An intercomparison of mesospheric ozone profiles determined by the UVSP and SAGE II solar occultation experiments

0032-0633/89 $3JXl + 0.00 Pergamon Press plc

Planet. Space Sci., Vol. 37, No. 1, pp. 97-104, 1989 Printed in Great Britain.

AN INTERCOMPARISON OF MESOSPHERIC BY THE UVSP AND SAGE II SOLAR

OZONE PROFILES DETERMINED OCCULTATION EXPERIMENTS

A. C. Aikin Laboratory for Atmospheres, NASA Goddard Space Flight Center, Greenbelt, Maryland, 20771 USA D. J. Kendig ST Systems Corp. (STX),

4400 Forbes Blvd., Lanham, Maryland, 20706 USA H. J. P. Smith

Visidyne Inc., 10 Corporate Pl., Burlington, (Received

17 November

Massachusetts,

01803 USA

1988)

Abstract - A comparison is made of individual UVSP and SAGE II mesospheric ozone profiles between 50 and 70 km altitude as determined by the solar occultation technique. The generally good agreement between the two data sets below about 57 km leads to the conclusion that they may be considered as complementary, thus extending the effective altitude range of both. Comparison of the long-term ozone trend at 55.5 km shows a systematic difference of a few percent between the two measurements.

INTRODUCTION The understanding of mesospheric ozone photochemical and dynamical processes requires a long-term well-calibrated data set. Six different techniques have been employed to measure mesospheric ozone: solar occultation (Aikin et al., 1982, Chu and McCormick, 1979), ultraviolet limb sounding (Rusch et al., 1983), backscattered ultraviolet from the nadir direction (Heath et aZ., 1975), 1.27pm emissions (Thomas et al., 1983), infrared limb emission measurement (Gille et al., 1980, Remsberg et al., 1984), and ground-based millimeterwave observations (Wilson and Schwartz, 1981). Each of these techniques has its own unique set of operational parameters, such as time, altitude range, and resolution, as well as sources of error. Comparisons within the mesosphere are affected by diurnal ozone variations, complicating the interpretation of data taken at different times of the day. In addition, there is the possibility that mesospheric ozone infrared radiation may arise from non-thermodynamic equilibrium processes (Solomon et al., 1986). If these processes are not included in the inversion technique, erroneous ozone profiles may be derived from infrared limb soundings. It is the purpose of this paper to intercompare solar occultation results obtained with the Ultraviolet Spectrometer and Polarimeter (UVSP) on the Solar Maximum Mission (SMM) satellite with results of the Stratospheric Aerosol and Gas Experiment (SAGE II) on the Earth Radiation Budget Satellite (ERBS). Only sunset profiles will be compared, eliminating the need to consider diurnal variations.

91

A. C.

98

INVERSION

AIKIN et al.

OF OCCULTATION

OBSERVATIONS

The UVSP aeronomy and the SAGE II experiments are both observations of the occultation profile of the solar intensity. Assuming a single absorber and a non-scattering medium, this may be described by the following equation:

where I = intensity at any point in the occultation profile 10 = intensity at the top of the Earth’s atmosphere ox = absorption cross section at wavelength X n(Or) = number density of ozone molecules along the path and the integral is to be performed along the path from the satellite to the sun. Equation (1) may be recast as an integral equation for n(O3):

J

In(fa/l) = QA

n(03) dI = 7

Note that the right-hand side is simply the optical depth. The inversion involves the solution of this integral equation. The UVSP experiment has been performed at a wavelength of 279 mn in the Hartley band of ozone. The product of the Rayleigh scattering cross section in this region and the total concentration is much smaller than the product of the Ilartley band absorption cross section and the ozone concentration. Thus one can ignore the Rayleigh scattering effects for the UVSP case. The SAGE II experiment, on the other hand, makes use of seven wavelength bands, near 385,448,454,525,600,936 and 1020 nm (Kent and Wang, 1986), allowing for the measurement of stratospheric ozone. This paper only deals with comparisons in the mesosphere. While ozone absorption is significant at 600 nm, there are also contributions from aerosol extinction and from Rayleigh scattering. These make the SAGE II inversion problem considerably more complex than the that for the UVSP, since the channels of the SAGE II instrument must be inverted simultaneously to obtain profiles of aerosol concentration and molecular density as well as ozone. The reader is referred to Chu (1986) for details. The UVSP inversion has been discussed in Aikin el al. (1982). The UVSP inversion needs accurate satellite and solar ephemerides during the occultation. These are used to compute the unit vector from the satellite to the sun center. From the instrument boresight and the satellite pitch, roll, and yaw, the unit vector describing the UVSP line of sight (LOS) is determined. The distance of closest approach to the earth center for each LOS determines the tangent point as well as its latitude. A simple model for the figure of the earth is used to compute the tangent height corresponding to each LOS. Assuming the atmosphere can be divided into concentric spherical layers defined by the observed tangent heights, the path length through each of the layers is calculated and used in the inversion. The UVSP aeronomy experiment uses a narrow rectangular aperture of 1 x 180 arc seconds. The orientation of the slit and its location on the solar disk are important, since there is considerable limb darkening at the wavelength used. Fortunately, this effect has been well measured by solar physicists using the UVSP, and a model of it is available for use in the inversion. This model is applied to determine the coordinates of the effective slit center on the sun and the pointing vector is modified accordingly and used in the tangent height calculation. The longer wavelength used by the SAGE II for the ozone determination has a solar flux about 20 times larger than the Hartley band wavelength used by UVSP. This means that the signal-to-noise ratio in the SAGE II experiment is correspondingly larger. For most UVSP profiles the slit is nearly tangent to the earth’s limb giving a geometric altitude resolution of M 0.012 km. For those few cases where the slit is more nearly perpendicular, the resolution is w 2.1 km. The SAGE II experiment uses a larger aperture, yielding a potentially larger signal-to-noise ratio.

99

Intercomparison of mesospheric ozone profiles

SOURCES

Obscuration

measurements

edge of an accurate UVSP instrument

instrument

to show that the precision atmosphere

are by their nature relative experiments, since they do not depend on knowlThis is clearly seen on the left-hand side of equation (2). The calibration.

uses photomultipliers

count rate range, the detectors increases.

as detectors,

and thus its results are given as counting

are known to be linearly proportional

of the measurement

Table

OF ERROR

1 illustrates

improves

this.

to intensity.

significantly

Equation

as the counting

rates.

In this

(2) may be used

rate at the top of the

In this table we have assumed that the uncertainty

in any

count is given by the square root of the count. One can see from Table 1 that the larger flux seen by SAGE at the top of the atmosphere range overlapping

makes it possible for that experiment

the lower part of the UVSP TABLE

l.Error

in AT as a Function of Count Rate

Optical

Depth

r = 1.0

?- = 0.5

7 = 0.1

7 = 0.01

‘T = 0.001

100

6.3%

7.9%

9.5%

9.9%

10.0%

500

2.8%

3.5%

4.3%

4.4%

4.5%

2.0%

1000 10000

II

altitude

range.

10

counts

to measure ozone over a larger

I

0.6%

Equation (2) may also be used to examine the behavior of the UVSP measurements (and by analogy SAGE II as well) at the lower end of the occultation profile. Let the uncertainty of a given part of an occultation

profile be written

as I * AI.

Then from Equation

(2) we can write:

The second term has been discussed in effect in Table 1. Notice that the error increases rapidly rate diminishes

and in fact is indeterminate

and places a lower limit on the altitude The previous concentration

discussion

This behavior

the limb darkening. The UVSP solar ephemeris

is concerned

with

the error in the determination

by use of the ephemerides, All of these contribute

satellite

ephemerides

used was the 23-term

for Computers

(1980-1988).

inversion

profile.

the UVSP

pointing

are accurate to within fl Chebychev The

of the value of the ozone

Each such point is associated information,

with a tangent

the slit orientation,

and

to the tangent height calculation.

UVSP

satellite pitch, roll, and yaw. The limb darkening The individual

as the count

has been observed in the UVSP

of the inverted ozone profile.

assigned to a given point in an occultation

height, aa determined

Almanac

at I=l.

polynomial

km (3a) fit obtained

in each Cartesian

component.

from the U.S. Naval

The

Observatory

p ointing is known within A.3 arc seconds expressed in curve fit also contributes an error term to the calculation.

effects of each of these is relatively

small, at least in comparison

to the effects

of the

three-point binning of the UVSP data that is used to simplify the numerical task of the inversion. The UVSP is theoretically capable of sampling ozone at intervals of approximately 0.1 km. The three-point binning reduced this to M 0.3 km. All of the pointing and ephemeris errors described above are subsumed in this procedure.

A. C. AKIN

100

et al.

65

50

lE+9

lEi8

lE+-10

Oj concentration

Fig 1. Comparisons of files close in time and space. UVSP raw data are shown as open cixles with a secondorder fit plotted aa a solid cume. SAGE Ef data are shown zw solid circles.

65 ‘2 23 fi

lE+fl

(cm-3

(a)

November 11, 1985 23:03 UT, 7’- N 78’ W UVSP SAGE II 23:X? UT, 6’ N 83’ W

(b)

July 13, 1986 19:09 UT, 0” N 18” W UVSP SAGE II 1953 UT, 3” N 25” W

(e,l

July 14, 1986 I&43 UT, 3O S 13’ W UVSP SAGE II 18% UT, 2” S 6’ W

60

5 a 55

1 EM 0,

(a)

65

50 1 CL+8

lEfl0

concentmth

(cm-‘)

0 .

I

‘

I

‘....*I

lE+S Os concentration

Fig 2. Further pro& comp&om. XI data are shown as solid circles.

(a)

.

lE+10

I

Ill**

IEd-

(cm-‘) WSP raw data are shown as open circles with a second-order fit plotted a~ s alid CUI*YC. SAGE

July 12,1986 WSP x1:43 UT, 5” N 950 E SAGE If

1EClf

22~54 UT, 8* N 69’ W

(6)

July 13,1986 tJVSP

l&O0 UT, I’ N 29” E

SAGE II

l&39 UT, 4” N 24’ E

Intercomparison of mesospheric ozone profiles TABLE

2. Data Corresponding to Figure 1

0s

Figure h (km) 66.5 65.5 64.5 63.5 62.5 61.5 60.5 59.5 58.5 57.5 56.5 55.5 54.5 53.5 52.5

0.935 1.320

CONCENTRATION

Figure

la SAGE

UVSP

UVSP

II

-

1.837 2.521 3.412 4.553 5.992 7.776 9.952 12.560 15.631 19.183 23.215 27.706 -

TABLE

2.485 3.187 4.047 5.091 6.341 7.824 9.559 11.568 13.863 16.454 19.341 22.517 -

Figure

lb

1.113 1.454

2.374 3.127 3.984 5.021 6.229 7.784 9.958 12.536 15.622 19.075 23.303 28.640 34.974

~

(10’

-

1.894 2.461 3.191 4.127 5.324 6.851 8.795 11.263 14.389 18.337 23.310 29.561 -

1.990 2.720 3.647 4.948 6.606 8.620 10.768 13.468 16.706 20.810 25.377 31.171 38.195

cmsS)

~~ ~

UVSP 1.001 1.395 1.919 2.601 3.477 4.583 5.955 7.629 9.636 12.002 14.737 17.842 21.298 25.066

2a

Figure

( SAGE 0.933

1.521 2.484 3.896 5.792 7.975 10.096 12.342 14.969 18.846 23.650 29.059 34.961

II

UVSP 0.857 1.164 1.580 2.141 2.895 3.910 5.272 7.097 9.538 12.799 17.147 22.936 30.630 40.841 -

2b

1 SAGE 0.752 1.097 1.643 2.517 3.789 5.721 8.425 12.242 16.644 21.528 26.439 32.111 38.429

lc SAGE

UVSP

II

3. Data Corresponding to Figure 2

Figure

66.5 65.5 64.5 63.5 62.5 61.5 60.5 59.5 58.5 57.5 56.5 55.5 54.5 53.5 52.5

cm-‘)

-

OS CONCENTRATION

h (km)

(10’

SAGE

1.468 1.919

2.155 2.861 3.773 5.046 6.393 7.969 9.780 12.458 15.904 20.004 25.032 30.931 37.974

i--

101

II

II

102

A. C. AKIN et al.

Data presented here are for the period after August 1985. During this entire period the UVSP wavelength drive has been fixed due to a mechanical problem. Due to an imprecise understanding of the failure, the exact wavelength

at which the data were obtained

is not precisely

as observed by different instrument slits, a wavelength nm being the most probable. The longer wavelength

known.

Based on an analysis

of the solar spectrum

range of 278.783 to 278.937 nm is possible, with 278.937 was used in the data analysis. The cross section was cm* was employed. Since the (1986) and 4.395 x lo-r8

adopted from measurements by Molina and Molina optical depth is the product of the total ozone amount and the absorption cross section, any uncertainy in wavelength translates directly into an uncertainy in absolute ozone values through the absorption cross section. COMPARISON

The satellites.

ability of comparison The Solar Maximum

SAGE

II, is in a nearly

occurs

in the equatorial

Although

UVSP

INDIVIDUAL

PROFILES

of UVSP and SAGE II profiles is contrained by the respective orbits of the Mission satellite is in a 28.5” inclination while the EBBS, which accommodates

polar orbit. region.

simultaneous

Individual

OF

profiles

The greatest

Tangent

likelihood

regions

were the most desirable,

ozone data points

of the two tangent

which fell within

regions

3” latitude

there was often a separation

are fit with a second-order

polynomial

being nearly

of one another of several

using linear

coincident

are compared. hours in time.

least squares.

In many

instances, However,

ozone profiles obtained from UVSP have a nearly constant scale height in the 53-67 km region. this is by no means universally true. Frequently, there is considerable variation in the slope and

curvature

of the altitude

Figure

1 displays

As mentioned

above,

comparisons

these

are sunset

in latitude

and longitude

the SAGE

II profile.

than SAGE SAGE

II. The

II profile.

in latitude

profiles.

three

and SAGE

In Figure

are 1” and 5” respectively.

Both

profiles

UVSP

exhibit

ozone profile

In Figure

and longitude,

of UVSP cases.

lb,

a changing extends

the scale height

respectively.

The

la,

II data which are close in both the time

UVSP

scale height

separation

profile

has better

with altitude,

more than one scale height is more nearly

In Figure

lc, profiles

constant.

for one day later

altitude

and UVSP

above

This

than

generally

Another

appears

comparison

and time.

Separations

resolution

than

shows less slope

the region

covered

is for a separation for Figure

The latitude and longitude separation is 1” and 7”. There are about 8 months between la and those in Figures lb and lc. Table 2 gives the values for the profiles in Figures agreement

space

is 9 minutes.

by the

of 3” and 7” lb are given.

the profile in Figure la, lb, and lc. The

to be M 10%.

is shown in Figure

2. Here, two additional

sets of ozone profiles

separated

in time and

space are plotted. Results are similar to the previous example. The profiles in Figure 2a are approximately 11 hours apart and are separated by 3” latitude and 164” in longitude. In Figure 2b the separation is about 39 minutes in time, 3” in latitude, and 5” in longitude. The agreement km and poorer at higher altitudes. Table 2 gives the profile data. Many presented here, all with similar results.

LONG-TERM

in these cases is best below x more cases have been compared

58 as

INTERCOMPARISONS

Weekly means of UVSP ozone at 10” N lt5” are plotted as a function of time from August 1985 to September 1988 in Figure 3a for an altitude of 55.5 km. Data exhibit an annual cycle with a pronounced minimum at the beginning of the year. There are two maxima, one in the spring and the other in the fall. A maximum seascnal change of 30% is observed. Also shown for the same altitude and averaging are data from SAGE II covering the time period November 1984 to November 1987. Although there are fewer data points for SAGE II, the outlines of the seasonal dependence are present. There is a systematic difference of about 10% between the two data sets, with SAGE II exhibiting larger values. One possible origin of this difference may be the altitude resolution of the two experiments. SAGE II can have an altitude resolution as poor as 4 km. A larger field of view for an instrument means that rnore absorption will be observed due to the fact that the range of the observed atmosphere will encompass a higher ozone concentration. Using a mean tangent altitude would then skew the profile toward higher concentration. The wavelength uncertainty of the UVSP instrument is another possibility. However, in order to account for the observed difference between the data sets, a cross section corresponding to approximately 279.7 nm .would have to be used. This is unlikely based on the previously mentioned analysis, although it cannot be ruled out.

Intercomparison of mesospheric ozone profiles

103

If the SAGE II altitude is adjusted d~w~w~d by 0.5 km, there is no systematic difference. This is illustrated in Figure 3b, where the UVSP zonal weekly means at 10’ N are p!otted with open circles w a function of time for an altitude of 55.0 km. The SAGE II data are plotted as solid circles. There is good agreement between the two data sets. In addition, UVSP data from early 1985 are plotted. These data were obtained at precisely known waveiengths and were obtained before the period of wavelength uncertainty, The consistency between the two sets of UVSP data suggests that our wavelength determination is indeed accurate. 2.4

1989 Year

2.4

Year

Fig 3. ZonaI weekly average ozone for 10” N. Open circles are IWSP data. Open squares represent UVSP data taken at various precisely known wavelengths. Filled circles are SAGE II data paints averaged in the same manner. (s,l UVSP and SAGE II averages at 55.5 km. lb) SAGE II data at 55.5 km versus UVSP data at 55.0 km

The ozone profile data sets from the UVSP and from the SAGE II show generally good agreement below about 55 - 57 km. Experience with the UVSP inversion leads us to conclude that the SAGE fI data begin to lose accuracy above about 55 - 57 km. It is concluded then that the two occultation experiment data sets can be used together to provide effective ozone profile rne~~ern~ts over a broader altitude range than either one alone.

Acknowledgement The authors wish to thank Dr. M. P. McCormick for providing SAGE II ozone profiles prior to public~tian.

A. C. AIKIN et al.

104

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