced a slight variation of the same maxima, curiously parabolic with a minimum around -60°C. For the minima, the absorption coefficient decreases with temperatures below 0°C. The importance of the effect varies with the wavelength with a maximum around 3200 A. E. Vassy had found a linear variation which was very convenient for computations, but Vigroux has observed a more complex variation. It is not impossible that these discrepancies have their origin in the different dispersions used by the authors, as E. Vassy was able to observe such a dispersion effect. It would be worth while to undertake new measurements of the coefficients in Huggins bands and of the temperature effect. This would provide an opportunity to investigate the small discordance between different results or even in the same series of measurements, which occurs very often around 3000-2900 A, as was noticed by E. Vassy in 1937. 2.5.2. Visible (see Fig. 1). In the visible spectrum, ozone has a large absorption band presenting some smooth undulations, known as the Chappuis bands. The more recent values of the coefficients have been given by Inn and Tanaka [23]. They agree (within a constant factor) with the values of Vigroux and without our own values, the later series not being absolute but adjusted with Ny and Choong coefficients in the Huggins bands. It will be quite easy to get reliable values when we have exact measurements for some wavelengths, as stated above. More confusing is the state of the temperature effect in the Chappuis bands. Discovered by Chappuis, and studied by E. Vassy, it was not observed by several other investigators. The question arises whether concentration or

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pressure affects these coefficients, because Chappuis and Vassy have used rather high concentrations. But, Dejardin [29], analyzing his measurements on atmospheric absorption, found results consistent with a temperature effect in the Chappuis bands. It seems important to study this problem again, as some satellite ozone studies are based on measurements in the visible range.

2.5.3. Infrared. The ozone infrared spectrum is rich in absorption bands. A series of bands appears between 7600 and 9910 A, but they are too weak to be used for the study of the earth's atmosphere. Further, we know bands a t 4.8 and 6.8 p, a strong one a t 9.6 p, another at 14.1 p, and a few lines between 126 and 600 p. All these bands exhibit a line structure when observed with sufficient resolving power. The strongest is the 9.6-p band, which was thoroughly investigated owing to its interesting properties and its applications to atmospheric studies. J. Strong has pointed out that absorption (besides the law in dx)varies according to p'14 for a given ozone amount. Other investigators obtained similar results. The 9.G-p band is also present in emission in the atmosphere. So we may conclude that if ozone absorption coefficients are rather well known from an academic point of view, their application to atmospheric research needs further study, which will require accurate, detailed, and patient investigations, 2.6. Units

A consequence of the diverse aspects of atmospheric ozone is a similar diversity in the units generally employed to represent the different parameters, aggravated by the fact that some are of chemical and others of physical origin. We will consider now the more common units and their relations, as well as the graphical representation of these data. We will refer mainly to different reports prepared by W. L. Godson, president of the Working Group on Atmospheric Ozone of the World Meteorological Organization. 2.6.1. Total Ozone. The total amount of ozone in the atmosphere above a station has been defined by Fabry as the thickness (reduced to standard pressure and temperature conditions) of the ozone contained in a vertical cylindrical column of unit cross section above the location; it is expressed in units of length: centimeters, millimeters, or microns. Recently the use of millicentimeter has been recommended ; as it applies to normal Conditions, it has been called milli-atmosphere-centimeter (m-atm-cm). The same definition

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is valid for the total ozone above a given pressure level for balloons and rockets.

2.6.2. Ozone Concentrationfor a Given Pressure Level, Particularly Surface Ozone. We have a wide choice of units, depending on the method of measurement, but all take account of the great dilution of ozone in the air: (a) Ozone density p-the mass of ozone per unit volume of air, expressed in pg/meter3, i.e., 1O-l' gm/cm3. ( b ) Ozone mixing ratio r-the mass of ozone per unit mass of air, expressed in pg/gm; it is a fractional number (formerly, it was called weight concentration). ( c ) Ozone mole fraction-the number of ozone molecules per molecule of air; this quantity is identical to the volume concentration c. It is also a fraction and may be measured, for example, by 3 x ( d ) Ozone partial pressure p-the product of total pressure p, by the mole fraction; it is expressed in micromillibars (pmb). Presently, the recommended units are those expressing the partial pressure or mixing ratio, especially a t ground level. But in the pioneer studies, which were undertaken chiefly by spectroscopists, a very convenient representation was used: the reduced thickness of ozone E , expressed in microns per kilometer of air (not reduced to standard conditions). When the pressure is known, E is easily converted to the volume concentration. This quantity is given directly in optical soundings. All these quantities satisfy the following relations: = c x p , (mb) x 10' (1) p (pmb) = 10.13~ x lo8 a t sea level p (pmb) = 1.657~ x 10' (2) r (pglgm) (3) p (pg/meter3)= 2 1 . 4 ~for standard pressure and temperature for optical soundings = E (plkm) x 3.71T x (4) p (pmb) T being the temperature 2.6.3. Vertical Distribution of Ozone. The vertical distribution is a representation of the variation of the concentration with altitude. The first experiments were made by optical methods and two kinds of diagrams were adopted; the ordinates were the altitudes and the abscissas either the reduced thickness per kilometer or the volume concentration. As stated above, reduced thicknesses are readily converted to partial pressures. Recently, the World Meteorological Organization decided to recommend the use of Godson's diagrams, called ozonagrams. The abscissas are ozone partial pressures, the ordinates are the air pressures in a logarithmic scale

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(which gives a rather linear scale for the heights). Isopleths of ozone mixing ratios are drawn on the diagram, and an auxiliary diagram (T-logp) gives the temperature versus the height a t the time of the ozone sounding. This ozonagram is a type of representation which is very familiar to meteorologists and offers the advantage that the vertically integrated amount of ozone (in m-atm-cm)is proportional to the area delineated by the distribution curve, the ordinate axis, and the two horizontal lines corresponding to the two limits of the layer. This property is very useful for checking the resulting curve and comparing the results with other measurements.

3. CONCENTRATIONAT GROUNDLEVEL 3.1. Measurements and Results Concentration measurements a t ground level, which were the first routine measurements (made in Paris since 1876) in the field of atmospheric ozone, appeared to be very informative as they showed an annual variation similar to that of the total amount. Since that time the number of measurements has increased, and it is ,notable that the interest in surface measurements is no longer the concern of the geophysicist alone. As a matter of fact, content a t ground level is strongly governed by local influences of meteorological or human origin. Therefore, as will be seen later, it concerns the meteorologist (studies concerning the exchanges in the troposphere, the part played by lightning in the local production of ozone) and, more particularly, the biologist and townplanner, since ozone is one of the elements of our ambient air, and pollution is a problem of alarming proportions. Let us recall that chemical measurements are representative of concentration in a given location only insofar as wind speed is sufficient to supply the air intake with fresh air (wind of over 3 knots a t Val-Joyeux, near Paris). All experimenters agree on this matter [30, p. 371. It is also essential to locate this air intake properly. The usual volume concentration is about lo-* (or 10 pmb) a t sea level. With altitude it increases, lo-' (100 pmb) a t Jungfraujoch Scientific Station (3467 meters). It is also higher a t isolated stations, about 5 x (Kerguelen Islands and Antarctic). A daily variation has been noticed, with ozone content decreasing during the night. It was impossible to find any correlation with total ozone, either in the daily changes or in the annual variation [31], which is inconsistent with the results recorded prior to 1910.

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Let us note mainly the very high concentration variability at ground level, where changes in a ratio of 1 to 15 can arise in less than 1 hr. Moreover, the limiting values range from 10-l' (almost zero) to lo-' a t sea level. An example is given in Fig. 4.

7 /5/54

7/6/54

7/7/54

7/8/54

7/9/54

FIG.4. Variations of surface ozone concentrations at Val-Joyeux (July 1954).

3.2. Exchange Phenomena The main ozone source is found in the stratosphere, and the supply of the lower layers is achieved by means of various mechanisms which are to be considered. Rasool [31] has shown that when subsidence periods with reversal occur, exchange stops and the ozone quantity a t ground level lowers considerably because of local destruction; moreover, little ozone is found in quiet air with fog.

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Various observers (E. Regener, Ehmert in Germany, Steinhauser in Vienna, A. Vassy in Paris, Teichert in Dresden) have noted the domination of local microclimate on the ozone content. A study extending over 7 years of measurements made in Val-Joyeux [32] has shown that ozone content increases as wind speed increases and then decreases a t a considerably slower rate than wind speed. This is obviously far less noted when there is a strong inversion which acts as a barrier to vertical exchanges. This phenomenon differs from the one reported above, which occurs with low wind and which consists of a rather poor ozone supply close to the air intake. Thus, turbulence (the scale of which varies in terms of wind velocity) is the most efficient factor supplying layers close to the surface with ozone. This ozone comes from higher layers where destruction is considerably slower than on the ground.

3.3. Storms Considering the annual ozone concentration change a t ground level, we noticed a maximum in summer. This led us to consider the part played by storms in ozone formation in the troposphere. Dobson and Brewer were the first [33] to note that a noticeable increase of the total atmospheric ozone content occurs during a storm. If ozone is produced in storms, the phenomenon is obviously concealed by ozone from the upper layers, which shows no increase. I n 1954 [34], we presented interesting correlations in this field, then Dave [35] reported some cases of ozone concentration increase in stormy periods. After a statistic survey extending over a 4-year period, we were able to show [36] that storms cause an important increaoe (3 to 10 times) in the ozone concentration. This increase starts before the first electric discharges, 3 hr before for the summer storms and 5 hr before for the winter storms. We observed also some cases of increase related to the approach of a cold front but without a local storm. Ozone is thus formed in the stormy cells a t the first stage of their development, i.e., by silent discharges and not by lightning. The alternative assumption of a supply from the upper layers through the downcoming flow external to the cumulo-nimbus does not appear to be reasonable for these specific cases.

3.4. Ozone and Atmospheric Pollution Ozone is a constituent of the medium in which animals and plants live. Its presence has two main consequences: a sterilizing action, which is useful, and, when present beyond certain amounts, an irritating or destructive action which is not thoroughly understood a t the present time. Studies are under way in this field, which will not be dealt with here. Nevertheless, we

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should recall here some of the deleterious effects that ozone has had on our environment in the past decade. Scientists were alarmed, in the Los Angeles district when it was noted, prior to 1950, that tires in that area were deteriorating faster than those in other localities. Measurements were carried out, which showed an abnormally high ozone content during the smog periods in Los Angeles. At the same time (1952) the ozone measurements made in Paris showed a n ozone concentration decrease during periods where traffic was heaviest. The Los Angeles smog also caused disagreeable eye irritations. Tobacco, grapes, and other crops suffered objectionable injuries. Similar phenomena were also noticed with Virginia tobacco. Therefore, an exhaustive study was carried out by the Air Pollut,ion Foundation which issued numerous reports in this field, the main points of which are summarized by Renzetti [37]. It was shown that ozone formation results from automobile exhaust gases. At the same time, we made some measurements in a field station close to Paris as well as in Paris a t various levels (Eiffel Tower). We noticed [38] that Paris air is very poor in ozone in the first 30 meters above ground level and that the annual average value of concentration is slowly decreasing as human and automobile presence increases in the surroundings (see Table I). TABLEI Volume concentration ( x 10-8)

Years

Volume concentration ( x 10-8)

Years

1.061 0.685 0.893 0.712 0.625

1954 1955 1956 1957 1958

0.385 0.207 0.168 0.313 0.145

1959 1960 1961 1962 1963

It was interesting to check the reason for automobiles causing an ozone increase in Los Angeles but a n ozone decrease in Paris. This apparent discrepancy has been found to be due to the different nitrogen peroxide contents of the air of these two cities. Actually, this component is the oxidizing agent which, under ultraviolet illumination, reacts on the unsaturated hydrocarbons, provided that its content ranges between 10-7 and 10-4. The volume concentration prevailing in Paris has been, up to now, below these limits, but has been slowly increasing, as shown by analyses made a t regular intervals [39]. This problem is obviously important insofar as big cities are concerned, and has to be closely watched by Health and Town Planning Authorities.

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3.5. Origin and Interest of Tropospheric Ozone It can be seen from the discussion in Sections 3.3 and 3.4 that ozone close to ground level shows only limited relations to geophysical phenomena proper. Where does tropospheric ozone come from? Considering that photosynthesis is not efficient, the only causes of formation are the storm cells and the photochemical oxidation of unsaturated hydrocarbons and alcohols by nitrogen peroxide. The latter is limited to big cities while the former is restricted to short intervals; their role in the production of ozone is, therefore, minor. Generally ozone comes from the upper layers. Concentration is the result of a changeable balance existing between ozone supply and destruct'ion. Destruction occurs a t ground level. It is caused by plants, animals, and industrial products, its efficiency being more or less pronounced according to the presence of these various factors. Thus, in Kerguelen Islands, in the Antarctic, and in the Sahara, destruction of ozone is low. I n this respect mountain stations are more representative insofar as tropospheric ozone is concerned. Moreover, in the whole troposphere, destruction occurs by spontaneous decomposition-very low in low temperatures-and photolysis by the visible solar radiation. Ozone is supplied from the stratosphere by diffusion and turbulence. It has been assumed that, on the average, mixing is good in the troposphere, as the ozone content is nearly constant under the tropopause. Thus, ozone may be used as an indicator in order to study the stratospheretroposphere exchanges. (Such an attempt was made by Junge [40] but with insufficient data.) However, a t ground level, measurements correspond mainly to the effect of local or transient phenomena. Ozone can make a valuable contribution in the study of these phenomena. Finally, we should not neglect the air we breathe; physicians and biologists have been investigating for the past few years the physical factors of our ambient medium in relation to our physiology, epidemic outbreaks, and sensitivity to disease. It would therefore be regrettable to discontinue the surface ozone measurements. This applies particularly to stations which already have a long series of measurements. +

4. REDUCED THICKNESS AND TEMPERATURE

4.1. Introduction Since the initial work of Fabry and Buisson in 1920, numerous measurements of the total ozone content of the atmosphere have been carried out. Some of them are grouped in small series of variable duration, intended for examining a specific aspect of the matter. The first, most systematic set of measurements was the patiently arranged network of G. M. €3. Dobson,

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which he started in 1925 and regularly extended from year to year; later on, an extensive effort resulted in the network of 60 stations of the International Geophysical Year and the International Geophysical Cooperation. We shall not record the results, now conventional, due first to Dobson and his co-workers: annual variations, variations with latitude, characterization of the origin of the air masses on a synoptic scale. They are reported in such books as those of Fabry [all and Paetzold [42]; Dobson [43] gives, more particularly, the relations of the daily changes with meteorological conditions. We prefer to give details of the latest results or the results recently brought forward which complete or improve the basic pattern of the first 30 years. These new results have been gathered thanks to the extensive effort during the IGY (International Geophysical Year). They allow more valid statistical studies than were made prior to 1057. Stimulation has also come from the interest in ozone shown by meteorologists because ozone is considered as a kind of tracer which facilitates the study of air motions. It is of critical importance to stress the double character of the ozone content of atmosphere: part of i t is conservative, ozone created elsewhere being protected from destruction by its low temperature and by absorption of the layers situated above, and another part is permanently subjected to the photochemical equilibrium law. This equilibrium varies with the factors controlling it. I n spite of difficulties arising in calculations, the lower limit of the region in which ozone is dependent on this equilibrium is approximately situated a t 25 to 30 km elevation. This dual aspect of the problem obviously did not facilitate previous work as the conservative part, the most important one, imparts its inertia to the variations. Moreover, ozone created by actions other than solar radiation superimposes its specific character, but to a lesser degree because only small quantities are involved. As in any other atmospheric phenomenon, ozone can be considered from two different viewpoints: one may use statistical studies or it is also possible to make a thorough study of some specific cases, the characters of which are intense enough to exclude the assumption of entirely coincidental occurrences. This depends on the scale selected for the study of the phenomenon. This notion is of a critical importance insofar as atmospheric physics is concerned. A large scale results in neglecting some transient or local effects in order to find a variation corresponding to a greater time or space range. These effects mutually cancel in the broad picture. However, the study of these accidental effects must then be resumed because they become the origin of new ideas and interpretations which represent physical phenomena. We want to consider here the minor (as presently considered) effects, keeping aside the matters related to vertical distribution which u7ill be detailed in a separate section.

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4.2. Ozone Distribution Irregularities at the Earth's Surface Though a minor constituent, the whole ozone content of the atmosphere is about 3 x loe metric tons. In order to have an idea of its distribution a graph in Fig. 6 shows the curves of equal reduced thickness, the abscissas being the months while the ordinates are the latitudes. The selected values are the monthly averages extending over a great number of years and stations. We were aware that the equator is the area where the smallest ozone content is recorded and that maximum values were found toward the upper latitudes in the spring. The significance of a figure such as Fig. 6 should not be overestimated, because local variations do exist, as will be shown. However, Fig. 6 clearly shows that the poles present a very noticeable deficiency. The maximum is situated a t about 76" latitude in the Northern Hemisphere and J

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N

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80 N

60 40

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Fig. 6. Distribution of ozone total amounts (the hatched are- correspond to amounts exceeding 400 m-atm-om).

a t about 50" in the Southern Hemisphere. The minimum line, which may be called the "ozone equator," is situated a t approximately 3"-4"N with a noticeable shift to the north during the southern summer period. An asymmetry between the two hemispheres results from these conditions. We already mentioned that in the Kerguelen Islands we noted the highest average ozone

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values known at the time [MI, and that the area of the largest ozone thicknesses covered a latitude belt ranging between 44" and 54" south. The ozone excess in the southern latitudes is not limited to this area. I n 1939 we were able to show [45] that the Southern Hemisphere had reduced thicknesses about 11 yo larger than the same latitudes in the Northern Hemisphere between 10" and 45" latitudes. This effect was recently noticed by Khrgian, and next by Kulkarni. Khrgian considers that this excess is compensated for by the fact that the polar cap deficit is more extensive in the South Polar area than in the North Polar region. If we take a smaller scale separating the longitudes, another distribution irregularity, recently noticed [46], appears, namely, a change with longitude. The distribution does not have a zonal character. Considering a geographical distribution for a given season (Figs. 6 and 7), rather steady areas with maximal ozone content can be observed in the Northern Hemisphere 1900

FIQ.6. Springtime geographical distribution of ozone (from J. London) of the total ozone concentration (m-atm-cm).

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FIG.7. Autumnal geographical distribution (from J. London) of the total ozone concentration (m-atm-cm).

throughout the year (London, Sekiguchi, Khrgian). These areas are the north of the American continent, Scandinavia, and the eastern edge of the Asiatic continent. A very noticeable ozone “valley” crosses Siberia, from India to the Pole. Such an effect is obviously a result of the continental areas and cannot be found in the Southern Hemisphere with small land area. The Southern Hemisphere provides an easy application of the theory of the general circulation of an atmosphere on a rotating sphere. This is why the austral stations are of critical importance. One can understand that this notable longitude effect is of such a nature that it must affect the interpretation of research work done before this phenomenon became well known. The seasonal variation and some specific cases will now be considered. The conventional diagram of the seasonal variation in reduced thickness is a curve with its maximum in spring and its minimum in autumn. However, this is not the general case.

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Considering, first, the case of the lower latitudes the following facts are apparent. I n 1941 we showed [47] that a t the lower latitudes the reduced ozone thickness was rather closely following the change in solar energy, the maximum being extended from June to September with an occasional minimum between these 2 months. Considering the IGY and IGC data reported for the Leopoldville (4"), Kodaikanal (lo"),Mauna Loa (19"), Mont Abu (24"),and Marcus Island (24")stations, it can be seen (Fig. 8) that a maximum is reported for Mont Abu, Marcus Island, and Mauna Loa in June and for Kodaikanal, in June, July, and August. For Leopoldville, the 1958 curve presented a noticeable dip in May; we did not establish the average value for the 2 years, but the annual change is very slight.

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Month Fro. 8. Seasonal variation for low latitudes.

ARLETTBl VASSY

j? 4.j

/

\ \. 1. 78O I I' -0 2. 49O21' -+ 3. 43O30'-

4. 5. 6. 7.

54O 76O 41°

Little Americo Port oux Fronsois Christchurch

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Mocquoris Holley Bay -Wellington 66O40'Dumont d'Urville 8, 650 .........Argentine Island

J

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1

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FIG.9. Seasonal variation for high latitudes showing the winter maximum.

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For the higher latitudes different patterns were noted. The latest measurements made during the IGY evidenced a new effect: a secondary maximum in winter [44]. We considered in particular, the Southern Hemisphere, where land and water distribution leads to a considerably simpler general circulation or, in a sense, closer to theoretical conditions. On the other hand, it is known that the circulation pattern is clearly zonal between 45" and 90" south latitude and that the polar stratospheric trough is very steady over the Antarctic. Figure 9 shows the annual change in ozone (reduced thickness) for the stations of Little .America (78"), Halley Bay (76"),Dumont d'Urville (67"), Argentine Islands (65"), Macquarie (54"), Port-aux-Franpais (49"), Christchurch (43"), and Wellington (41O). All the data concern 1958 and 1959, and for some of them 1957, except for Christchurch where they correspond to old measurements which have been corrected in order to be made homogeneous with the other ones. On all these curves one can observe, in mid-winter, a very noticeable maximum which is not noticed in latitudes below 40". This maximum value also exists in the Northern Hemisphere; on the curves presented by Ramanathan [48] a t the Arosa Symposium a secondary maximum in December was noticeable for Resolute (75"N), Tromso (70"),and Reykjavik (64') stations. Also, Sapporo, although a t 43" only, is situated in an area of high thicknesses (edge of Asiatic continent); in 1959, a double maximum was recorded here. This secondary maximum reaches an exceptional value in the Kerguelen Islands and in Little America. We now turn to the case of the spring maximum. I n the Northern Hemisphere, a t the upper latitudes and even in Sapporo, it appears rather prematurely, in January, February, or March, according to the year and in connection with the breakdown of the polar vortex and the stratospheric warming. It appears in April in middle latitudes. In contrast, in the Southern Hemisphere, in the high latitudes the maximum appears rather late, a t the end of spring (November), but in middle latitudes it appears, as in the Northern Hemisphere, a t the spring equinox (September). 4.3. Diurnal Variation Old measurements made by Chalonge and Vassy in Arosa failed to show any change between day and night. But in 1952, measurements made in India and Pakistan showed an apparent increase of the total ozone thickness during the hour following sunset, compensated for by an almost as fast decrease after sunrise. The various investigators agreed that this change, both for experimental and theoretical reasons, was located between 40 and 60 km elevation. Resuming his old measurements, Ramanathan was unable

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to confirm his results, which could be ascribed to a fog effect. Examining the measurements made in the Antarctic, we were unable to find a change. However, Diitsch, although not finding a change during sunset or sunrise, noticed in Arosa a change during the night, with an increase prior to midnight and a decrease later. Though this may be interpreted as resulting from reactions in the mesosphere, no quantitative agreement has been found [49]. On the contrary, however, a diurnal change has been detected by Khrgian and Kouznetsov [50] who considered the data available from 3 stations (Reykjavik, Vigna di Valle, and Elmas). An increase of 8 m-atm-cm was noticed between 9 and 16 hr local time, with a minimum a t noon. In Japan, Horiuchi also found an ozone excess in the afternoon for the summer months, up to 10 m-atm-cm. I n Afghanistan, Khalek [51] in a systematic study of the diurnal change, noticed a minimum a t 14 hr local time, as well as an ozone excess in the afternoon as compared to the morning. The noon dip was considerably larger in summer than in winter. These results were obtained during days of very fine weather only, i.e., about 20 days per year. A t the same time, Khalek showed that the temperature was a t a maximum a t noon. Although all these results can be explained along the lines of the photochemical theory, it is not possible to arrive a t quantitative values.

4.4. Ozone and Terrestrial Magnetism Various authors looked for a correlation between magnetic activity and the reduced thickness of atmospheric ozone, without arriving a t conclusive results. However, most of these studies were related to middle latitudes where the daily fluctuations of the reduced thickness have a very great amplitude compared to the effects of magnetic activity and a comparable variability. Taking advantage of IGY results, we studied [52] various cases of magnetic storms showing a minimum ozone thickness during the 24- or 48-hr period prior to the storm minimum, followed by a steep rise. The maximum limit generally occurs a t the same time as the storm maximum. We noticed a time shift with latitude. An additional statistical study covering the monthly averages appeared to show, also, a positive correlation between the reduced thickness and the magnetic character figure, C. A more thorough investigation by Sekihara [53], extending over a 3d-year period and distinguishing the lower latitudes from the auroral zone, confirmed our study. Sekihara finds a t the lower latitudes a minimum of ozone 1 or 2 dzys before the magnetic disturbances and within the auroral zone a minimum of ozone on the day of the disturbances. Moreover, in the latter zone, a second minimum occurred 5 days later.

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145

A recent work of Kulkarni [54], considered from a rather different point of view, shows also an influence of magnetic storms on ozone. Thus the existence of a correlation between ozone and terrestrial magnetism appears to be well established; obviously this does not mean that it involves a cause and effect relationship. I n connection with these observations, it should be recalled that Murcray [55] noticed a large increase of ozone infrared radiation emission immediately after a polar aurora; the question arose whether this was related to an increase of ozone thickness or of ozone temperature. It seems, according to Sekihara, that a thickness increase exists, but not to the exclusion of the temperature increase. If a direct relation seems to exist within the auroral zone between the two phenomena, through the corpuscular radiations which cause the aurora, it still remains to be explained why the phenomenon occurs earlier a t lower latitudes. No doubt we must search in solar activity other than corpuscular radiation to find the origin of these effects. A thorough analysis of the matter seems to be worth while.

4.5. Relations with Solar Activity The oldest method of characterizing solar activity is the application of the number of sun spots which presents the 11- or 22-year cycle and 27-day recurrences. Various attempts have been made to find the 11-year cycle in the ozone measurements. A major hindrance has been encountered, namely, series of records extending over several decades are very scarce. The only report of a correlation is due to Paetzold, who considered the ozone amount above 35 km, but for only 8 years. However, a similar difficulty does not apply to the 27-day cycle. But another objection arises. It has been said that the major part of ozone is conservative; short-term solar influences are active only on the upper ozone layers where the photochemical equilibrium is quickly established. Thus, proceeding over the full thickness, the changes investigated are attenuated by the predominating part of low ozone and are concealed by its changes in connection with atmospheric motions. Thus it is not surprising that the 27-day cycle has not been discloeed. At the present time the solar activity is also characterized by the solar flux a t 10.7 cm, which is also considered as representative for the short wavelength ultraviolet radiation. An intervting study has been made by Godson [56] who investigated the correlations with this solar index by dividing the ozone into layers of 5 km in thickness. He recorded positive correlations for the upper layers, up to 40 km in winter, and 30 km in summer, i.e., for the layers where the photochemical equilibrium is established in a few hours.

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Attempts have been made to reach a correlation with the flares [63]; no clear result has been obtained. This confirms the earlier remarks of Fritz who still considered the total thickness.

4.6. Relations with Meteorology Relations between the daily changes in ozone thicknesses and the motions of air masses are too well known to require detailed description. They have been specifically studied by Dobson and his co-workers [43]. Ozone acts here as a tracer. As a matter of fact, in spite of the slow decomposition suffered by stratospheric ozone, its gradual degeneration is not faster than that of the air mass conveying it. This phenomenon is noticed more particularly a t the lower latitudes where daily changes are less important than in the middle latitudes. We have observed several such cases in Morocco. Also, in the U.S.S.R. and Japan a noticeable influence of the jet stream has been observed on the ozone distribution. The jet stream develops a barrier preventing any transfer along a meridian. North of the jet the ozone content is greater than to the south. This is still more accentuated in winter. Khrgian explains it by a circular ozone flow around the jet stream, going downward to the north and upward in the south. Besides of its interest as a tracer, ozone contributes to the thermal balance of the atmosphere, and it is thus closely associated to the stratospheric changes. Goduon [67] obtained remarkable correlations between the 10-day running means in ozone thickness and temperature a t 100 mb for various times and stations. Correlation is also excellent with temperature a t 30 mb. I n Arosa, Diitsch [68] also obtained, for the winter, a good correlation between ozone content and temperature in the lower stratosphere. I n the lower latitudes (down to 40"), a 2-year cycle in the values of ozone thickness has recently been recorded in Australia [69]. A similar investigation has been presented by Ramanathan a t the Berkeley I.U.G.G. Meeting (1963). This effect is probably bound to the oscillation of about 26 months reported for the equatorial circulation (with wind reversal). The Berlin effect appears to be an interesting case. I n 1962, Scherhag discovered a stratospheric warming phenomenon of explosive character occurring in February. Experiments made with missiles a t Fort Churchill established that changes of dynamic nature start in the upper altitudes, within the mesosphere, and are propagated, in a few days, down to the lower layers. At about 3640 km, one can note a large temperature rise. This phenomenon has now been thoroughly studied. We know that i t is connected with a breakdown of the polar vortex. Diitsch [60] has shown that ozone concentration increases strongly during these sudden warmings. Temperature a t 10 mb reaches its maximum a t the same time as ozone

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concentration a t 30 km. We note, however, that the ozone content of the layer a t 35 km starts to increase about 5 days before this maximum. Dutsch's maps also show that the increase in the total Ozone thickness starts in the very high latitudes 3 days earlier than in middle latitudes. The vertical distribution measurements of Paetzold [61] confirmed and completed Dutsch's results. Although dynamic motions connected with these warmings appear to be well explained, ozone formation in January a t about 70" latitude is still difficult to understand, with a photochemical origin hard to imagine. We noticed another example of ozone increase accompanying a high temperature between 40 and 70 km [62]. In February of 1961, a t Hammaguir, rocket firings enabled us to measure mesospheric temperature. We noticed a n abnormally high temperature between 40 and 70 km, corresponding t o ozone reduced thicknesses, exceeding the regular value by about 30 %. Before ending this section it is worth recalling that we compared the secondary winter maximum in the seasonal change of ozone content with the winter anomaly of the D layer as well as that of the electronic density of the F layer. Godson did notice that solar activity does not explain the absorption phenomena within the D region, but that there is a matching relation between the D-region anomalies and the appearance of baroclinic waves a t 100 mb; however, this relation is not noticed in all cases. To summarize, it can be seen that numerous phenomena in the field of dynamic meteorology are associated with important ehanges in atmospheric ozone. Even a lunar influence, similar to tidal effects, has been reported [63]. However, up to now, there is no satisfactory relation between cause and effect which can be proposed, and the connecting mechanism is still obscure. I n order to explain these problems we shall have to wait for additional data on ozone variations a t high altitudes.

4.7. Average Ozone Temperature Obviously, all the reduced thicknesses or vertical distribution changes of ozone are accompanied by changes in its average temperature, as defined by E. Vassy in 1936 (see Section 2.4.2.).This parameter is rather easy to obtain with spectroscopic methods but, in spite of this, it has not been measured very often. Nevertheless, its measurement permitted the calculation, well before the missile measurements, of the seasonal change in the polar stratosphere [64, 651. From simultaneous meteorological soundings and ozone soundings, the mean temperature permits one to obtain data on temperature above the ceiling of the sounding. But the most serious problem is the risk of recording some effects which are, a t least partly, temperature changes and not total thickness changes when the influence of this temperature effect on the minimum absorption coefficients is omitted. Thus, in the use of Dobson's spectrophotometer, pairs

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of wavelengths B and C are both affected by temperature, one of these wavelengths for C (3322.9 A) being exactly a t a minimum and for B (3291.6 A) between a maximum and a minimum. The measurements made under conditions in which stratospheric temperature may have changed (diurnal effect, solar eclipse) must be carefully scrutinized. Khalek’s records, already quoted [61], are pertinent in this connection. The ozone temperature is also obtainable through infrared measurements; the method was developed by Adel [66] who used an infrared spectrometer with a rock-salt prism and a thermoelement. He measured the blackbody temperature corresponding to the infrared emission of atmospheric ozone, which he called ERTOR (effective radiation temperature of ozone region) ; obviously, it is an average temperature. The measurement is feasible by day as well as by night, and no diurnal variation was observed; but rapid changes, such as may be caused by turbulence, occur rather frequently. We must keep in mind that the pressure effect on the 9.6-p band is able to give more weight to the lower layers. A statistical study, dealing with 3 years of measurements, has shown interesting periodicities in ERTOR values: a 19-day period in summer and winter, a 10-day period in summer only; the 19-day period is also noted for the absorbed infrared energy and for the total ozone amount, this last quantity being also affected by the 10-day period. 6. VERTIUAL DISTRIBUTION

Vertical distribution of ozone and, more particularly, its changes are among the most valuable pieces of information for geophysicists and meteorologists. The general aspect of this distribution is known. Starting from ground level partial pressure rises with altitude until i t reaches a maximum which is observed between 20 and 30 km, and which is of 200-300 pmb (sometimes more). It is often noted, as will be seen later, that 2, or even 3, maxima are present. I n order to determine this vertical distribution, various methods have been used; some are direct ones, i.e., observation is made a t various altitudes by an instrument carried aloft, the measurement being made by either chemical or optical procedure. Others are indirect methods, with the instrument remaining on the ground in a fixed position. The latest procedures are especially interesting insofar as the mesosphere and altitudes above 30 km are concerned. These methods are to be considered prior to considering the results.

5.1. Direct Methods The instrument is installed aboard a carrier in order to collect the required information. Hence, it is necessary to recover the records or the plates after

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the ascent or to transmit the data in flight by means of some type of telemetry. The carrier can be a manned balloon. This was the case in 1935 when the Explorer I1 stratostat carried 2 quartz optic spectrographs, recording every 15 min, the first one the direct sunlight and the other the sky light, a little above the horizon. Each spectrum allowed the quantitative measurement of the ozone above the balloon altitude. This method was used on that occasion only. The same experiment can be performed with unmanned balloons equipped with spectrographs which are recovered after descent. This method was used for the first time by E. Regener in 1934 and has been used several times by his co-workers, in particular by V. H. Regener. The measuring instrument can be carried by an aircraft. This has been done several times by Ehmert and his staff and by Brewer and his assistants. The instrument was a chemical one. Each measurement gives directly the ozone content a t the altitude a t which the measurement is made. However, the sole routine method consists of using a radiosonde transmitting the information during the ascent. Two main types are available, i.e., the optical sonde and the chemical or electrochemica1 sonde. Most of them have been developed for IGY. Obviously, the two major requirements for these instruments are low weight and moderate cost. Moreover, the apparatus must be simple enough to be handled by nonskilled operators. Brewer’s sonde [67]-a chemical device-uses the potassium iodide method described in Section 2.3. The electric signal generated is applied to the transmitter of the radiosonde regularly used by the British Meteorological Office or, more precisely, to a transistorized version of this instrument. Accuracy is estimated to 16% for variations versus altitude. Regener’s sonde [12] is a luminescent-type sonde using the above-described principle (see Section 2.3.5). This instrument is coupled to the radiosonde of the U.S. Weather Bureau. Since water vapor can cause some errors, the air must be dried a t the inlet. Calibration must be made prior to start. Accuracy seems to be better than 5%. The Japan Meteorological Agency developed a type of chemical sonde [68] using an electrometric method similar to those of V. H. Regener or A. Vassy, where measurement is made by measuring the time interval between two refills. A calibration is made before launching. Accuracy is considered to be better than 5 yo. The first of the optical sondes originated with Coblentz and Stair [69]; it has, however, scarcely been used. The two types now in operation, i.e., the Paetzold sonde [70] and the Vassy sonde [71], closely resemble each other. The intensity of solar radiation is measured in two spectral ranges, one ranging from 2800 to 3600 A and the other one from 2800 to 3300 or 3500 A. The ozone amount above the balloon is measured from the ratio of these

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two intensities, taking into account the absorption by molecular scattering. The Vassy sonde is coupled to the French standard meteorological sonde; the Paetzold instrument is provided with its own transmitter and a pressure gage. Since their development, both instruments have undergone numerous modifications in order to improve the accuracy, which was 15-20 yo only. The Japan Meteorological Agency also developed an optical sonde using the device described in Section 2. It is not yet in operation. An optical sonde was also developed in Sweden by Stranz but has not been used. When comparing chemical and optical sondes, it can be seen that each type has its own advantages and limitations. Instruments using liquid solutions cannot be used a t high altitudes where an excessive evaporation disturbs the measurement. Moreover, it is remarkable that these instruments give, generally, a very low ozone content in the troposphere, contrary to other observations. The optical sondes cannot be used a t night since they necessitate the presence of the sun; by way of compensation, however, they are capable of giving the ozone amount above the balloon-bursting altitude. The latter information completes usefully the recorded distribution. Optical sondes are not sensitive to the other oxidizing agents existing in the atmosphere, as is the case with the other types of sondes; however, the readings are affected by the presence of aerosols insofar as proper narrow band filters are not yet available. As a matter of fact these aerosols are rarely present above 16 km. The optical sondes do not, require a highly trained staff for handling and operating. However, the processing of the data is more difficult and can be made a posteriori only. Finally, the optical sondes require great precision in making measurements in order that the final distribution curves bc of reasonable accuracy. This has been, up to now, one of the major hindrances, but remedies appear to be in sight. Nevertheless, the optical sondes are the only ones which can be used for the upper layers, thanks to their integrating power over an optical path multiplied with the zenith distance of the sun. Thus, the type of sonde has to be selected according to the intended scope of research, and it is fortunate that instruments which complement each other are available.

5.2. Indirect Methods The first experiment which gave valid vertical distributions used an indirect method based upon the Gotz effect. This method was used first in 1932. It is the so-called “Umkehr effect” or Gotz effect method. This is now a conventional method. Let us simply recall that the distribution curve is obtained in proceeding by successive approximatiQns, with simplifying assumptio&, so that it is possible to calculate a curve matching the experimental results. The method can be used either with a spectrograph or with a

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Dobson spectrophotometer. In the latter case the major hindrance is the high sensitivity to aerosols, the influence of which can be eliminated with a spectrograph. Another hindrance was the difficulty involved in the mathematical analysis, which was done with the aid of Ramanathan tables. Diitsch [72] attempted to overcome these difficulties by using a method of analysis fitted to electronic computers. Mateer [73] also gave a calculation procedure making use of matrices. Obviously, the Umkehr method is sensitive t o changes in the distribution as measurements proceed. Much more critical are the simplifying assumptions adopted during the IGY, which automatically lead to inaccurate results any time the distribution is rather uncommon, for example, when there are 2 or 3 maxima or a high concentration in the mesosphere. This is evident since the variation law between 36 and 64 km is kept unaltered (following a preset variation), and only 3 parameters are determined in order to plot the whole distribution curve. Indeed, when an abnormal distribution is detected, it is always possible to avoid these restraints and to attempt to find the proper distribution by means of tedious approaches. However, experience shows (as may be easily realized) that a single solution cannot be found, as it would require an accuracy far greater than that obtainable by present measurements. A detailed mathematical analysis made by Walton [74] leads to similar conclusions. Moreover, the role of multiple scattering is still unknown, and, here again, temperature acting on certain wavelengths alters the results. These restrictions must be kept in mind when handling the numerous results obtained by this method, which was the only practical one for several years. Another indirect method is the infrared method. It is derived from the Strong method proposed in 1939; it consists in using the pressure influence on the infrared band of ozone a t 9.6 p (see Section 2.5.3). The interest of this spectral band lies in a transparent window of atmospheric absorption. The process is as follows: The atmospheric absorption is measured for three spectral ranges: ultraviolet, infrared a t 1.14 p, and infrared a t 9.6 p. The first gives the ozone reduced thickness, the second the precipitable water, the third yields the ozone “mean pressure” after having substracted the absorption due to water vapor. The value of the energy W (or better, the power) absorbed by atmospheric ozone within the 9.6-p area is obtained by comparison with a blackbody a t known temperature. Let z be the altitude of a layer containing a reduced ozone thickness x, under pressure p . The energy absorbed (within a constant) is

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ARLETTE VASSY

and, in the whole atmosphere, the energy absorbed will be

W

=IOm

xp114dz

Total thickness has been measured and is (6.3)

X

=jam x

dz

Assume an ozone layer X to be a t uniform pressure P,. In order to absorb the same energy it should have a pressure defined as follows:

The average P, pressure corresponds to an altitude H which can be called the average altitude, keeping the definition (6.4)in mind. This procedure was used again by Walshaw and Goody [76], [76] employing a double monochromator. They were able to observe variations of this average altitude. Adel and his co-workers [77] improved the method by measuring not only its absorption but also its emission within the infrared bahd. This allows one to obtain the vertical distribution. Goody and Roach developed also an alternate procedure [78]. The Adel method is as follows: The reduced thickness is obtained by a measurement within the ultraviolet range. Absorption of the solar radiation by ozone as well as ozone infrared emission is measured within the infrared band. From these results it is possible to obtain the total ozone thickness, ERTOR temperature (see Section 4.7) and the “effective” By dividing the atmosphere into thicknesses of each layer, i.e., x(p/p0)0.296. layers, one attempts to find by successive approximation as in the “Umkehr” method a distribution capable of matching the measurements. The number of layers is 13, between 0 and 47.6 km. In order to proceed to a complete analysis, data on temperature and pressure supplied for the lower layers by meteorological radiosondes are used. The Goody and Roach method is very similar, but involves some simplifications. They neglect the mean pressure measurements when considering Goody and Walshaw’s results [76], which give a rather small variation in thie mean pressure. Zero value is obtained by measurement on liquid air, which gives an absolute measure of intensity. Finally the atmosphere is divided into 3 layers only (4 later on), the accuracy being insufficient for greater refinement. Here again, distributions involving 2 maxima are excluded, and distribution within a layer follows a preset law. Generally speaking, all the methods intended to reconstitute a distribution curve from

ATMOSPHERIU OZONE

153

observations integrating the whole atmosphere suffer from the same limitations and lack of determination reported above in relation with the “Umkehr” method. Moreover, a detailed analysis made by Ooyama [79] shows that 1 % error in absorption has, as a result, a 15 % error within the 300-50 mb region and 10 % error within the 50-30 mb layer. Walshaw [80] upgraded the method with an improved resolution which permits elimination of the wings of the water vapor bands. It appears that the method is particularly applicable to the region between ground level and, say, 15 km. The author has used it in that region. Goody’s method has been used by Migeotte and Vigroux [81]. They introduced a correction for carbon dioxide. Distribution laws are slightly changed but still remain predetermined, thus eliminating all the fluctuations, the actual existence of which is known both in the troposphere and the stratosphere. A third indirect method, held in high esteem in thc past, is that of moon eclipses. Its theory has been exhaustively studied by Link [82]. The method consists in taking spectra of the moon within the shadow of the earth and its atmosphere. The sunlight having passed through the atmosphere passes obviously very high due to the considerable absorption that a grazing sun ray would suffer. Refraction plays an important part in the path of the rays, imparting an important curvature as well as energy losses; minimum altitude of the sun’s rays is obviously dependent, upon the wavelength. This method is interesting mainly for the upper atmospheric layers, however calculations are very difficult. The observation technique must also be very carefully studied. The measurements are carried out within the Chappuis bands where water vapor can lead to some errors, which, fortunately, are reduced by the fact that the troposphere is not crossed by the light studied. Moreover, the temperature effect may be objectionable. Finally, in order to avoid the secondary diffusions, Mahmoudian [83] showed that it is possible to use the Gotz effect procedure. Observation is made in the Chappuis bands and when the sun is beneath the horizon. An invergion similar to the Gotz effect is noticed.

5.3. Methods Applicable to the Mesosphere For various reasons, all the methods considered above are not used beyond a certain altitude. The sounding methods are limitcd to thc ceiling of tlic balloons, which is about 30 km for reasonably priced carriers. The “Umkehr” method implies, by basic absumption, that there is no more ozone above a given altitude; it has been noted that the distribution law is determined for layers above 36 km. The infrared method, as secn, is mainly applicable to the troposphere and the lower stratosphere. The moon eclipse method, on the

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ARLETTE VASSY

contrary, could be extended toward the upper layers; however, due to the rarity of this phenomenon, it loses its interest in the present era of space research. We will therefore now consider the new methods which are available for the study of mesospheric ozone. The interest in this has already been stressed in the discussion of photochemistry and upper atmosphere meteorology. These methods are based on the use of rockets or satellites (even high altitude balloon&).Tlic instrument for observation can be situated either on the ground or on board the missile. The instrument can be placed in a rocket. The first experiment was done by R. Tousey and his co-workers of the U S . Naval Research Laboratory [84]. The Aerobee rocket carried 2 spectrographs which recorded the solar spectrum during the ascent a t carefully preset time intervals on a film calibrated the day before the firing. Obviously, cameras have to be recovered. Thanks to the conventional, slightly amended method of Fabry and Buisson, the ozone thickness above the rocket can be computed for each spectrum. The aocuracy is 20% a t the highest altitudes, which on the whole provides a rather fine result. Other distributions have been obtained by means of the same method but using cells or photoncounters [86]. The spectrographic method has been also used in USSR [86]. Since that time, space research has been extensively developed and single experiments are no longer considered but, as far as possible, series of experiments are prepared which require simplification of the equipment. Thus, in the United States a project of the Naval Ordnance Test Station [87] has been planned, using Arcas rockets, small rockets reaching 60km with a 5-kg payload. The principle is the same as for optical sondes but with different filters because of the smaller amounts to be measured. Interferential filters are fitted on a rotating drum; Cs or R b telluride cells are used. These missiles have not yet been launched. In France, too, a special rocket is being developed, intended for measuring the vertical distribution of ozone. It is a Belier missile carrying a 32-kg payload up to 80 km. Here again the optical system has been adopted, the filters being WG.3 and WG.5 and the receiver a photomultiplier. Future models will be equipped with ultraviolet interference filters. An alternative process consists in locating the instruments on the ground. Pittock [88]suggested the observation of high altitude balloons a t sunset. This method is derived from the moon eclipse method, but the geometry of the optical rays is simpler. Here again, we benefit from the path increase due to the sun’s great zenith distance; moreover it is possible to measure accurately the reflectivity of the balloon in terms of the wavelength. This may be determined in the laboratory, prior t o launching the balloon. Here again calculations are rather intricate. Accurate knowledge of altitude and

ATMOSPHERIC OZONE

155

position of the balloon are required, which have to be determined by auxiliary measurements. The background due to the twilight sky is rather troublesome for observations; it must be eliminated by diaphragms or screens. The operation has to proceed as fast as possible, but the ground equipment can be as accurate as desired. The common limitation to all methods derived from moon eclipses, is that they integrate ozone over a wide geographical area. Measurements are carried out within the Chappuis bands. It has been feasible to use a satellite. This method was used by the Naval Ordnance Test. Station [89]. Echo 1 (1960, Iota 1) has thus been observed from China Lake for 3 weeks in August-September 1960, in NovemberDecember 1960, and in January 1961. Echo 1 is made of polyester coated with aluminum in order to provide a high reflective power of 0.90 which is assumed to keep a constant value within the whole visible spectrum. For the analysis of the results, the atmosphere beneath a given arbitrary height, h, is divided into layers of equal height Ah; atmospheric refraction is neglected. The paths covered within the various aii layers are calculated (see Fig. lo), variation in zenith distance of the satellite

FIG.10. Method of analysis of Echo I rneasuromcnts (NOTS).

for the various heights, as long as this zenith distance is smaller than 60°, is also neglected; as a matter of fact, the validity of this assumption is controlled by the distance of the satellite. Beer’s law must be valid for the densities to be additive. Taking account of the number of absorbing particles per cubic centimeter, it is possible to calculate readily the ratio between the energies for two wavelengths received by the instrument. For the reverse calculation,

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ARLETTE VASSY

i.e. for computing the ozone distribution from the data, a reasonable value of total ozone amount must be available; measurements must begin before the sun's rays, impinging on the satellite, have reached the absorbing layers. It is implicitly assumed that the vertical distribution of the absorbing medium is the same for the trajectory between the observer and the satellite and for the vertical of the point where the sun's rays are tangent to the earth. The authors indicate that calculation is not affected by this variation for layers under 60 km. The absorption due to molecular scattering, for instance, or any other kind, may be taken into account, if desired. The influence of refraction is calculated. It is shown that the rays passing above 40 km do not suffer from a noticeable refraction. Between 40 and 26 km the effect is not too objectionable. Several pairs of wavelengths are selected for ozone distribution purposes. Indeed, the choice is dependent on the altitude of the sun-satellite ray. It must be noted that, when the rays are passing between 70 and 46 km,the variation of total absorption is small, as it is due mainly to the satelliteobserver path; this is obviously a limitation of the method. When upper regions are concerned, it would be better to proceed with rather high absorption coefficients, i.e., around 3100 A for the two wavelengths. Obviously, measurements must also be made when the sun-satellite ray passes above the atmosphere for due comparison. When rays croas the atmosphere between 26 and 46 km, the sun-satellite path prevails; the experiment has been conducted with visible light, within the Chappuis bands, a t 6000 and 6000 A. Although operating in the shadow, one must obviously take account of the sky background, and of the scintillations of the satellite. It should be noted that the results obtained are for the location of the setting sun, not for the observing point. This method is not likely to reveal a sunset effect, due to the distance covered by the rays. However, in spite of its limitations, this method is very attractive. , An idea of Blamont [go] is worth recalling. On occasion of a double explosion a t high altitude (110 and 91 km) a visible cloud had been created and the 0un light diffused by it a t 91 km was studied with a spectrograph; solar depression was 9". Comparing these spectra with that of the direct sunlight showed the presence of the Chappuis bands. Unfortunately, the reflectivity factor of the cloud is unknown, and its determination during an experiment of this type would increase noticeably the analysis of the data; moreover, this factor varies as the cloud changes. It is essential that the sky background be black, because the cloud is transparent. Obtaining the vertical distribution of ozone can only be a by-product in such a n experiment pursuing other aims.

ATMOSPHERIC OZONE

167

Finally, equipment can be placed on board the satellite. Several methods are derived from one suggested by Singer and developed by Singer and Wentworth [91]. The satellite observes the atmosphere from above and measures the spectral distribution of the energy in the light received from the atmosphere, which diffuses in the upward direction the ultraviolet radiation from the sun. It is assumed that t h e diffusion factor is due to the atmosphere alone (without ozone), and that absorption is due to ozone alone. This point should, however, be carefully checked. It was made by Sekera. We shall refer to this matter again. Light received by the satellite has twice crossed the ozone situated above the diffusing layers. The complexity of the calculations, similar to those of the Gotz effect but with a simpler geometry, are notable. Obviously, the layers below about 20 km remain out of reach. Twomey [92] developed the mathematics of this method. First a model atmosphere has to be selected and various simplifying assumptions made. It is assumed that the atmosphere contains a preponderance of polarizing particles-therefore diffusing-and a small amount of absorbing particles. Diffusion of absorbing particles and absorption of diffusing particles are neglected. The direction of the incident light as well as the direction of the observation are assumed to be vertical. The instrument operates in a fixed solid angle and explores a given spectral range. Atmosphere is assumed to have a n unlimited height. A calculation expedient consists of taking as variable the total ozone content above the level a t pressure p, instead of the pressure p, since the new variable varies monotonously in terms of p. One must assume that the ozone amount never becomes nil, even a t infinity. Calculation uses the Laplace transformation and some approximations. Oblique incidence is not considered, as the method becomes rather inaccurate under those circumstances. The author does not introduce the absorption due to molecular scattering, even though the spectral range considered is 2800-3000 A. By simply neglecting the secondary diffusion a noticeable error is introduced, mainly for the more penetrating wavelengths (down to 20 km). The required experimental accuracy is estimated to 1 yoin intensity and 1 A in definition of the wavelength ; however, the technical aspect of the experiment is not considered. Kaplan [93] suggested a method very similar to the one mentioned above. The explicit computations have been made by Sekera and Dave [94] with simplifying assumptions which are unavoidable but quite reasonable: plane homogeneous atmosphere, infinite in the horizontal direction but of a finite optical thickness along the vertical line. The Chandrasekhar

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ARLETTE VASSY

method is then used. The atmosphere is divided into two layers, the lower layer being perfectly diffusing, the upper layer being rarefied enough so that only primary diffusion enters into the calculation. It has been possible to show that diffusions of higher orders contribute to the received light with a maximum of 8 to 16% when the sun’s zenith distance is less than 60”. Numerical values have been obtained for the wavelength 2990 A. However, it is indicated that the absorption coefficient of ozone varies considerably faster than the diffusing optical thickness, and only the former factor needs to be taken into account when the wavelength is changed. It can be shown that the assumption (that the contribution of the lower layer be less than 2 %) is valid even with clouds, or snow on the ground, provided that wavelengths longer than 3070 A are excluded. The spectral range shorter than 2800 A is interesting only for studying the upper layers a t 60 km. This study rcsulted in considerable advance; it was even suggested that measurements be taken for 10 wavelengths between 2800 and 3070 A with a band width not larger than 10 A and that balloon tests be carried out prior to satellite experiments. We suggested [96] extending this method to the determination of the mean ozone temperature, and, furthermore, that by means of a few conventional radio soundings, one could obtain the vertical distribution of the temperature. In this case, one should use the minimum and maximum in the Huggins bands. Neither of the above suggestions has as yet been translated into action but may be close to realization. An ozone experiment aboard a satellite now circling has been undertaken by the British Meteorological Office [96]. The instrument is placed in the satellite; it does not use the solar light diffused by the atmosphere but the direct sunlight. The measurements are made a t sunrise and sunset (for the satellite) in order to take advantage of the considerable thicknesses which are traversed. Since the satellite completes one rotation in about 2 hr, about 30 measuring opportunities per day arise, distributed over two earth parallels. Because of precession these parallels are shifting, and in about 1 month a wide field of latitudes is covered. The satellite now operating is the second UK-2 satellite. The same method was used in an Air Force satellite launched in July, 1962 [96 a]. The satellite is stabilized so that the axis of the instrument is vertical; it has a polar orbit, with a mean altitude of 200 km. The instrument records observations a t an angle 90” to the axis direction; thus it receives light from the sun a t sunset and sunrise, with an aperture angle of 30’. Measurements are made in the Hartley band, The total duration of the apparent “sunset” is about 20 sec, and the apparent diameter of the sun is taken into account.

ATMOSPHERIC OZONE

169

What can be expected from these various projects? The measurements made with an instrument carried by a rocket or a satellite are not dependent on the meteorological conditions of the launching site or of an observing station. This circumstance is highly favorable when no station is available in a region with propitious climate. It is also interesting and profitable to place instruments in rockets as long as the unit costs are low since distributions derived from sporadic observations cannot lead to definite conclusions. Instruments placed in satellites are, obviously, advantageous in this respect. However, calculations rest, unavoidably, on approximations (this does not refer t o the complexity in this age of electronic computers); moreover, it is still a major task to place in a satellite an instrument with the required first-class qualities. In methods in which apparatus is on the ground level, nothing stands in the way of using first-class equipment, because weight and power requirements are no longer important. Moreover, the instruments can be operated by observers, and it is well known that, in the research field, man is considerably superior to automatic devices. Obviously, investigators will have to acquire in the laboratory basic data of higher accuracy than are presently available, SO that progress matches data acquisition.

5.4. A Few Results Numerous determinations of vertical ozone distribution have been made by means of the Gotz effect and the infrared method, as well as by a few ascents of balloons equipped with spectrographs and a limited number of radio soundings. Moreover, 7 distributions by means of rockets were successful, as well as several series of observations with Echo. The very first experiments, either direct or indirect (prior to 1938), showed that the partial pressure of ozone a t ground level was about 10 to 20 pmb but increased with altitude up to values of about 200 pmb a t an altitude of about 25 km and then, decreased, becoming very low in the vicinity of 45 km. Figure 11 shows a conventional distribution. It was quickly noticed that vertical distributions were highly variable. Nevertheless, the direct methods are the ones which can show us in what way we can make use of the numerous data collected by means of the “Umkehr” or infrared methods. Let us consider the results of the direct methods. Since the first measurements made by the E. Regener team [97], it has been noticed that the distribution within the troposphere often shows a minimum of zero around 5-8 km altitude. It can also be noted that the layer where concentration is a t its maximum may be very thin, as was the case for the ascent of the US

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ARLETTE VASSY

Partial pressure of

ozone (pmb)

FIQ.11. Ozonagram of the average vertical distributions (Tamanrasset and Arosa).

ATMOSPIIERIC OZONE

161

stratostat [98]. Further, radio-sounding results have shown that the distribution curve can have 2 or even 3 maxima; in middle latitudes the typical distribution has been observed only in about 5 0 % of the cases [99]. This result, reported also by other authors (Paetzold, Ramanathan) has recently been found again in a set of radio soundings made in Bedford (United States). Even though nonconventional distributions are lacking in the results acquired by indirect methods, numerous interesting conclusions can, nevertheless, be drawn regarding the ozone variations with altitude. Some of these conclusions may have to be revised when radio soundings are extended. It is known that vertical distribution has no connection with total thickness. Adel has given an example in which the maximum rises from 15 to 30 km while the total content remains unchanged. It is also known that very important changes of the vertical distribution are noticed within very short time intervals (a few hours). Statistical analyses have shown that the highest daily variability is noted between 10 and 20 km [77]; indeed, 15 km is the level where a secondary maximum appears often [loo]. These variations have been studied in connection with atmospheric motions. These studies have shown how ozone is brought downward by subsidence into the lower stratosphere where i t is stored and “frozen.” It seems that a difference exists between the distribution in higher and lower latitudes. I n the lower latitudes, all authors agree on a location of the maximum a t rather high altitude and on the very low contents in the troposphere; this is obviously connected with the high altitude of the tropopause. Figure 11 shows an average distribution for Tamanrasset station and for a period extending from March to May. The maximum is a t about 30 km; below 15 km very little ozone is found. At Colomb-Bechar where, however, noticeably greater changes are observed than a t Tamanrasset, the upper maximum is placed very high, and frequently 314 of the total amount lies above 27 km. I n the high latitudes we are mainly struck by the high ozone content between 10 and 20 km; if we recall that the tropopause is low, one can see that ozone is concentrated between the maximum and the tropopause [99, 101, 1021. The studies made by Ramanathan and his assistants show that the vertical distribution changes on the average between latitudes 30 and 40”. Seasonal changes a t various altitudes have also been studied; Diitsch [ 1031 showed that the seasonal variation in Arosa is a t its maximum between 14 and 24 km; above these layers, it is very low. However, as already pointed out, the analytical procedure applied to the Umkehr measurements tends to provide constant results insofar as the upper layers are concerned. If we refer to optical soundings which can give us the

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ARLETTE VASSY

quantity above the maximum altitude reached, important changes of the reduced thickness above 30 km are noticed. Among the rocket measurements, made on January 25, 1950, June 14, 1949 (United States), and June 15, 1960 (USSR), a partial pressure of 1.5 pmb was observed a t 50 km, while other firing tests gave only nonmeasurable thicknesses. Measurements made with the Echo satellite in December 1960 gave a value more than 20 times larger than these last ones (see Fig. 12). 70

60

50

km

t

40

URSS 1960

-'

30

20 10 OL

I o9

.

' . . . * . . I

Fxo. 12. Eximiplw of

woiic

10'0

.

. * . . . . . I

-

10" mol/crn3

' a . * - a * l

10'2

'

n

'

'

'

Y

Ioi3

vcrtictll tliHtrihutioii obtaiiiotl with rockets and satellitcs.

This sliows the importancc of continuing the measurements in order to check the realitics of Ruth changes in the upper altitudes, because the presence of ozone is one of tho factors controlling the atmospheric temperature a t about 50 km as well as the chemical reactions by day or night in the upper mesosphere, as we will see in Section 6. I n conclusion, let us state that all the methods which can be used above, say, 30 km are optical methods; the most important problem for these regions is knowledge of the accurate ozone content; therefore, the computations arc based on the absorption coefficients. Hence, as already stated, it is cssent,ial to have reliable values of these coefficients available within the spectral ranges in which some inconsistencies are still present. 6. OZONEIN

THE

UPPERATMOSPHERE

Oxygen atoms, produced by solar ultraviolet radiation, acting upon the oxygen molecules, are present in the upper atmosphere. Considering this

ATMOSPHERIU OZONE

163

important fact, scientists suspected the existence, in the same regions, of ozone molecules, which could offer various possibilities of chemical or photochemical reactions. Today, some of the advanced theories have proved to be a valuable tool for investigators, but many unknown aspects remain, and the character of the theories is often provisional. Therefore, we will present an outline of the possible part played by ozone in the upper atmosphere while limiting our attention to the more representative theories presently available. Ozone appears as a dominant factor in the emission of the OH bands (Meinel bands) in the nightglow. This emission takes place from 60 km up to 100 km (these results were obtained with rockets, other methods give higher values) and shows a good correlation with the nightglow emission of sodium lines. The primary mechanism would be the formation of excited OH molecules, according to reaction (1). (1)

H

+ 0 3 + OH* + Oz*

Then hydrogen is recycled from lower energy OH molecules according to reaction (2): (2)

OH

+ 0 + H + Oa

This mechanism was proposed independently and simultaneously by Bates and Nicolet and by Herzberg; in spite of the lack of knowledge concerning coefficients of production and destruction rates of the different elements involved, Bates and Nicolet [lo41 were able to give a very complete survey of the problem. This theory is rather generally accepted, first because it leads to emission rates in reasonable agreement with nightglow intensity measurements, next because it explains why the bands originating from levels v 2 10 are nct detected in the nightglow emission, and finally because laboratory experiments have shown the emission of Meinel bands in a mixture of ozone and hydrogen [105]. Still further, the theory is able to account for the covariance with Na lines, but here complex aspects of the problem must be considered. Let us start first with a pure oxygen atmosphere illuminated by solar ultraviolet radiation. Its components will be, besides ordinary 0, molecules, what S. Chapman called odd oxygen atoms, i.e., free 0 atoms and 0 atoms combined to the molecules to give ozone 0,. Bates [lo61 gave a complete discussion of the composition of such an atmosphere and computed the ozone concentration a t noon for the different heights. It appears that the computed values are smaller than the observed ones (see Section 5.4)near 50-60 km. Bates also considered the possible diurnal variation. A t sunset, when ultraviolet radiation vanishes, the oxygen atoms are no longer supplied by the

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photodissociations of 0,and 0, molecules; hence the ozone concentration increases by recombination according to (3): (3) O a + O + M + 0 3 + M

At the altitude of maximum concentration, this reaction takes place immediately, but its contribution to the total amount is very small, owing to the small number of available oxygen atoms, even during daytime. This is the tentative explanation of the nocturnal variations (see Section 4.3)of the ozone amount; but as stated, quantitative values are not in good agreement, the observed changes being far greater than the computed ones. This is a rather simple case, but our atmosphere is not composed of pure oxygen; it contains hydrogen (which is responsible for OH bands by combining with ozone) and sodium. Recently, Ballif and Venkateswaran [107, 1081, suggested that the nightglow emission intensities of the OH bands and Na lines are both controlled by the concentration of ozone molecules. The involved reactions are rather complex, but the explanation is attractive. Other reactions may occur in the atmosphere, in the absence of illumination, due to collisions. For example: (4)

0

+ 0 3 +2Oa

This reaction compensates for reaction (3), and releases a very important quantity of energy, 78 kcal/mole. We also have to take account of nitrogen oxides and of possible reactions such as: (5)

NO

+

0 3 + N0a

+ Oa

Our knowledge is very poor concerning the eventual occurrence of all these reactions and their rate coefficients; nevertheless, they have been considered by Dutsch [log], who was able to find a nocturnal maximum in the ozone concentration near 70 km. We need to remember, from these tentative explanations, that the ozone molecules are an active element in the chemistry of the atmosphere above 60 km, by day as well as by night. Besides this photochemical point of view, ozone is able to contribute largely to the thermodynamic equilibrium of the atmosphere. Ozone absorbs solar energy in the whole spectrum and yields it back to the atmosphere as thermal radiation. But we know that the ozone concentration, as far as photochemical formation is concerned, is inversely dependent on temperature. Therefore, ozone has a stabilizing and damping action on temperature: When temperature increases, ozone concentration and heating rate decrease; simult.aneously, infrared radiation increases, and temperature will soon return to the initial value. Computations were made by Craig and Ohring

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165

[ 1101; but they assumed that ozone is controlled solely by photochemical equilibrium. Supposing that ozone concentration is increased by an alternative process, the resulting rise in temperature will not be balanced by the change in ozone concentration. Finally, the initial temperature will be restored, but a t a slower rate and only when the creating agent has disappeared. Obviously such considerations are valid only for altitudes above 46 km. I n the cold stratospheric regions, the phenomena are entirely different. The above survey is intended, as we already said, to point out that the ozone problem is not limited to the stratosphere and that the importance of ozone is much greater than its very small amount might suggest. The theories which have been related still involve unverified assumptions and speculative concepts, but they open interesting and new prospects and furnish an excelllent stimulus for further studies.

7. ORIGINOF ATMOSPHERIC OZONE Having thus summarized the different aspects and characters of atmospheric ozone, we should now look for the source of this ozone, as its presence in our atmosphere requires the intervention of a n external cause. As a matter of fact, the destructive agents are numerous a t ground level and a t high altitudes: spontaneous (or dark) decomposition, photolysis by ultraviolet and visible radiation (the rate of which is increased by water vapor molecules), and chemical reactions. Indeed, the general outline of the distribution curve with its maximum implies the existence of two opposing effects, production and destruction. There the question arises: What is (or are) the agent(s) creating ozone in our atmosphere? In the troposphere, as shown in Section 3, ozone exists through advection from higher levels; additional amounts are produced by storm cells (this is not a t all negligible, owing to the great number of storms in the world) and to a lesser degree by a photochemical reaction restricted to polluted atmospheres. There is no difficulty in explaining its presence and there is little point in dwelling on tropospheric ozone. But above the tropopause, the problem is more complex. Two kinds of agents are capable of creating ozone in an oxygen atmosphere: (1) ultraviolet radiation, which originates from the sun only (for X-rays, we apparently know little about their efficiency and (2) corpuscular radiation coming from the sun or from the outer space (cosmic rays). Let us examine first the photochemical production. The most recent studies on the photochemical theory of ozone were done by Dutsch [lo91 and by Paetzold [42]. The computations concern the equilibrium between ozone and oxygen under the opposing influences of creative and destructive radiations.

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This equilibrium depends on the inverse of the temperature, and both radiations have different penetration into the atmosphere, the destructive wavelengths reaching to lower levels of the troposphere. The most probable reactions are the following: Production of 0 atoms Photodestruction of ozone Ozone production by triple colliaions Ozone destruction by triple collisions Recombination Dark decomposition

0s

+ hv + 0 + 0

h < 2100 (experimental value)

+ hv + 0 8 + 0 for h < 1 p 0%+ 0 + M + 0 3 + M

0 3

+ + 2 0% 0 + O+M-+Oa + M 0

20

0 3

3 +3

0%

We have to consider, of course, other reactions with minor components of the atmosphere, such as OH, N,O, and Na, but they do not have an important influence on the equilibrium in regions below 60 km. Dutsch has pointed out the difficulty in carrying out the computations and the serious hindrance resulting from inadequate knowledge of basic experimental data. Moreover, photochemical theories generally neglect the temperature effect. In spite of simplifications, the theory is capable of yielding a vertical distribution curve fitting roughly the average experimental results between 15 and 40 km. However, we will call attention chiefly to the time that is required for equilibrium to be established (or restored): The lower the altitude, the longer is this time delay. Authors differ on the numerical results, but, roughly, the time of recovery is expressed in years a t 26 km, in days between 30 and 40 km, and in hours near 60 km. These results show without any doubt that the ozone amount measured below 30 km has a rather loose connection with the solar energy reaching thesc regions, which is why this fraction of the total amount is called “conservative”. These computations also explain why the photochemical theory failed to account for the scasonal variations of total amount, and primarily for the spring maximurn in polar regions. However, a t low latitudes, we showed years ago [47] that the seasonal variation of ozone is well explained in terms of variations of incident solar energy. This was confirmed by the IGY measurements (see Section 4.2) for latitudes between 0 and 24”. Although the photochemical theory is capable of accounting for the existence of the maximum concentration, it is in serious conflict with experimental results insofar as the altitude of the maximum is concerned; theories give a height of 20 km near the equator and 27 km near the polar circle, yet observations show the reverse, a high maximum for low latitudes. Moreover, the theory indicates a nocturnal maximum in the mesosphere near 70 km, but is unable to explain the high diurnal values recently measured. To account for seasonal variations of the total amount, variations which

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167

are mainly located in the layers between 12 and 24 km, without being forced to give up with the photochemical theory, we can take recourse to advection mechanisms, ozone being brought from the regions in which production is active, i.e., equatorial regions, to the high latitudes. The first attempt was based on a poleward meridional transport in the stratosphere. Several objections have been advanced. For one it is well known that meridional circulation is far less important than zonal circulation; next, computations by Reed and Julius have shown that the assumption of a meridional transport requires that both hemispheres be included in the same circuit. This is in confiict with the observational results on radioactive fallout, which have proved the existence of an equatorial barrier in the lower stratosphere [lll]. Instead of transport, large-scale mixing has been assumed, but this also seems objectionable. Such a mixing can hardly result in having the lowest values in those regions in which production is active, except if the rate of exchange were very large; but here again radioactive fall-out indicated that this is not the case. Finally, we must recall that the jet stream (see Section 4.6) appears as a barrier preventing exchange between poles and tropics, either by transport or by turbulence. Therefore, the more recent explanations have considered vertical motions. Downward motions connected with the displacements of the polar vortices could explain the spring ozone maximum. By such a subsidence ozone coming from high layers accumulates in the lower stratosphere where it is stored, the ozone in the high layers being immediately replaced by photochemical action. This mechanism is valid for middle latitudes, but it becomes unlikely in winter for high latitudes where photochemical production, even a t 50 km, is negligible, as solar radiation does not reach these layers. In this respect, it seems difficult to explain by dynamic effects the increases in the ozone amount connected with the sudden warmings (Berlin effect) ; we have pointed out that the increase in ozone starts a t high latitudes 3 days prior to middle latitudes. It is my opinion that we have to find the source of this increase on the spot where it is observed. Furthermore, the secondary maximum observed during winter in the Southern Hemisphere is not consistent with the photochemical theory, even in conjunction with dynamic motions. It seems therefore worth while to consider the second agent capable of creating ozone: corpuscular radiation. It is well known from laboratory experiments that ozone is produced by low speed electrons impinging on oxygen molecules, the minimum energy required being 6.3 volts. Fast electrons are also efficient, as well as a-rays. Among the electrons entering into our atmosphere, well known from observational evidence and with a spectrum extending from cosmic rays t o Van

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Allen belt electrons, it is quite easy to discover electrons normally reaching 80 km. Occasionally, electrons of high energy, 200 MeV, have been detected down to 30 km. I n addition, secondary electrons, half of which have energies exceeding 36 ev, may contribute to the formation of ozone. On several occasions, it was suggested that ozone was produced by the solar corpuscular radiation responsible for the polar aurora. And in 1932, Dauvillier proposed a complete theory ascribing the origin of atmospheric ozone to auroral activity. We have seen (see Section 4.4) that the polar aurora shows a relation to increases in the ozone amount and that magnetic activity has a slight correlation to ozone. Also, the winter maximum has an analogy to the winter anomaly in ionospheric absorption, which takes place around the 80-km layer. Another winter anomaly is observed for the electronic density of the F layer. Recently, Gregory [112] could measure large increases in the electronic densities above 60 km, mainly during the winter season. It appears therefore that the assumption of production of ozone, a t least occasionally, by corpuscular radiation cannot be neglected. This possible origin needs consideration for the vertical distributions. Special attention ought to be devoted to the changes in the vertical distribution of ozone during sudden warmings a t high latitudes and primarily during the polar winter. The observational data now available are too scarce to furnish conclusive evidence. REFERENCES

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