Ozone as a cause of tobacco injury

Agricultural Meteorology - Elsevier Publishing Company, Amsterdam - Printed in The Netherlands

O Z O N E AS A CAUSE OF T O B A C C O I N I U R Y 1 E. I. MUKAMMAL Canadian Meteorological Branch, Toronto, Ont. (Canada)

Received April 6, 1964

SUMMARY Damage to tobacco plants, known as "weather fleck", is now recognized as being due to ozone. Over the northern shores of Lake Erie ozone is believed to be mainly generated by photo-chemical processes from oxides of nitrogen and hydrocarbons emitted in industrial areas to the south. However, thunderstorm activity and stratospheric ozone might at times contribute to total ozone concentration, although only in small quantities. Indications are that on most occasions of high ozone concentration, lake breeze surges, and meso-scale systems forming as a result of the physiographical features of the Great Lakes area, dominated the weather picture and were the key mechanism by which ozone was advected and brought down to the surface. Severe flecking generally occurred with a particular combination of amounts of ozone dose and evapotranspiration. When the hourly ozone concentrations were modified by a coefficient of evapotranspiration derived from a simple form of the mass transfer equation, a much improved correlation between flecking severity and modified ozone dose was obtained, and several hitherto obscure results could be explained.

INTRODUCTION Millions of pounds of flue-cured tobacco have been rendered commercially valueless in southwestern Ontario, Canada, as a result of a disorder known as "weather fleck". Black flecks appear first as groups of palisade cells die, but soon the flecks turn brown as the affected tissues dry out. Since 1958 the Canadian Meteorological Branch, in close collaboration with the Canada Department of Agriculture, the Canada Department of National Health and Welfare, and the Ontario Research Foundation has carried out field studies of the causes of weather fleck at experimental sites near Lake Erie, where flecking is usually most severe. 1 Submitted with the approval of the Director, Meteorologic~d Brartch, Department of Transport, Canada. Agr. Meteorol., 2 (1965) 145-165

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Preliminary observations and experimental work indicated that increase in surface ozone concentration might in some measure be responsible for tobacco fleck. In fact, tobacco plants inside greenhouses or polyethylene tents, fumigated wi~h artificially generated ozone, had injury symptoms similar to those observed in the field. That ozone could be the cause of such injury has been confirmed by later studies, the meteorological aspects of which are presented below.

INSTRUMENTATION

In order to implement the meteorological programme a main microclimatological station, located 400 m from the northern shore of Lake Erie near Port Burwell in 1960, and 2.4 km inland in 1961, was established on a tobacco farm and equipped with extensive instrumentation. Continuous observations of temperature, humidity and wind at four levels to a height of 30.5 m were taken, using aspirated shielded wet and dry thermocouples for temperature and M.S.C. type 45 anemometers (a threecup type anemometer manufactured by the Canadian Meteorological Branch) 1or wind. Profiles of temperature, humidity and wind within and outside the tobacco crop at 20, 40, 80, 160 and 320 cm were also recorded, using Thornthwaite and Sheppard anemometers for wind° and shielded aspirated thermocouples for temperature. Soii temperature and soil moisture at three sites were also measured at 5, 10, 20, 30 and 50 cm, using fiberglas units Ibr soil moisture and thermocouples for soil temperature. Continuous measurements were also made of illumination, using a Beckman and Whitley illuminometer, of direct and reflected solar radiation, using Eppley pyrheliometers, and of net radiation with a Suomi type net radiometer. Evapotranspirauon was measured by means of a floating lysimeter of 1.52 m diameter and 1.83 in depth. similar to that developed by KING et al. (1956). A study of the stability of the ai~ in the first 330 m of the atmosphere, using a tethered balloon, and between the surface and 400 mb using radiosonde instruments, was undertaken on several occaslo~, during the 1960 and 1961 seasons. At five auxiliary stations, the furthest 24 km inland, thermographs, hygrographs and M.S.C. type 45 anemometers were installcd. In 1960 and 1961 a Kruger "72" double analyser was used by Canada Department of National Health and Welfare for the continuous monitoring of ozone concentrations. Details of instrumentation will be given in a subsequent report.

GENERAL CONSIDERATIONS

Although flecking was observed with many different synoptic weather pattern~, examination of frontal contour charts indicated a close association between flecking and certain air masses. The most striking feature on most occasion of flecking ~va~; the presence of either a maritime tropical, or a modified maritime polar air mas~

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over the region. A few occasions of slight flecking occurred in a maritime polar air mass with a southern trajectory over the region. During the summer months the area to the south and southwest of the tobacco growing region is normally under the influence of a weak pressure configuration with very little circulation, as a result of a large amplitude ridge extending from the Bermuda High. Occasionally a south-southwesterly flow is established behind anticyclones or broad ridges which move in from the west or northwest and occasionally from the southwest, bringing into southern Ontario warm and moist air which has been stagnant for some time over southern areas. With such an influx flecking followed only when coupled with other conditions, involving the stage of development of the plant, a mechanism for bringing down ozone present in the atmosphere, and other micro-meteorological factors favourable for intake of ozone by the tobacco plant. Certain characteristic weather conditions were associated with the critical pressure systems and air masses. Maximum and minimum temperatures were above normal. There usually were more than five hours of sunshine during the day prior to flecking, and in many cases more than ten hours prior to occasions of severe flecking. However, on a few occasions of moderate to slight flecking less than one hour of sunshine was recorded the previous day. Varying amounts and types of cloud were observed 24 hours prior to flecking. Nevertheless, on most occasions of severe flecking the sky was either cloudless or had a few patches of high or medium cloud. Poor visibility, generally less than two miles, caused mostly by haze or smoke, often accompanied conditions conducive to moderate or severe flecking. Measurements of the turbidity coefficient of the air, a measure of the quantity of haze, taken in 1959 using a Michelson-Biittner actinometer (MICHELSON-BiJTTNERand ALBRECHT, 1929), showed that all severe flecking was preceded by a high turbidity coefficient. Comparison of hourly and daily incoming solar radiation under cloudless skies two days before, and the day preceding flecking, showed that solar energy was generally reduced on days of flecking by about 8-10 ~ for daily values, and by as much as 16 ~ for hourly values.

S O U R C E OF O Z O N E

Ozone, generated locally by a photo-chemical process from oxides of nitrogen and hydrocarbons emitted in industrial areas to the south or by motor vehicles, is believed (under favourable meteorological conditions) to be the main cause of the contamination of surface air by ozone. A distinct diurnal variation of mean ozone concentration was observed, as shown in Fig.l, and although this might be taken as reflecting the corresponding diurnal variation of turbulence, as an agent for bringing down pollutants from higher levels, the decrease in ozone concentration with northwest-northnortheast winds, which generally feature a high degree of turbulence, would invalidate such an interpretation. Moreover, examination of mean ozone concentration for all hours with favourable wind direction (southeast-south-southwest) grouped at

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4-hourly intervals, showed that it was lower for the period 09h00-12h00 E.S.T. than between 17h00 and 20h00 E.S.T. when turbulence is normally greatly reduced. This is believed to be due to insufficient local generation of ozone during the earlier period. The suggestion is appealing that ozone increased with the approach of c~id fronts due mainly to tropospheric ozone being brought down along isentropic surfaces following entrainment of stratospheric ozone at the break of the tropopause in the warm air near the jet stream. On such occasions stratospheric ozone could perhaps have contributed to the total observed ozone, although it could not whoity account for concentrations as high as 10-12 p.p.h.m. (parts per hundred millio~t observed on some of these occasions. There is no apparent evidence that, on other occasions, when there were ~o large scale frontal surfaces around the area, stratospheric or tropospheric ozone was ever responsible for surlhce increase in ozone, as most days of high ozone concentration were characterized by a high pressure centre just to the east or southeast of the area, with a weak pressure gradient and a subsidence inversion over the area. 'lhe latter would scarcely permit the penetration of a significant amount of ozone from above, even though according to BALL (1960) some entrainment of air from above the subsidence inversion would be possible with strong convection. Analysis of conditions prevailing during the 1961 season which were characterized by frequent atmospheric disturbances, suggests that with thunderstorm activity additional sources of ozone might become available. During the period August 1 to September 13, 1961 there were seventeen days when thunderstorms occurred around the area, and each time ozone concentration was high, although in some cases advected polluted air was also involved. Furthermore, sharp ozone increases were observed whenever a thunderstorm occurred over the experimental site. For instance, ozone which had only been 4.3 p.p.h.m, during the day rose sharply during the night of Axr. MeteoroL, 2 (1965) 145 i 65

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OZONE AS A CAUSE OF TOBACCO INJURY

August 25-26, 1961, to 5.5 p.p.h.m, after the occurrence of thunderstorms over the area. It is not altogether clear whether either lightning flashes or downdraughts accompanying thunderstorms are capable of increasing ozone to the higher values observed. However, the fact remains that the high ozone concentrations and the frequent variations in surface ozone must be locally attributed to the photo-chemical process.

REGIME FREQUENTLY ASSOCIATED WITH HIGH OZONE CONCENTRATION

Increase in ozone at ground levels occurred under such diverse meteorological conditions that no general simple model could be constructed. The most frequent occurrences of high ozone, responsible for most occasions of severe flecking, occurred under the usual effective conditions for the accumulation of atmospheric pollutants, such as persistent anti-cyclones, with very light winds, and large scale subsidence inversions at about 850 mb. However, there were additional complex local contributing factors leading to the build-up of higher ozone concentration a short distance from the northern shores of Lake Erie. Marked lake breeze surges or meso-scale discontinuity surfaces accompanied by exceptionally unstable conditions in the first 30 m above ground were observed on most of the above occasions. Richardson's criteria (SUTTON, 1953) between 30.5 and 6.2 m gave unrealistically high negative values (--1.5 to --2.4), although lapse rates were no higher L---0.7° to --0.8°C) than on other days. On the other hand, wind shears were exceptionally low. Tethered balloon observations taken at the experimental station indicated the existence o f isothermal conditions above 60 m with shallow weak inversions from 180 to 250 m. With such low level stratification it is likely that small wind shears would result from convectional turbulence in the isothermal layers or under inversions. With little or no downward fluxes of horizontal II.O

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m o m e n t u m from above, turbulent mixing would dissipate horizontal momentum z~t the surface and eventually equalize the velocities in the lower levels. Examination of successive tethered balloon observations revealed that the above low level inversions seemed to disappear and reappear later, suggesting thai they might have been linked with lake breeze, which is thought to be a series of pulsations, as suggested by WALLINGTON(1959), rather than a steady movement. Attempts were made to ascertain the extent of the influence of lake breeze pulsations upon ozone concentration by examining variations with distance from the lake of wind speed at t0 m above ground level, using mean hourly wind measurements from the auxiliary stations. Differences in horizontal southwesterly wind speeds between each auxiliary station and that next further inland varied from one period to another in a gradually changing fashion. Fig.2 gives the differences of the horizontal wind between a station only 200 m from the lake and the experimental stati(m (2.4 km inland), as well as the variation of ozone concentration at the experimental station on August 18, 1961, when marked lake breeze was experienced and when the highest ozone concentration of the 1961 season was observed. Of considerable interest is the similarity between the shape of the ozone con° centration curve and the difference in the horizontal wind, as well as the marked increase in the positive gradient after 14h00 E.S.T., when a large increase in ozone was observed. Moreover, the decrease in the gradient after 17h00 E.S.T. corresponded to the decrease in ozone.

L O W - L E V E L SUBSIDENCE

On the above occasions successive tethered balloons up to 330 m indicated the exis!ence of small-scale subsidence at a relatively fast rate. Examination of a sequence of temperature profiles as given in Fig.3, curve F - H . for August 25 and 26, 1960, reveals that a lapse condition with a slight inversion above existed over the tobacco area on August 25, 1960, at 15h45 E.S.T. However, twenty hours later at I lh30 E.S.T. on August 26 the base of the inversion had descended to 220 m at a rate of 0.1 cm/sec, as indicated by curve G. Four hours later, curve I~. the base of the inversion had lowered to 140 m at a rate of about 0.6 cm/sec. The highest mean hourly ozone concentration observed on August 25 was 2.5 p.p.h.m. compared with 6.5 p.p.h.m, on August 26. It would accordingly seem reasonable t~, assume that the rate of descent is related to the surface ozone concentration. Fig.3, curve A--E, gives five temperature profiles for September 7. 1960, whe~ the highest concentration of ozone of the season was observed and severe flecking followed. Curve A shows wiresonde observations in the early morning, with the usual radiational inversion in the lower layers. It will be seen from curve B that 45 mi~ later the lower inversion had been destroyed, that another inversion had been established between 30 and 150 m, and furthermore, that the layer had cooled by abotH 2~C, suggesting the onset of a southerly flow from the lake, possibly a lake breeze.

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Fig.3. Temperature profiles on August 25 and 26, and September 7, 1960. Such a change is substantiated by surface observations taken at the same time, which show that winds veered from northeast to south and dewpoint rose about 5°C. At 14h00 E.S.T. (curve C) a quasi-isothermal layer existed between 90 m and 300 m, with several shallow layers of inversion in between. As shown in curve D, 45 min later, the layer between the surface and 300 m had warmed and the inversion layers were at lower levels, suggesting that subsidence had taken place at quite a considerable rate (about 0.9 cm/sec). On that day an anti-cyclone was centred southeast of Lake Erie with a very weak pressure gradient over the area in question. Surface observations indicated that two major lake breeze surges had occurred, one after 08h00 E.S.T. and the other after 13h00 E.S.T. On each occasion ozone increased. However, after the onset of the latter surge, ozone concentration rose sharply from 2.6 to 6.0 p.p.h.m, and continued to rise until 16h00 E.S.T., when it reached 10-12 p.p.h.m., after which ozone decreased appreciably to only 3 p.p.h.m, at 20h00 E.S.T.

MECHANISM OF D O W N W A R D T R A N S P O R T OF O Z O N E

While neither the exact mechanism by which ozone is brought down to the surface nor to the atmospheric levels at which ozone concentrations are high are altogether

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understood, it is apparent that on most occasions of high ozone, lake breeze surges were an important contributing factor to the increased ozone concentration, at least along the northern shores of Lake Erie, where flecking is generally very heavy. This is illustrated by the schematic diagram of the postulated normal and vertical velocities components associated with lake breeze presented in Fig.4. [t i,

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Fig.4. Schematic diagram of the postulated normal and vertical velocity components associated with lake breeze.

assumed that an internal boundary layer forms behind the lake breeze l¥ont, with subsidence above and convectional turbulence below. The former would be partly due to difftuent flow over the lake resulting from strong lake breeze surges north and south of the lake, and partly to downward motion normally observed behind suci~ fronts. It is further assumed that these vertical velocities are at their maximum near the front. On the other hand, upward vertical velocities and strong turbulent mixing+ resulting from progressively greater heating inland of the air, are assumed to exis~ in advance of the front. With low level descending motion behind lake breeze fronts ozone trapped under a large scale subsidence inversion below the 850 mb level would be brought down, with greatly increased ozone concentration at lower levels. However, such subsidence is not thought to extend below the internal boundary layer, the depth of whici~ increases inland from the lake, where low level convectional turbulence would brine down ozone to the surface. With the advance inland of the leading edge of a lake breeze front the zone ot maximum vertical velocity and thus of maximum downward transport of ozone would be displaced, resulting in ozone concentration above the internal boundary layer being higher further inland behind the front than near the shore. Surface ozone would be correspondingly higher, as a result of progressively increasing low level convectional turbulence inland. During July, August and September lake breezes do not seem to penetrate very far inland north of Lake Erie. Observations of wind, temperature and humidity at five auxiliary stations as far inland as 24 km revealed that lake breeze surges extended more than 5-7 km inland on a few occasions only. Thus, it would A+gr. Meteorol., 2 (t965) 145 1(4

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OZONE AS A CAUSE OF TOBACCO INJURY

follow that on these occasions the maximum surface ozone concentration would be about 4-6 km inland rather than near the shore. Since there was no supporting observational evidence available to verify the latter, ozone data observed at the 1960 experimental site, 400 m inland, and at the 1961 station, 2.4 km inland, were examined, although it is recognized that inferences drawn from data not taken simultaneously at two sites are inadequate.

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Fig.5 gives frequency distribution of ozone concentration for the period July 10September 13 during each hour between 13h00 and 16h00 E.S.T., when ozone concentration is usually high, at the 1960 and 1961 sites, with wind direction (southeastsouth-southwest) most favourable for high ozone concentration. It is evident that ozone concentration in 1961 was much higher than in 1960. Moreover, the 1961 frequency distribution exhibited bi-modal characteristics not evident in the 1960 distribution. It is likely that had the oscillation of lake breeze surges not been more pronounced at the 1961 site than at the 1960 site no secondary mode would have been apparent, and the frequency distribution curve would probably have taken the form indicated by the dash curve. Indirect evidence in support of this latter explanation could perhaps be inferred f r o m a comparison of hygrograph traces at a station only 200 m from the lake with those at the central station 2.4 km inland, which showed that fluctuations in humidity, believed to be due to lake breeze surges, were more pronounced at the central station and even 5 km further inland than near the lake. In fact, there were many instances

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when no changes were apparent at the station near the lake even though distinct humidity changes took place further inland. It might be expected that in conditions such as those described above surface ozone concentration should generally decrease ahead of the leading edge of the lake breeze front, due to the existence of upward vertical velocities in advance of the front and strong turbulent mixing resulting from progressively greater heating inland of the air. There are no ozone measurements available for another site by which t~, verify the above hypothesis, but the mere fact that flecking decreased inland from the shore of Lake Erie does tend to substantiate it, particularly since ozone is now kwowll to be responsible for tobacco fleck, although, as shown later, physiological and micrometeorological factors are also involved.

OTHER REGIMES ASSOCIATED WITH HIGH OZONE CONCENTRATION There were several instances when ozone increased to a high level without all the above features being well defined. Quite frequently neither small scale subsidence in the lower layers nor lake breeze surges were evident, although large scale subsidence inver~ sions at about 850 mb were observed, and in the lower layers unstable conditions, due mainly to relatively strong lapse rates, were experienced. Average Richardson s criteria between 6.2 and 30.5 m from 13h00 to 16h00 E.S.T. on these occasions ranged from --0.40 to ---0.60, while ozone mostly increased with increased turbulence. It is believed that in these instances increased ozone was due to increased turbulenl mixing which brought down pollutants from higher levels, possibly under a subsidence inversion, where pollutants are usually trapped. High ozone concentration was at times also associated with relatively weak instability in the lower layers and moderate to strong south-southwesterly surface winds (8-12 m/sec) arising from advancing troughs, both with and without fronta! surfaces. Richardson's criteria ranged from --0.10 to --0.25, while ozone tended t~ increase with decreasing instability and increase in wind speed. On these occasions the increase in surface ozone was perhaps due to strong advection of polluted air and less upward ozone diffusion as the air moved from polluted sources to the area i~ question, since the upward diffusion of pollution by turbulent mixing is a major factor in limiting surface pollution; the more stable the air the less dilution and the greater the surface concentration. The above features by no means encompass all the conditions under which ozone increased. There were occasions when the simultaneous action of several of the above factors, among others, rendered impossible even a partial understanding oi" the exact mechanism by which, or the sources and levels from which, ozone was brought down. Perhaps a few of the highlights of the occasion of September 10, 1961 will best illustrate the complex nature of the problem. On several occasions a remarkable association was noted between the rise in dewpoint temperatures and increase in ozone, and although this was, as might be

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expected, as a result of advection from the south of moist air laden with pollutants, this could not account for the occasion of September 10, having regard to the circumstances described below. i.,,I.

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Fig.6 gives variation at 6-rain intervals of ozone and dewpoint temperatures at the experimental station for September 10, 1961. It will be seen that the fluctuations of ozone between 10h00 and 14h00 E.S.T. correspond to the changes in dewpoint temperatures, with a lag of about fifteen minutes. The latter is presumably due to a time lag difference in the response of temperature and ozone sensors. Of considerable interest is the drop in humidity to very low values at short intervals. Some doubt was at first cast on the accuracy of these observations, but readings of wet and dry bulb temperatures recorded with two different potentiometers, taken at two different positions at the experimental site, confirmed the drop in dewpoint temperatures. Furthermore, hourly observations taken from mercury thermometers in a ventilated Stevenson screen showed a similar drop in humidity. Wiresonde observations were also taken at about the same time, and are given in Fig.7. It will be noted from the ascent at 12h06 E.S.T. that cells or layers of dry air, with nearly the same dewpoint temperature as that observed at the surface at the time of the sharp drop in humidity, existed at about 240 and 315 m. Similar cells with relatively higher dewpoint temperatures were again observed at the succeeding ascents of 12h50 and 14h02 E.S.T. However, the ascent of 14h50 E.S.T., when ozone concentration dropped appreciably, shows that the cells or layers had disappeared. No satisfactory explanation of these events has been arrived at, although the suggestion is put forward that air advected inland had a high ozone content and that the intermittent drop in ozone concentration was a result of purer air with low dewpoint temperatures being brought down from the dry cells, by either turbulence

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RELATIONSHIP OF OZONE TO TOBACCO FLECK

During the years 1958-1961 a great deal of effort was devoted to measuring concen-~ trations of ozone effective in producing tobacco fleck. Uncertainties arose when statistical attempts were made to correlate ozone dose (daily accumulated mean hourly ozone concentration in p.p.h.m, during daylight hours of the previous day~ with flecking indices the following morning. Fig.8, which gives flecking indices on ~ scale from 0 (no flecking) to 5 (severe flecking) against daily ozone dose on sevcrai occasions during the 1961 season, shows such a wide scatter of points that no significant correlation can be obtained. For instance, a relatively low daily ozone dose (35.5) was observed on September 6 but was followed by the most severe flecking of the season (fleck index 5) on the morning of September 7. Moreover, two consecutive days of high ozone concentration were often accompanied by heavy flecking on the first day but very light flecking the second day, even though the second day ozone Agr. Meteorol., 2 (1965)

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dose was higher than the previous day. For example, on August 19 moderate to heavy flecking (index 3.5) was reported when the daily ozone dose on the previous day had been 70.3. On the other hand, the next day (August 19) the ozone dose rose to as much as 87.6, while flecking was light (index 1.5) the following morning. This would seem to suggest that there were some physiological parameters inhibiting the absorption of ozone the second day, due to most of the susceptible leaves having been previously injured, or to the plants protecting themselves against further injury, or to meteorological factors directly or indirectly affecting the intake of ozone by the leaves. There were no physiological parameters available which could be introduced into the relationship between flecking and ozone dose. However, certain meteorological parameters known to influence plant behaviour were available for several days, I

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FLECK INDEX AGAINST DALLY OZONE DOSE AS MEASURED DURING 1961

Fig.& Fleck index against daily ozone dose as measured during 1961.

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and attempts were made to find whether improved relationships could be obtained if either a single meteorological parameter or a combination of several could be introduced into the relationship. Illumination was first tried but without much success, since flecking occurred with both very high and low intensities of illumination. Further attempts were made to introduce wind velocity, temperature and humidity, but again with very little SLICGeSS.

O Z O N E DOSE A N D E V A P O T R A N S P I R A T I O N

Hourly evapotranspiration data from a floating lysimeter were available Ibr 22 days~ during which time the soil had a plentiful supply of rain water. Daily measuremems of soil moisture with fiberglas units and with several oven-dried samples taken a~ different locations showed that during the whole 1961 season soil moisture did not drop very far below field capacity. The amount of evapotranspiration was expressed in recorder units rather than in depth of water per unit area. This was because of the difficulty in determining the effective area of the lysimeter, due to the placing of expanded polystyrene in the upper part of the lysimeter soil in order to allow it ~ float properly. Nevertheless, several tests were made by placing known weights o~cr the lysimeter to check the linearity of the recorder scale, which was found to be linear with no significant change throughout the season. The choice of an appropriate method of relating flecking intensity to both ozone and evapotranspiration presented some difficulty. As a first attempt daily accumulation of hourly values of both ozone and evapotranspiration was plotted against daily flecking intensity, and although the points were widely scattered and too few to give an exact picture of the relationship, some broad features of an obviously complex distribution were apparent. Indications were that severe flecking occurred only w'ith a certain combination of evapotranspiration and daily ozone dose. Within certain limits of these two variables there appears to have been a trend towards the occurrence of severe flecking as evapotranspiration increased and ozone dose was relatively low, provided the amount of evapotranspiration was high enough to reach the required level, and similarly with low evapotranspiration and high ozone dose. Moreover. with a constant amount of evapotranspiration flecking severity generally increased with ozone increase, reaching a maximum at a certain limit of ozone dose, then decreased with a further increase in ozone dose. While the latter may perhaps be somewhat difficult to accept, it occurred mostly on days following severe flecking the previous day. Events of August 18 and 19, as given in Table I, illustrate the circumstances of one such occurrence. It will be noted that daily evapotranspiration was ahnosl the same on both days, while, although vapour-pressure differences between the two levels (340 and 180 cm) were slightly higher on August 18 than on August 19, wind speeds at 340 cm and wind shears between 340 cm and 180 cm were much higher on August 19 than on August 18, suggesting relatively higher evapotranspiration on Agr. MeteoroL, 2 (1965) 145-165

OZONE AS A CAUSE OF TOBACCO INJURY

159

TABLE I EVAPOTRANSPIRATION~

WIND SPEED AND VAPOUR-PRESSURE DIFFERENCES, OZONE CONCENTRATION AND

COEFFICIENT OF EVAPORATIONON AUGUST 18 AND 19, 19611 Time (E.S.T.)

August 18, 1961

7 8 9 10 11 12 13 14 15 16 17 18 19

Total

Eva,ootranspiration in recorder scale 0.1 0.2 0.4 1.1 2.0 2.4 2.5 2.6 2.8 2.4 2.0 1.5 0.8

e2 - el (mb)

U2 (cm/see)

Uz - U1

Ozone (p.p.h.m.)

Coefficient of evaporation (A )

OA

0.15 0.25 0.50 0.85 0.97 0.92 0.95 0.92 0.84 0.68 0.71 0.64 0.60

_~65 151 146 189 239 228 240 249 260 291 265 223 138

65 55 48 69 88 80 82 92 95 110 101 89 67

1.4 1.4 1.6 2.0 3.0 3.5 3.7 5.6 6.4 8.5 9.2 10.0 7.5

0.004 0.005 0.006 0.007 0.009 0.011 0.011 0.0114 0.0130 0.0121 0.0107 0.0100 0.0095

0.005 0.007 0.010 0.014 0.027 0.039 0.041 0.064 0.083 0.102 0.098 0.100 0.070

20.8

August 19, 1961

Total

8 9 10 11 12 13 14 15 16 17 18 19

0.2 1.5 2.0 2.2 2.6 2.6 2.6 2.8 1.8 0.8 0.5 0.2

0.660 0.22 0.64 0.72 0.74 0.92 0.76 0.80 O.96 0.83 0.65 0.60 0.31

62 234 279 308 341 403 393 391 371 287 201 214

21 91 109 114 128 168 155 152 146 106 75 81

2.4 6.0 5.5 6.9 7.7 8.3 8.5 7.0 7.0 7.2 8.0 6.7

0.014 0.010 0.010 0.0096 0.0080 0.0080 0.0085 0.0080 0.0060 0.0040 0.0040 0.0030

0.0336 0.0600 0.0500 0.0662 0.0616 0.0664 0.0720 0.0560 0.0420 0.0288 0.0320 0.0201 0.5937

19.8

1 e2-el vapour-pressure difference between 340 and 180 cm; U2 = wind speed at 340 cm; U 2 U1 = wind shear between 340 and 180 cm; E -- evapotranspiration; A = coefficient of evaporation derived from A = E/U2 (e2 - e0; p.p.h.m. = parts per hundred million. =

A u g u s t 19. H o w e v e r , t h i s w a s n o t t h e case. M o r e o v e r , f l e c k i n g w a s m u c h m o r e severe o n A u g u s t 18 t h a n o n A u g u s t 19. The above events would certainly seem to support the second inference, at least on occasions with two successive days of high ozone concentration. This has b e e n o b s e r v e d r e p e a t e d l y w i t h s u c c e s s i v e h i g h o z o n e d o s e s i n t h e field a n d a l s o c o n firmed experimentally by F. D. MacDowall Events of August

18 a n d A u g u s t

(personal communication,

1960).

19 w o u l d a l s o s e e m t o s u g g e s t t h a t w h e n Affr. Meteorol., 2 (1965) 145-165

160

~. I. MUKAMMAI,

evapotranspiration is Jess than the wind and moisture gradient would indicate, the ozone is less effective in producing flecking. This is perhaps due to some kind of physiological protective response on the part of the plant. 1V[AcDOWALL (1963) has shown the inhibiting effect of ozone damage on stomatal opening. An improved relationship was obtained when fleck index was plotted against 180 I70

a:

t60

~.

150

cc

s/,

t40 > u~

130 J20

jJ

~ilO u_¢ O~

fl-

ZlO0 ~Z OO

90

a-al

8/U

uJz 80 ro ~-N u_ ° 70

o#

/

0Z 4

60

J

50

f.~""

/

e~,.

g
7~

40 3O

20

PLECK INDEX

Fig.9. Fleck index against daily summation of the product of hourly ozone concentration an~i evapotranspiration.

daily summation of the product of hourly ozone and evapotranspiration, as given in Fig.9. It will be seen that the scatter was somewhat narrowed, and that certait~ critical points such as that of September 7 were better placed. However, other points, such as those of September 8 and August 4, with no flecking, were disappointing. Because of the earlier finding that evapotranspiration was less than the wind and moisture gradient would indicate, attempts were made to introduce these latter parameters by dividing the product of hourly ozone and evapotranspiration by the product of the wind and moisture gradient. This is really equivalent to modifying the hourly ozone dose by multiplying it by a corresponding hourly coefficient of evapotranspiration derived from an empirical form of the mass transfer equation for evaporation. Wind speed in cm/sec at 340 em and vapour-pressure gradient in mb between 340 cm and 180 cm were used. Daily accumulation from sunrise to sunset of hourly modified ozone dose Ajr. Meteorol., 2 (1965)

145-105

OZONE AS A CAUSE OF TOBACCOINJURY

161

0.7! ii/ee

/

0

/,

0.7( 31/8

./

h- b- 0.65 i.-O za.

/

6/8e

/.,,9/

014/9 029/8

04/9

HOIg

o61

~ p 0.60 II:z ~w I_ u. u-

~3/9

e2o/8

4/B

~.o D ~ 0.fib oo z

• 31e

5/9 ~

/

! ,

zm

i.-uj 0:

g~ o.~o

J

Zo

~D

,30/8

0.45 ~IB/B

0.40 0,38

~9/9

1

NOTE: NUMBERSBESIDE POINTS INDICATE OATEI AND MONTH

O 2 3 4 FLECK INDEX FLECK INDEX AGAINST DALLY MODIFIED OZONE DOSE (OA) DURING 1961

Fig. 10. Fleck index against modified ozone dose (OA) during 1961.

(OA) was plotted against the corresponding fleck index, and is given in Fig. 10, where O is ozone concentration in p.p.h.m, and A is coefficient of evaporation. It will be seen that the scatter of points was narrowed considerably and that a fairly good correlation between the two variables is indicated, in contrast to Fig.8, which gives unmodified ozone dose against the fleck index for the same points. It is worth noting the change which occurred in some of the more widely scattered points in Fig.8. For example, point 7/9 as given in Fig.10, which had a relatively low ozone dose and a high fleck index, was brought up by the new approach to a relatively high level of modified ozone dose, and similarly with the points on the two consecutive days 19/8 and 20/8. Point 20/8 was brought down considerably, to below that of 19/8 in accordance with the observed corresponding flecking severity. This latter is extremely significant, in that this technique seems to account for the low injury on the second day of two consecutive days of high ozone dose. Although

Agr. Meteorol., 2 (1965) 145-165

162

E.I. MUKAMMAI

the above results are encouraging, additional data were not available to check the relationship. It would appear that one might assume that the coefficient of" evaporauon derived from actual evapotranspiration is an empirical index reflecting the physiological conditions of the tobacco plant rather than the physical conditions of the atmosphere, although the latter also seems to be involved.

POSTULATED PHYSICAL PROCESSES INVOLVED

Although the above coefficient of evaporation is generally regarded as empirical, surprisingly enough it correlated fairly well with the corresponding hourly drag coefficient for three days when reliable wind data at five levels taken beside the lysimeter and humidity measurements were available. On at least one day meteorological conditions prevailing were totally different from the other two days. From observed evapotranspiration, wind shear and humidity gradient, frictional velocity was computed, which, combined with the average wind speed at the two levels, gave the drag coefficient. If in this instance the coefficient of evaporation actually was proportionate ~, the drag coefficient over tobacco plants, as is the case over open water as stated by PRIESTLEY (1959), then the suggestion is put forward that with a high drag coefficient there is more mixing and greater exchange of gases between the air and leaf: a,d consequently ozone absorption is high, while with a low drag coefficient the lca~ would be in less contact with the contaminated air and thus absorption would be low and the ground would act as a sink for ozone destruction. Examination of the hourly values of A shows that in general A was relatively small when ozone concentratio~ was high without much flecking. However, when ozone concentration was low w , h heavy flecking, A was relatively large. A few ozone profiles were taken by the Canada Department of National Health and Welfare using one Mast instrument for measuring ozone, with switching device-, allowing air to be drawn in at each of the five levels in turn for about 10-15 rain. Since there were no simultaneous observations for all levels, corrections had to be made to the mean concentration at each level according to changes in the ozone concentration with time, which was measured by the Kruger instrument at one level during the above period. Fig.ll gives ozone, wind and water-vapour pressure profiles from l lh00 ~ 12h10 E.S.T. for September 3, 1961, on semi-log paper. It will be noted that ozor~c gradients at such levels could be detected during the day, even with a Mast instrumenl, which is not generally considered suitable for this purpose. It is possible that in thi,~ and other instances the absolute values and the magnitude of the ozone gradient arc not accurate, but there is no doubt that gradients were observed in all cases. The most striking feature of the diagram is that when the scales for wind, vapour A~r. Meteorol., 2 (1965) 145 165

163

O Z O N E AS A CAUSE OF T O B A C C O I N J U R Y

OZONE ( p . p . h . m ) 0 WIND

tI I I00

S P E E D (cm/sec) 5 0

150

200

I VAPOOR PRESSURE(mb]

250

500

350

I

I

I

I

30

29

28

27

--"

31

400

( p . p h.m.) (s~ale of o i l t h r e e e l e m e n t s

plotted

on t h e s o m e s c a l e )

340cm

/ ,¢;,~ - -

/

iill 6 0 cm

it"

~

/

I

•

X

Fig. 11. Ozone, wind, and vapour-pressure profiles between 1lh00-12h00 E.S.T., September 3, 1961. pressure and ozone were adjusted so that the profiles drawn by hand coincided at 60 and 340 cm, as shown in dotted lines in Fig. 11, the three profiles had practically the same form, particularly above the plant level. If, as seems to be the case, an ozone gradient does exist, then there must be downward fluxes of ozone, which in turn might be one of the main determining factors in the amount of ozone available for absorption. In order to verify the above, continuous measurements of ozone at at least two levels are necessary to estimate ozone fluxes. However, since only one level of ozone concentration was available a quasi-empirical approach was used. Encouraging results were obtained by correlating fleck index with the daily accumulation of hourly ozone flux, assumed to be proportional to OA 3/~. The latter function was again obtained from the mass transfer equation by assuming that the product of wind and ozone gradient divided by ozone at one level was proportional to A 1/'. However, since the validity of the above assumption could perhaps be disputed, and can only be verified by reliable ozone gradients and other micrometeorological parameters, it will not be further discussed, particularly since details of the derivation are perhaps not essential to the subject matter.

Agr. Meteorol., 2 (1965) 145-165

164

E. 1. MUKAMMAI

CONCLUSION

It must be emphasized that the above findings are by no means sufficiently verified as not to require later amendment. However, this is a first attempt at providing at~ explanation of observed events. Further advances in our knowledge of actual c~,:ditions in the atmosphere and of the exact flecking mechanism are more likely to come from a better understanding of the dynamics of meso-scale circulation and from m~'e comprehensive physiological, meteorological and ozone gradient data. From such studies as have been possible with the data available, it would appear evident that although tobacco fleck occurred under diverse meteorological conditions a certain characteristic synoptic weather picture, while not sufficient in itself, was a necessary condition of the most frequent and damaging occurrences of flecking. l h e stage of development of the plant was important, as was the presence of ozone in the lower atmosphere and the mechanism of intake of ozone by the plant. The advection of maritime tropical or modified maritime polar air masses mt~ the area appears to have been a prerequisite condition of flecking. Trajectories of air masses over the region about the time of flecking showed that on most occasions the air had passed over polluted areas to the south, where oxides of nitrogen and hydrt~carbons could have been instrumental in producing ozone by means of photochemicai processes. However, increased ozone can also result from either thunderstorm activity or from downward fluxes from the upper atmosphere, although these are thought to be of lesser importance. Although there were several recognizable regimes in which low level ozone c(;w~centration could have increased, that with a weak pressure gradient favourable to the onset of lake breeze surges seems to have been the most frequent and to have resulted in the greatest increase in ozone. Severity of injury appears to depend not so much on the absolute magnitude of atmospheric ozone concentration as on the physiological and micrometeorological factors which determine the amount of downward flux of ozone in the lower layers for absorption by the plant. The combination of ozone concentration with the corresponding coefficient of evaporation derived from actual evapotranspiration seems l~ correlate fairly well with the degree of tobacco damage, in contrast to absolute ozone concentration and severity of flecking.

ACKNOWLEDGEMENTS

I am extremely grateful to Mr. C. C. Boughner, Chief, Climatology Division, to Mr. G. W. Robertson, Meteorologist seconded to the Canada Department of Agriculture, and to Dr. R. E. Munn, Pollution Meteorologist, for their helpful criticisms and comments upon the ideas advanced in this paper and constant encouragement during its preparation. The author wishes to express his indebtedness to the Instrument Division of Agr. Meteorol., 2 (1965) 145-[ 65

OZONE AS A CAUSE OF TOBACCO INJURY

165

the M e t e o r o l o g i c a l Branch for the design a n d installation o f the e q u i p m e n t for the project, to Dr. F. D. M a c D o w a l l o f the C a n a d a D e p a r t m e n t o f A g r i c u l t u r e a n d to Dr. M. K a t z a n d Mr. A. F. W. Cole o f the C a n a d a D e p a r t m e n t o f N a t i o n a l H e a l t h a n d W e l f a r e for their valuable discussion, a n d p e r m i s s i o n to include flecking indices and ozone c o n c e n t r a t i o n data. Special m e n t i o n m u s t be m a d e o f Mr. H. F. Cork, M e t e o r o l o g i c a l Officer, for his assistance in the analysis o f data.

REFERENCES

BALL, F. K., 1960. Control of inversion height by surface heating. Quart. J. Roy. Meteorol. Soc., 86 (370) : 483. KINC, K. M., TANNER,C. B. and SuoMl, V. E., 1956. A floating lysimeter and its evaporation recorder. Trans. Am. Geophys. Union, 37 (6) : 738-742. MACDOWALL, F. D., 1963. Midday closure of stomata in aging tobacco leaves. Can. J. Botany, 41 : 1289-1300. MICHELSON-BLITTNER,K. and ALBRECHT,F., 1929. Beitr. Geophysik, 22 : 13. PRIESTLEY,C. I-I[.B., 1959. Turbulent Transfer in the Lower Atmosphere. Univ. Chicago Press, Chicago, Ill., 93 pp. SUTTON, O. G., 1953. Micrometeorology. McGraw-Hill, New York, N.Y., 152 pp. WALLINGTON,C. E., 1959. The structure of the sea breeze front as revealed by gliding flights. Weather, 14 (19) : 263.

Agr. Meteorol., 2 (1965) 145-165