Submicron aerosols over Indian Ocean: some meteorological characteristics

Atmospheric Research, 27 ( 1 9 9 2 ) 2 9 1 - 3 0 3

291

Elsevier Science Publishers B.V., A m s t e r d a m

Submicron aerosols over Indian Ocean: some meteorological characteristics M. Lal a,~ a n d R.K. K a p o o r b aCentrefor Atmospheric Sciences, Indian Institute of Technology, New Delhi, India bRain and Cloud Physics Research Centre, Indian Institute of Tropical Meteorology, New Delhi, India (Received February 1, 1991; revised and accepted July 5, 1991 ) ABSTRACT Lal, M. and Kapoor, R.K., 1992. Submicron aerosols over Indian Ocean: some meteorological characteristics. Atmos. Res., 27:291-303. The concentrations of submicron aerosols in the size range 10 - 7 t o 10 - 5 cm, also called Aitken nuclei (AN) were measured over the Indian Ocean enroute India-Antarctica-India within the 10 ° E 70°E longitude zone from about 10°N to 70°S latitude on board MV Thuleland during the period from November 26, 1986 to March 18, 1987 as part of the scientific activities on the Sixth Indian Antarctic Expedition. Our analyses showed that only in about 25% of the cases, AN count fell below 1000 cm-3. Throughout the tropical trade wind region, the concentrations of AN were relatively stable with an average of about 3000 cm -3 (medians of ~ 2600 and ~ 1700 cm -3 in Northern and Southern Hemispheres, respectively). Large AN concentrations were found to be associated with higher sea surface temperatures and stronger surface winds in this region. In contrast, the scatter of single observations was found to be remarkable over South Indian Ocean and in Antarctic waters. The average AN concentration over the Indian Ocean to the south of 30 °S was of the order of 1500 cm-3. No definite correlation could be established between large AN concentration and sea surface temperature, wind speed or wave height. Period with very low concentrations were, however, associated with clear sky conditions and calm winds or light breeze. Many events of sudden short-lived but large increase in AN concentrations were observed over the south Indian Ocean and in Antarctic waters and these were always associated with the approach of frontal systems. It is likely that particle production by bursting bubbles and sea spray as well as photochemical reactions and gas-to-particle conversions play important role in the observed high concentration of AN over South Indian Ocean. R~um6 On a mesur6 les concentrations en a6rosols sous-microniques dans la gamme des dimensions de 1 0 - 7 ~t 10 - 5 cm, 6galement appel6s noyaux d'Aitken (N.A.), au-dessus de l'Oc6an Indien sur le trajet

Inde-Antarctique-Inde entre les longitudes 10°E-70°E et les latitudes 10°N-70°S,/l bord du MV Thuleland entre le 26 novembre 1986 et le 18 mars 1987 dans le cadre de la Sixi~me Expedition Antarctique Indienne. Les analyses montrent que la concentration en N.A. tombe au-dessous de 1000 cm-3 dans seulement environ 25% des cas. Dans la r6gion tropicale des aliz6s, les concentrations sont relativement stables avec une moyenne de l'ordre de 3000 cm -3 (~ comparer aux valeurs m6dianes de 2600 et 1700 cm -3 respectivement dans les h6misph~res nord et sud). On observe que les fortes concentrations sont associ6es ~t des temp6ratures plus 61ev6es de lamer, et ~ des vents en surface plus rapides dans cette r6gion. A l'oppos~, la dispersion des mesures est 61ev6e dans le sud de l'Oc6an Indien et darts les zones antarctiques. La moyenne des concentrations au-dessus de l'Oc6an Indien au ~Corresponding author: M. Lal, C e n t r e for A t m o s p h e r i c Sciences, I n d i a n Institute o f Technology, New Delhi, I n d i a

0 1 6 9 - 8 0 9 5 / 9 2 / $ 0 5 . 0 0 © 1992 Elsevier Science Publishers B.V. All rights reserved.

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M I_A[.AND R.K. KAPOOR

sud de 30~S est de l'ordre de 1500 cm 3. On n'a pas pu 6tablir de correlation evidente entre les tortes concentrations et la temp6rature de lamer, la vitesse du vent ou la hauteur des vagues. Cependant, les p6riodes h tr~s faibles concentrations sont associ6es ~ des situations de ciel clair et de vent calme ou brise 16g6re. De nombreux 6pisodes d'augmentation importante brutale et de courte dur6e des concentrations en N.A. on 6t6 observ6s dans le sud de l'Oc6an Indien et dans les zones antarctiques, toujours ~t l'arriv6e de syst6mes frontaux. I1 est probable que la production de particules par 6clatement de bulles et par embruns, ainsi que les r6actions photochimiques et la transformation de gaz en particules jouent un r61e important dans les fortes concentrations en N.A. observees dans le sud de l'Oc6an Indien.

INTRODUCTION

Atmospheric aerosols are characterized by their size from about 10-7 to 10--3 cm in radius and also by variations of the nuclei concentrations upto about 4 X 106 cm-3. Among these, aerosols in the size range 10-7_ 10-5 cm are called Aitken nuclei (henceforth abbreviated as AN). These are relevant to the formation of cloud droplets in saturated or supersaturated air and thus have an important role in the precipitation process. An examination of the AN measurements over sea (Hogan, 1981 ) reveals that compared with the vastness of the Oceanic regions, the number of concentration measurements (about 900) and also their sporadic variations are far from satisfactory. In the recent past, a number of papers have appeared on AN concentrations measured over Pacific and Atlantic Oceans (Hoppel et al., 1985; Parungo et al., 1986; Hoppel and Frick, 1990) but practically no data exists over the Indian Ocean and in the waters off East Antarctica. Only few observations on aerosol concentration at the surface of the Southern Indian Ocean and around the periphery of the Antarctic continent are available in records (Meszaros and Vissy, 1974; Elliot et al., 1974; Hogan and Mohnen, 1979 ). In the course of the Sixth Indian Scientific Expedition to Antarctica (November 26, 1986 to March 22, 1987 ), an attempt was, therefore, made to monitor the concentrations of AN to study their geographical and temporal variations over the Indian Ocean as well as in the Antarctic waters and to ascertain their importance to meteorological processes. Besides these, measurements of AN concentration both at the surface and in the boundary layer were made at Maitree Hills station in East Antarctica, the results of which have been discussed elsewhere (Lal and Kapoor, 1989). Our analysis on the AN data set based on measurements made over the Indian Ocean enroute India (Goa)-Antarctica ( Dakshin Gangotri )-India ( Goa ) from 10 ° N to 70 ° S ( edge of the Antarctic continent) within 10°E-70°E longitude zone during the austral summer of 1986-1987 is presented in this paper.

SUBMICRON AEROSOLS OVER INDIAN OCEAN

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SAMPLING DETAILS

A Gardner small-particle detector (serial no. 1262) was used to measure the two hourly AN concentrations in the size range from 0.001 to 0.1/tm. The detector is a volume-controlled one. For each observation, three instrument readings were made and averaged before determining the nucleus concentration from the calibration curve of the particle detector (Hogan et al., 1975 ). The standard used for the calibration curve is the nucleus counter developed at The School of Cosmic Physics, Dublin Institute for Advanced Studies (Gardner Assoc. Inc., 1980). Over 250 observations were collected during the period from November 28, 1986 to January 1, 1987 while sailing from India (Goa) to Antarctica (Dakshin Gangotri) on board MV Thuleland. Enroute Antarctica to India, about 200 observations were made during the period from February 21 to March 18, 1987. The particle detector on board the ship was installed in a cabin at 6 m above the deck level ( 15 m above the sea surface) away from the exhaust and towards the bow deck. The air samples were taken through a 1.5 m polythene tube upstream inlet (rinsed at a weekly interval in order to strip ANs deposited therein from the sampled air) projected 45 ° downward outside through a window of the cabin. All the measurements on board the research ship were scrutinized and anomalous values of AN due to possible local contaminants were eliminated from our data set. As such, the AN data set considered in this study is assumed to be free from any contamination (the effluents released by the ship while cruising at a speed of 8-10 nautical miles per hour were believed not to affect the AN count unless the prevailing wind direction is same as that the direction of ship's cruise). Sampling of aerosols was also done on 0.8/lm millipore filters through a Cascade Impactor with a calibrated air p u m p of known suction rate. At the intake, a holder made of steel along with a perforated disc was fitted between two plates. The perforated holder was of an appropriate size to hold membrane filters, of 47 m m in diameter and 0.8/tm in pore size. The p u m p was placed on top of the deck forward of navigation bridge in upwind direction. Care was exercised to avoid the soot particles coming from ship's exhaust. On selected dates the p u m p was run with the ship in motion for 2 hrs ( 1100Z to 1300Z) so that the samples collected were representative of the mid-day conditions. Simultaneous surface observations of air and sea temperatures, pressure, wind and fractional cloud cover were also made during the entire cruise. The position of the research ship M V Thuleland, while cruising from Goa to Dakshin Gangotri through the Arabian Sea, Indian Ocean and South Indian Ocean and back to Goa, during which AN observations were made, is shown in Fig. 1.

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The 201 surface AN observations selected after scrutiny from measurements on board the ship during Goa-Dakshin Gangotri cruise when grouped together in different concentration classes revealed that AN concentrations between 100 and 1000 cm -3 comprised 23% of all the observations. On the return journey, 31% o f the 165 observations recorded AN concentrations be-

SUBM1CRONAEROSOLSOVER INDIAN OCEAN

295

low 1000 cm -3. On average, a cumulative frequency of about 75% covered all AN concentrations below 5000 cm -3. Over the entire Indian Ocean, a dominant part of AN measurements fell in the concentration range 100-2500 cm-3. These are of the same order as those over the Atlantic and Pacific oceans (Parungo et al., 1986, 1987; Clarke et al., 1987). Table 1 illustrates the latitudinal distribution of AN concentrations over the entire Indian Ocean as monitored during our cruise to and from Antarctica. In the northern equatorial region of Indian Ocean, a median of 2610 c m - 3 was obtained obviously under the influence of continental air. The sampling sites in the north equatorial region are not very far from the land masses of Africa and India. Moreover, it is along one of the main shipping lanes across the Arabian Sea. The effect of the continental air becomes less pronounced in the south equatorial region where the AN concentration had a median of 1680 cm -3. Composites of AN concentrations within the Tropical Indian Ocean ( 1 0 ° N - 3 0 ° S ) revealed that in the trade wind zone, the average AN concentration was about 3000 c m - 3 (AN concentrations of the order of 3000-6000 c m - 3 between Islands of Tahiti and Moorea in trade wind region of Pacific Ocean have also been recorded as reported in Hoppel et al., 1989). The weather observations for this portion of the cruise (trade wind zone) reported partly cloudy (20-40%) with stratus and fair weather cumulus clouds; wind and sea states were not high and there was no local precipitation. No TABLE1 Latitudinal distribution of AN concentrations N (cm -3) over the Indian Ocean. Nlo refers to the concentration below which lay only 10% of the observations, Nso is the median; N90 is the concentration above which lay 10% of all the observations. Also included here are the mean air temperature ( ° C ) and wind speed (knots) Region

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diurnal variation was discernible in the AN concentration under clear weather conditions in this region. Even during non-stationary weather conditions, no definite correlation could be established between AN concentration and wind speed/wave height. The non-existence of a definite correlation between AN concentration and either wind speed or wave height can be explained by assuming that any increased production of AN in strong wind conditions is largely counter-balanced by their increased upward transport to higher levels. Further, it is likely that the very low settling velocities and long atmospheric residence times ( ~ 50-70 hr in the absence of significant precipitation) of these fine particles would smooth out any short-term wind speed/wave height effects. The larger air-sea temperature differences on such an occasion also .z.0 30 20

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tion possibly from the oxidation products of DMS (dimethyl sulfide) in tropical oceans could also be responsible for high AN concentration in this region. During our cruise between 30°S-60°S, we encountered a diversity of weather conditions. Composites of concentrations based on all AN measurements during this portion of the cruise gave an average value o f 1500 c m - 3 (median ~ 620 cm-3 between 40 °S and 60 °S ). AN concentrations in modified maritime air at Cape Grim (40 oS) were found to have mean distribution within the range 300 < N < 3000 c m - 3 (Gras and Ayers, 1983 ) with summer medians ~ 500 c m - 3 (Bigg, 1980). Our observations on AN concentration are rather high considering that there were no continental influences (the continental air during its course is expected to be sufficiently scrubbed by the

299

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time it reaches this area). This region was characterized by increased wind speed, high sea states and cloudy skies (30%-80% cloud coverage). The high concentration of AN observed in this region could have been associated with surface-generated sea salt particles caused by higher seas and wind speeds in addition to a gas-to-particle conversion process which is more favourable for fine mode AN generation. The periods with very low concentrations ( 100250 c m - 3 ) w e r e associated with clear sky conditions and calm wind or light breeze. Further to the south in Antarctic waters (south of 60°S) the average AN count calculated from all observations collected in this region gave a number density of 850 cm-3; less than one-third of the mean AN concentration measured over the tropical region. The high median value of 980 c m - 3

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Fig. 6. Surface weather analysis chart for South Indian Ocean at 1200GMT on Dcember 11, 1986.

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Fig. 7. Variation of surface aerosol concentrations sampled on 0.8/~m millipore filters during the period 9-13 December and 26-30 December 1986 over the South Indian Ocean and in Antarctica waters.

SUBMICRON AEROSOLS OVER INDIAN OCEAN

301

in this region could be attributed to the influence of the Antarctic Convergence Zone (air blowing from Antarctica towards lower latitudes). A greater concentration of AN is expected over the snow surface which is colder than the air above and thus acts as a thermal precipitator. Interestingly both in the South Indian Ocean as well as in Antarctic waters, a close association between the concentration of AN and the intensity of vertical mixing was observed. In particular, a small nucleus count was found to be linked with shallow cumulus clouds whereas occurrence of stratus cloud was typical of high concentrations of AN at the surface. The AN concentrations measured in the South Indian Ocean and in Antarctic waters, particularly south of 40 °S was highly variable. There was a dramatic increase in AN concentrations invariably a few hours before the passage of frontal weather systems. Large increases in the aerosol concentration associated with frontal passages and advection of warm moist air in the Pacific Ocean have also been reported (Hoppel et al., 1989). Our observations revealed that, about 8-12 hr in advance of an approaching front (depending upon relative speed and direction of the ship's m o t i o n ) near the ship's location, AN increased sharply. The frontal weather systems documented themselves by an increased cloud cover at the site. These observations are substantiated in Figs. 2-5 wherein increases in AN counts are shown along with surface pressure, air temperature, wind and cloud cover. The sudden increase in AN number density at 0600Z on December 1 lth (Fig. 2) was linked with the frontal system identified as (A) on the surface weather chart (Fig. 6) of 11 December 1986 (1200Z), which crossed the ship at about 1600 U T C on the same day resulting in overcast cloud conditions. The observations on sudden increases of AN were recorded on a number of other occasions during the period of AN observations in Antarctic waters and they were always followed by overcast skies at the observation site (Figs. 3, 4 and 5 ). The AN concentrations of about 50,000 cm -3 recorded at the time of sudden increase on all these four occasions and lasting only for < 4-6 hr are, though convincingly real (no local contamination sources on board the ship were identified at the time of these observations), but difficult to explain in the absence of comprehensive measurements on gas and particulates. Increases in aerosol concentration over the oceans are known to increase the a m o u n t of low level cloudiness through a reduction in drizzle - - a process that regulates the liquid water content and energetics of shallow marine clouds (Albrecht, 1989 ). The concentration of particles in air masses associated with fronts were also examined by sampling of aerosols on 0.8 a m millipore filters collected on middays during the period 9-13 December and 26-30 December 1986. Figure 7 demonstrates that the aerosol population of dimensions larger than 0.8 a m also increased sharply on December 1 lth and December 27th in conformity with the sudden increases of AN concentrations in association with frontal passage as shown in Figs. 2 and 4. We believe that, while photo-oxidation in

302

M. LAL AND R.K. KAPOOR

the atmosphere of precursor gases under the strong westerly winds in Antarctic waters could be the main source of high AN concentrations, the generation of sea-salt particles under conditions of disturbed sea and higher wind speeds in association with weather fronts might also have contributed to the observed sudden increases in aerosol concentration. With the existing data it is not possible to quantitatively evaluate the formation of AN due to gas to particle conversion. Further verification of such observations together with chemical analysis of aerosol samples in future cruises would lead to a better understanding of the atmospheric processes in the marine atmosphere of the South Indian Ocean. ACKNOWLEDGMENT

The authors express their appreciation to the Indian Air Force and Indian Navy pilots for their cooperation in the collection of AN data. The first author (ML) is indebted to Dr. S.Z. Qasim, former Secretary to the Govt. of India, Department of Ocean Development for providing the opportunity to conduct scientific work in Antarctica.

REFERENCES Albrecht, B.A., 1989. Aerosols, cloud microphysics and fractional cloudiness. Science, 245:12271230. Bigg, E.K., 1980. Comparison of aerosol at four baseline atmospheric monitoring stations. J. Appl. Meteorol., 19: 521-533. Clarke, A.D., Ahlquist, N.C. and Covert, D.S., 1987. The Pacific Marine aerosol: Evidence for natural sulphates. J. Geophys. Res., 92:4179-4190. Elliott, W.P., Ramsey, F.L. and Johnston, R., t 974. Particle concentrations over the globe. J. Rech. Atmos., 8: 939-945. Gardner Assoc. Inc., 1980. Preliminary instructions for small particle detector type C.N. (Cat. 70004G2, Ser. No. 1262), Schenectady, NY. Gras, J.L. and Ayers, G.P., 1983. Marine aerosol at southern mid-latitudes. J. Geophys. Res., 88: 10,661-10,666. Hogan, A.W., Winters, W. and Gardner, G., 1975. A portable nucleus counter of high sensitivity. J. Appl. Meteorol., 14: 39-45. Hogan, A.W. and Mohnen, V.A., 1979. On the global distribution of aerosols. Science, 205: 1373-1375. Hogan, A., 1981. Meteorological variation of maritime aerosols. In: Proc. 9th Int. Conf. Aerosols, Condensation and Ice Nuclei, Univ. College, Galway, pp. 503-507. Hoppel, W.A., Fitzgerald, J.W. and Larson, R.E., 1985. Aerosol size distributions in air masses advecting offthe east coast of the United States. J. Geophys. Res., 90: 2365-2379. Hoppel, W.A., Fitzgerald, J.W., Frick, G.M., Larson, R.E. and Mack, E.J., 1989. Atmospheric aerosol size distributions and optical properties found in the marine boundary layer over the Atlantic ocean. Nay. Res. Lab., Rep. 9188. Hoppel, W.A. and Frick, G.M., 1990. Submicron aerosol size distributions measured over the tropical and south Pacific. Atmos. Environ., 24 A (3): 645-659.

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Lal, M. and Kapoor, R.K., 1989. Certain meteorological features of submicron aerosols at Schirmacher Oasis, East Antarctica. Atmos. Environ., 23 (4): 803-808. Meszaros, A. and Vissy, K., 1974. Concentration, size distribution and chemical nature of atmospheric aerosol particles in remote oceanic areas. Aerosol Sci., 5:101-109. Parungo, F.P., Nagamoto, C.T., Rosinski, J. and Haagenson, P.L., 1986. A study of marine aerosols over the Pacific ocean. J. Atmos. Chem., 4:199-226. Parungo, F.P., Nagamoto, C.T., Madel, R., Rosinski, J. and Haagenson, P.L., 1987. Marine aerosols in Pacific upwelling regions. J. Aerosol Sci., 18: 277-290.