Aerosol size distributions in the north and south Indian ocean during the northeast monsoon season

Atmospheric Research 65 (2002) 51 – 76 www.elsevier.com/locate/atmos

Aerosol size distributions in the north and south Indian ocean during the northeast monsoon season C.G. Deshpande *, A.K. Kamra Indian Institute of Tropical Meteorology, Pune, India Received 18 February 2002; received in revised form 12 August 2002; accepted 12 August 2002

Abstract Measurements of aerosol size distributions (3- to 1000-nm diameter) were made over the Indian Ocean (15jN, 75jE to 70jS, 11jE) during the onward (December 12, 1996 to January 6, 1997) and return (March 9 to April 5, 1997) cruises of the XVI Indian Scientific Expedition to the Antarctic. Observations show that during the January to April period, the environment over the Indian Ocean undergoes a transition from a relatively clean to a fairly polluted one. Our observations of the large concentrations and the North-to-South positive gradient of aerosol concentration over the northern Indian Ocean strongly support the transport of aerosols and trace gases with the seasonal northeasterly winds from the Indian subcontinent to the Indian Ocean. The results also indicate the production of new particles over the oceanic region by the gas-to-particle conversion processes. During this season, because of the persistent northeasterly winds and the shift of the Inter-Tropical Convergence Zone (ITCZ) to the Southern Hemisphere in this region, the air pollutants over the northern Indian Ocean are carried into the Southern Hemisphere with the cross-equatorial flow and reach up to the southern limit of the ITCZ. Some pockets of very high aerosol concentrations have been observed in and around the ITCZ. The nucleation mode particles are observed in great abundance up to 30jS. From the Indian coast to 5jN, aerosol particles are observed to have bimodal size distributions with a maximum in accumulation mode at 133-nm diameter and a minimum for nucleation/Aitken mode particles of < 74 nm in diameter. The size distributions gradually change to trimodal ones with another minimum appearing for the coarse mode particles>422 nm in the 5jN-Equator belt and are generally trimodal in the Southern Hemisphere. Transport of the nucleation mode particles from the free troposphere to the marine boundary layer (MBL) associated with the large-scale subsidence and the subsequent North-to-South transport of these and continental aerosols with the seasonal northeasterly surface winds over the Indian Ocean is proposed to explain the high concentration of the nucleation mode particles observed up to the ITCZ. Size distributions change to Junge’s powerlaw type in regions of high aerosol concentration inside the ITCZ. Comparatively small concentrations with almost uniform size distributions that are typical of the pristine air in the *

Corresponding author.

0169-8095/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 8 0 9 5 ( 0 2 ) 0 0 1 2 1 - 7

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Southern Hemisphere are observed from 42jS to 56jS. Some high peaks in aerosol concentration that can be associated with the low-pressure systems that encircle Antarctica are observed in the 60 – 70jS latitudinal belt. Further, our observations strongly demonstrate the effect of wind direction on the land-to-ocean transport of the atmospheric aerosols. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Tropospheric aerosols; Marine aerosols; Marine pollution; Aerosol size distribution; Aerosols over Indian Ocean; Aerosols during northeast monsoon

1. Introduction Marine aerosol particles range in size over four orders of magnitude and play important roles in atmospheric processes such as cloud formation, ice nucleation, radiative transfer, chemical reactions, wet and dry removal, etc. Their role in estimating the radiation budget of the atmosphere is one of the largest sources of uncertainty in validating model predictions of climate change (IPCC, 1995). Over oceans, part of the marine aerosol originates directly from ocean, but a large fraction of it, could be advected from the continents or be formed within the atmosphere. The submicron size aerosols play an important role in the formation of cloud droplets in the precipitation process and are of vital importance in assessing the background air pollution over oceans. The number, size distribution and composition of the marine aerosol particles vary with time and space in response to a number of different processes such as the transport of continental air, the bursting of whitecaps, the oxidation of gases emitted by the oceans, etc. Several experiments have been conducted in the Pacific and Atlantic oceans to study the physical, chemical and optical properties of submicron aerosols (e.g., Hoppel et al., 1985, 1994; Hoppel and Frick, 1990; Quinn et al., 1993, 1996; Covert et al., 1996; Bates et al., 1998, 2000; Kaufman et al., 1998; Russell et al., 1999; Raes et al., 2000). As a result of these studies, several features in the unperturbed marine boundary layer (MBL) size distributions and the processes that control them have been identified. However, the background aerosol size distributions are substantially altered and the radiative and cloud nucleating properties are modified if the air masses are influenced by some continent. The region of the Indian Ocean during the winter monsoon season is one such area. In spite of some measurements reported by Prodi et al. (1983), Savoie et al. (1987), Lal and Kapoor (1992), Rhoads et al. (1997), and Jayaraman et al. (1998), detailed data on the spatial and temporal distributions of the aerosol concentrations over the Indian Ocean is lacking. The Indian Ocean, south of the Equator is virtually a data void region. While compiling the available data all over the globe on the concentration and size distribution of marine aerosols, Heintzenberg et al. (2000) point out that major gaps occur in the Indian Ocean region. The main objective of the recently conducted Indian Ocean Experiment (INDOEX) was to assess the extent of the aerosol and pollutant transport over the Indian Ocean in the northeast monsoon season. From the data obtained during a pre-INDOEX cruise, Krishnamurti et al. (1998) have studied the aerosol and pollutant transport and their impact on radiative forcing over the tropical Indian Ocean. Lelieveld et al. (2001) report aerosol mass loading over the entire northern Indian Ocean toward the ITCZ, comparable

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to suburban air pollution in North America and Europe. The aerosol optical depth measurements of Krishna Moorthy et al. (1997), Satheesh et al. (1998), Parameswaran et al. (1999), and Krishna Moorthy and Saha (2000) and airborne measurements of de Reus et al. (2001) show that in the marine boundary layer (MBL), aerosol can be characterized by high number concentrations of submicron and accumulation mode particles, which gradually decrease with distance from the Indian subcontinent. The North-to-South gradients of aerosol concentrations over the northern Indian Ocean are also observed in measurements of the atmospheric electric conductivity (Kamra et al., 2001) and submicron aerosol particles (Kamra et al., submitted for publication). Bates et al. (in press) have examined the aerosol number and volume distributions in eight regions which are distinctly different in respect of different aerosol sources, meteorological conditions and time spent in the MBL. Two meteorological features of this region of the Indian Ocean are of much relevance in interpreting the results of the measurements presented in this paper. First, the atmospheric circulation over the northern Indian Ocean has predominant North-to-South low-level flow during the northeast monsoon season (January to April). Second, the ITCZ shifts during this season to the southern hemisphere at about 10jS. In this paper, we report our measurements of the submicron aerosol size distributions made onboard Polar Bird along the cruise track in the Indian Ocean during the onward (December 15, 1996 to January 6, 1997) and return (March 9 to April 4, 1997) journeys of the XVI Indian Antarctic Expedition. Results are studied in terms of the local meteorological and synoptic conditions.

2. Instrumentation and sampling The aerosol measurements are made with a TSI 3030 Electrical Aerosol Analyser (EAA) System. It measures the size distribution of particles of diameter 3 to 1000 nm in 10 different size ranges, at atmospheric relative humidity. Liu and Pui (1975) find that the sensitivity of this instrument is a strong function of particle size, varying from very high for large particles to low for particles < 7 nm. For particles < 7 nm, the calibration in their experiment curve is obtained by linear extrapolation on a log –log plot. Further, the loss of small particles by diffusion in the inlet tube may be appreciable in this system. Accuracy of the contributions from the lowest two channels, i.e., for 3- and 7-nm-diameter ranges, is not sufficient due to the limitation of operating the instrument where the rate of generation of the photochemically generated aerosols is highly variable and, hence, as discussed later in this paper, the contributions from these channels are not considered in this analysis. The EAA system is operated every 3 h to collect five size distribution samples and the data is stored in a PC. An average of five samples taken at an interval of 3 h is used in this analysis. The inlet of the EAA system is cleaned at least once a day to minimize the accumulation of sea salt, snow or dust at the inlet. This cleaning frequency was found to be adequate and effective as no appreciable difference was observed in the measurements taken before and after the cleaning. The air is sampled at a height of 12 m above mean sea level through a 1-cm-diameter stainless steel tube of 1-m length, projects outward from a cabin and is properly grounded.

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The EAA system is installed inside the cabin. The position of the inlet to collect the air sample is so selected that the exhaust from the chimney on the ship does not pollute the aerosol measurements. Simultaneous measurements of the electric conductivity of both polarities are made throughout the cruise with a Gerdiens’ apparatus described by Dhanorkar and Kamra (1992). Gerdiens’ apparatus is installed on deck and is located about 3 m below the inlet of the EAA system. The electric conductivity is a measure of the background aerosol pollution and has been used by Cobb and Wells (1970), Misaki et al. (1972) and Kamra and Deshpande (1995) to study the secular changes in the background aerosol pollutants over open oceans. However, the validity of the conductivity – aerosol concentration relation is based on certain assumptions which are not strictly met over the northern Indian Ocean and in the ITCZ during the northeast monsoon season and also near the continent of Antarctica around which the low-pressure systems circulate in the 60 – 70jS latitudinal belt. Therefore, the region of 55– 41jS has been selected for intercomparison of the two techniques. The 10-min averaged conductivity-derived values of aerosol concentrations did not differ by more than 15% from the measured values. The agreement is very good considering that the ranges of particle size measured in the two techniques may differ.

3. Cruise and weather Fig. 1 shows the route followed and position of the ship at 1200 UT on the dates marked along the cruise. The cruise started on December 12, 1996 from Goa and the ship berthed at Mauritius from December 20 to 22, 1996, for helicopter loading and bunkering. After crossing the zone of mid-latitude westerlies and the Antarctic circle, the ship encountered the pack ice. After the ice cutting/breaking operations, the ship reached the Antarctica coast on January 4, 1997. After a stay at the Indian Antarctic station, Maitri, from January 10 to February 24, 1997, the return journey started on March 9, 1997. The ship was berthed at Durban, South Africa, on March 20, 1997 and at Mauritius on March 27, 1997, then continued her journey toward India and reached Goa on April 5, 1997. During the winter monsoon season from December to March, the northeasterly surface winds originating from the subtropical high over continental regions prevail over the North Indian Ocean. The position of the secondary convergence zone, the Northern Hemispheric near-Equator Trough (NHET) is near 5jN in December, shifts toward the Equator by January and reappears at 3– 5jN in March. The southeasterly trades over the tropical South Indian Ocean originating from the subtropical high region near 35– 40jS blow toward the Equator and converge in the region of the ITCZ or the Southern Hemisphere near-Equator Trough (SHET). Both the SHET and the NHET are the regions of organized convection. NE trades are pulsatory in nature and turn to a N/NW direction near the Equator, crossing it as a NW monsoon up to 5– 10jS into the SHET whose position varies between 7 and 15jS in the lower troposphere. These features repeat every year from December to March such that the peak in their intensity occurs in January– February and the reversal of the annual cycle begins in March. Under their influence, the NE trade winds carry the polluted continental air from the Asian subcontinent and deserts of Arabia and meet the pristine air brought by SE trade winds from the Southern Hemisphere in the NHET/SHET region.

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Fig. 1. Cruise tracks of the ship, MV Polar Bird during the onward and return journies during 1996 – 1997 Indian Scientific Expedition to Antarctica. Solid circles and squares along the tracks show the daily ship position at 0000 UT.

Strong mid-latitude westerlies dominate the region of the 40 –60jS belt. The region south of 60jS, the Antarctica water, is often frequented by the low-pressure systems circulating around the Antarctica continent. The three-hourly values of meteorological parameters measured on the ship are typical of this season over this region (Fig. 2). The air temperatures and the sea surface temperatures drop below 10 jC southward of 45jS. The atmospheric pressure shows a

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Fig. 2. Three-hourly meteorological data taken on the ship.

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sharp fall at 50 –55jS. High winds of about 25 –30 knots are observed in the 40– 50jS belt along with sea states of 5 –6. The total cloud coverage is 6 –8 octas below 45jS. During the present cruise, the ITCZ is located south of the Equator in the month of March, and the winds in the Northern Hemisphere are persistently northeasterly. The ship observations show an increase in cloud cover from 1 –3 to 6 – 7 octas as the ship sails from 10jS toward the Equator. The cloud cover again reduces to 1– 3 octas north of the Equator.

4. Aerosol concentrations and size distributions over the Indian Ocean Fig. 3 shows total aerosol concentrations (10 < d < 1000 nm), and the aerosol concentrations in the Aitken mode (20 – 100 nm) and accumulation mode (100 – 1000 nm) obtained by adding concentrations in the respective channels of the EAA system observed along the onward and the return cruise tracks. On the onward journey, no data could be obtained from 15jN to 4jN due to some technical difficulties, and from 32jS to 46jS due to very rough sea conditions. An important feature of the observations is the higher total aerosol concentrations of 8  102 – 25  103 cm 3 observed north of about 30jS during the return journey in March –April 1997 as compared to those of 8  102 – 6  103 cm 3 observed in the same region during the onward journey in December 1996– January 1997. The higher values of concentrations, of course, appear only as peaks and the average values on the return journey

Fig. 3. The total number concentration of the aerosol particles and concentrations in Aitken and accumulation modes along the onward and return cruises.

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are only 2– 3 times higher than on the onward journey. Also, aerosol concentration shows greater variability with latitude on the return journey than on the onward journey. These values of aerosol concentrations are more than an order of magnitude higher than those observed over other open oceans (e.g., Fitzgerald, 1991) including those observed over the South Indian Ocean (Meszaros and Vissy, 1974; Savoie et al., 1987). However, these values are of the same order as observed near coastlines (Hoppel et al., 1989) or in the airflow from continents into the MBL during the Aerosol Characterization Experiment (ACE-2) (Bates et al., 1998, 2000; Johnson et al., 2000). Such large concentrations in this season are also supported by the recent observations of (i) Aerosol loading of the Indian Ocean north of the ITCZ observed with a high-volume sampler and a low-pressure sampler (Parameswaran et al., 1999), (ii) Aerosol optical depths measured with a multi-wavelength solar radiometer and an EKO sun-photometer (Krishnamurti et al., 1998), and (iii) Measurements of the atmospheric electric conductivity (Kamra et al., 2001) and aerosol size distribution (Bates et al., in press; Kamra et al., submitted for publication) during the INDOEX. The higher concentrations reported here should be considered keeping in view that the northeastern winds transporting the continental pollutants prevail over the Indian Ocean persistently for 4– 5 months and extend up to 10 –15jS. Our values of aerosol concentrations need not be consistent with those of Heintzenberg et al. (2000) who generated marine aerosol data for aerosol modelling purposes and reported seasonally averaged values for a coarse grid of 15j  15j. We shall further discuss the possible cause of this seasonal difference in the aerosol concentrations observed in this region. Meszaros and Vissy (1974) and Parungo et al. (1987) observed a diurnal variation with a maximum during daytime in the concentration of Aitken particles over the South Atlantic, Indian and Pacific Oceans. Haff and Jaenicke (1980) observed a pronounced daytime maximum in the concentrations of particles of radius < 6 nm. However, Hoppel et al. (1989), Hoppel and Frick (1990), Haff and Jaenicke (1980) did not find any diurnal variation in the concentration of particles>6 nm over the remote Atlantic and Pacific Oceans. Our observations also do not show any systematic diurnal variation in total aerosol concentration over the Indian Ocean. These conflicting observations of the diurnal variations may not be inconsistent in view of the specific conditions under which the mechanism for generation of new particles by photooxidation of the dimethyl sulphide (DMS) emitted by the sea can operate in the MBL. These conditions show large variability in both space and time. Total aerosol concentration increases as the ship approaches the landmasses of South Africa, Mauritius or India and decreases as the ship goes away from the coasts of South Africa or Mauritius. While such changes are observed only up to about 200 km from the coastlines of South Africa and Mauritius, they are observed for 2000 –3000 km in the case of the Indian coast. Such changes in aerosol concentration near the landmasses are generally associated with the transportation of the aerosols from the continents (e.g., see Hoppel and Frick, 1990; Deshpande and Kamra, 1995; Kamra and Deshpande, 1995; Bates et al., 2000). Our observations also show some peaks near the Antarctica coast on both the onward and the return cruises. Such enhancements in aerosol concentration have also been observed by Voskresenskii (1968), Hogan (1975), Gras and Adriaansen (1985) and Lal and Kapoor (1989) over coastal stations of Antarctica. The peak concentrations in observations mostly consist of enhancement in the nucleation mode particles and are associated with the low-pressure areas which form a ‘wall of storms’ around the continent

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of Antarctica. The atmospheric surface pressure observations made on the ship do indicate the presence of some low-pressure systems in this region. During the onward journey, three peaks showing an increase of 2– 4 times in aerosol concentrations and lasting for a few hours, are observed between 1jN and 8jS. On the return journey also, two peaks of much greater magnitude, showing an increase of about an order of magnitude in aerosol concentrations and lasting for several hours, are observed between the Equator and 9jS. These peaks are located within the ITCZ during these periods. The satellite pictures of cloudiness in this area suggest that the ITCZ is weak and not well organized during the onward journey, but is well developed and intense, and has somewhat shifted to the South during the return journey. Comparatively low and nearly constant values of aerosol concentrations are observed from 8jS to 30jS and to some extent, also from 47jS to 65jS (no observations are available from 30 to 47jS) on the onward journey and from 57jS to 42jS on the return journey. During these periods, the ship is in the open sea and experiences (as the ship observations and the NCEP reanalysis data of surface winds show) the mid-latitude westerly surface winds which have a long journey over the ocean. The aerosol concentrations recorded during these periods are typical of the pristine air in the Southern Hemisphere. The ship continues to be in the zone of westerlies up to about 41jS when the surface wind directions suddenly change from westerly to easterly or north-easterly. These winds may transport aerosols from the African continent. Consequently, aerosol concentrations remain comparatively high and variable up to Durban. About 1750 size distributions of aerosol particles were obtained during the onward and the return cruises from 15jN to 70jS. Vastly different meteorological conditions prevailing over such a large latitudinal belt are likely to influence and change the aerosol size distributions. Therefore, we have grouped together the size distribution curves which remain identical continuously for several hours in a particular latitudinal belt. Figs. 4 and 5 show some typical size distribution curves observed in different latitudinal belts during the onward and the return cruises, respectively. Size distribution shows sequential changes with respect to the different latitudinal belts. Three types of size distributions appear. (i) Trimodal distributions with one maximum in the accumulation mode at 133 nm and two minima, one for the nucleation/Aitken mode particles < 74 nm and another for coarse mode particles>422 nm. These curves are open ended with aerosol concentrations increasing on both sides of the minima. (ii) Bimodal distributions with one maximum at 133 nm and one minimum at 74 nm. (iii) Power-law distribution—Size distributions are bimodal north of 5jN, start changing to trimodal in the 5jN-Equator belt and are generally trimodal in the Southern Hemisphere except in the 70 – 60jS belt and also in the 10 –5jN and 40– 30jS belts on the return cruise. All size distributions, especially those within the ITCZ and north of it, show very large concentrations of fine particles in the nucleation mode. Moreover, the size distribution curves in the Northern Hemisphere show large variability among themselves even within a period of a few hours. On the other hand, in the Southern Hemisphere, size distributions remain nearly similar continuously for several hours or even days. The size distributions observed from 70– 40jS to 30 – 10jS are nearly similar in shape to those observed in these regions of the Southern Hemisphere during the onward journey. In the 70 –60jS belt, particle concentration in each size range increases and the increase in the nucleation mode is so large that the maximum in the accumulation mode almost

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Fig. 4. Aerosol size distributions which remained nearly similar continuously for several hours in different latitudinal belts during the onward cruise.

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Fig. 5. The same as in Fig. 4 for the return cruise.

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disappears. Also, the aerosol concentration of the particles in nuclei mode (13 –74 nm) is much higher in the 5jN to 18jS region as compared to that in the 60 – 20jS region. In sharp contrast to the onward journey, the size distributions observed in the 10 – 5jS belt, where the ITCZ is located during the return cruise, change from trimodal to the powerlaw type. In the 5– 15jN region, concentrations of particles in all sizes and particularly of larger sizes are higher by about an order of magnitude than in all other latitudinal belts. Moreover, instead of showing a minimum at 422 nm, concentrations keep decreasing in this region. The same trend in size distributions is observed in the 40 –30jS belt. These trends indicate the transportation of the continental aerosols with the prevailing northeasterly winds over the northern Indian Ocean and local generation of large particles by breaking waves on the sea surface due to strong winds in the 40 –30jS belt. Further, while size distributions in the 20– 10jS belt are of the trimodal type, typical of those observed over open oceans, they change to the power-law type in the 10 – 5jS belt where the ITCZ is located during this period. Moreover, the particle concentration of all sizes increases in the ITCZ. Average modal parameters in different latitudinal belts very well demonstrate the large concentrations and the north-to-south gradients of the Aitken and accumulation mode particles from 5jN to 1jS on the onward journey (Table 1). The concentrations and age of the particles in this region are comparable to those observed in the Northern Hemispheric Table 1 Average modal parameters for the measurements made during XVI Indian Antarctic expedition 1996 – 1997 Position latitude-longitude

Modal parametersa

15jN, 73jE – 5jN, 69jE

N DgN r N DgN r N DgN r N DgN r N DgN r N DgN r N DgN r

5jN, 69jE – Equator, 66jE

Equator, 66jE – 5jS, 64jE

5jS, 64jE – 10jS, 61jE

10jS, 61jE – 30jS, 30jE

30jS, 30jE – 60jS, 25jE

60jS, 25jE – 70jS, 11jE

a

Onward journey Aitken numberb

Return journey Accumulation numberb

_

_

950 F 126 85.2 F 62.4 1.684 F 0.022 864 F 405 85.3 F 25.4 1.692 F 0.004 434 F 214 47.1 F 24.8 1.678 F 0.017 603 F 224 88.8 F 37.2 1.692 F 0.006 767 F 260 89.2 F 45.6 1.690 F 0.009 1194 F 1230 89.2 F 36.9 1.691 F 0.007

378 F 48 194.7 F 111.8 1.696 F 0.006 318 F 81 177.9 F 52.6 1.696 F 0.004 244 F 66 131.5 F 35.4 1.693 F 0.003 150 F 40 192.7 F 110.1 1.696 F 0.005 195 F 87 392.6 F 246.4 1.701 F 0.006 287 F 339 528.5 F 533.4 1.702 F 0.004

Aitken numberb

Accumulation numberb

2142 F 1641 77.9 F 15.6 1.691 F 0.004 2145 F 2103 103.2 F 28.7 1.694 F 0.008 2089 F 2391 102.1 F 23.4 1.695 F 0.003 8691.5 F 5995 76.1 F 10.1 1.691 F 0.002 967 F 482 92.6 F 48.5 1.692 F 0.006 806 F 425 86.7 F 18.7 1.693 F 0.003 764 F 425 87.9 F 32.3 1.691 F 0.007

932 F 163 88.9 F 22.9 1.686 F 0.005 391 F 357 276.9 F 208.4 1.698 F 0.004 271 F 174 248.9 F 134.4 1.698 F 0.005 1166 F 932 157.5 F 54.1 1.694 F 0.005 248 F 136 312.9 F 172.6 1.701 F 0.003 158 F 76 386.1 F 279.6 1.701 F 0.003 126 F 55 518.2 F 340.4 1.701 F 0.009

N (cm 3), D (geometric in nm), r (standard deviation of the size distribution). Modal parameters are calculated from lognormal fits to the size distribution. F refers to the atmospheric variability. b

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Continental Tropical airmass during the INDOEX (Bates et al., in press; Kamra et al., submitted for publication). Chemical measurements of Quinn et al. (in press) show that 70% (by mass) of the submicron aerosol advecting off the coast of the continent consists of non-sea-salt (nss) sulphate aerosol which includes SO4 , NH4 + and H2O at 55% relative humidity. The additional presence of black carbon and nss potassium indicate a biomass or biofuel combustion source (Ball et al., in press; Quinn et al., in press). On the return journey, although the accumulation mode shows a north-to-south gradient, the Aitken mode particle concentrations are very large but almost uniform from 15jN to 5jS, indicating thereby an additional source for such particles. Surprisingly high concentrations in both the Aitken and the accumulation modes are observed in the 5 – 10jS belt on the return journey. The particles in this latitudinal belt are less aged as compared to those observed either north or south of it except to those observed very close to the Indian coast (the 15– 5jN belt). Keeping in view that the surface wind speeds are not high and the cloud coverage is 3 –6 octa during this period, the high concentrations in the Aitken mode may be associated with mesoscale subsidence accompanied with the deep convection in the ITCZ. These particles can grow to accumulation mode by the cloud processes. In agreement with observations of Covert et al. (1996) over the South Pacific and of Bates et al. (in press) over the South Indian Ocean, the aerosol number distribution, south of the ITCZ, is split between the Aitken and accumulation modes. South of 10jS, the number size distribution is dominated by the Aitken mode, and the accumulation mode consists of the particles which are more aged than those in the ITCZ or north of it. Comparatively higher concentrations in the Aitken and accumulation modes in the 60 to 70jS belt on the onward journey can be associated with the passage of two low-pressure systems during that period. Figs. 6 and 7 show the ratio of the particle number concentration in the nucleation mode (d < 100 nm) to that in the accumulation mode (d>100 nm) for the onward and the return cruises, respectively. The increasing trend of the ratio from North to South on the onward cruise emphasizes the relatively decreasing contribution of the accumulation mode from the polluted air over the northern Indian Ocean to the pristine air in the Southern Hemisphere. Such a trend over the whole latitudinal range of the cruise is not found on the return cruise. However, a large positive gradient of the ratio is noticed on the return journey from 15jN to 8jS. Removal of continental aerosols by wet deposition is less likely because of the dry season which is marked by scattered, shallow nonprecipitating cumulus clouds over the northern Indian Ocean from January to April. The above observation therefore indicates the growth of particles by coagulation and cloud processes and their consequent gravitational settling as the highly polluted continental air moves away from the Indian subcontinent with prevailing northeasterlies. The contribution of the accumulation mode is much higher north of 18jS on the onward cruise and north of 8jN on the return cruise. Although the nucleation mode always dominates, its dominance is particularly very large during the periods of peaks in total aerosol concentration. In the absence of local generators in such regions, particles in the nucleation mode may be either advected and accumulated or may be transported from the free troposphere where the precursor gases for the gas-to-particle conversion processes can be convected up with deep convection in the ITCZ and get distributed horizontally with zonal winds at those altitudes, as suggested by Raes (1995) and supported by Covert’s et al. (1996) experimental data. Homogeneous nucleation of the oxidation products of precursor gases, specially DMS, emitted by the oceans can also contribute to the nucleation mode,

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Fig. 6. Plots of the difference in number concentrations of particles of diameter 42 nm and 24 nm, N * = dn(42)/ dlogDp dn(24)/dlogDp, the ratio of the number concentrations of particles < 100 nm to particles>100 nm, and the sea level pressure along the onward cruise.

particularly in those regions of clean air in the Southern Hemisphere where relative humidity and solar radiation are sufficiently large. These particles can grow to the size of the accumulation mode particles by heterogeneous nucleation or cloud processes (Hoppel and Frick, 1990; Hoppel et al., 1994). The age of aerosol is determined by the processes of coagulation, condensational growth and the cloud droplet nucleation and growth. The concentration of 42 nm particles is relatively stable as it is not subject to rapid changes due to the removal by the scavenging processes or to production by new particle nucleation. Therefore, the difference between the particle concentrations at 42 and 24 nm, dN(42)/dlogDp dN(24)/dlogDp (abbreviated as N*) is also plotted in Figs. 6 and 7. Negative values of N* indicate dominance of the nucleation mode in aerosol which had an ageing time of only a few days or less in the MBL. Positive values of N*, on the other hand, indicate an aerosol of relatively longer ageing time of several days or more and which has remained isolated from the sources of new particles. Although N* is negative through most of the return cruise, it has comparatively much larger negative values north of 30jS as compared to south of 30jS. These values of N* north of 30jS on the return cruise are also much larger than those observed in the same region on the onward cruise. The values of N* south of 30jS except for some large values associated with the low-pressure areas around the Antarctica coast on the onward cruise are almost similar on the onward and the return cruises. Covert et al. (1996) examined the difference between the concentrations of 50 and 20 nm particles. In conformity with our results, they found N* to be

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Fig. 7. The same as in Fig. 6 for the return cruise.

negative from 20jS to 40jS and in the air mass advecting from the continent of North America. However, in contrast to our results, they found N* to be positive in the tropical region from 20jS to 15jN and nearly zero or to vary positively or negatively poleward of 40jS. Thus, the result of Covert et al. (1996) that the aerosols of greatest age are observed in the tropics is confirmed from our observations made in December – January when the northeast monsoon season is about to start. However, the observations made during the northeast monsoon season in March – April show that the aerosols of shortest age prevail between 11jN and 30jS, thereby indicating the transportation of particles from the free troposphere during this season. Over remote oceans, the submicron aerosol are generally composed of sulphate material derived from DMS emissions from ocean. It was believed that the sea-salt aerosol produced by the bursting of bubbles on the sea surface during the periods of high winds contributes only to the coarse mode particles. However, observations of O’Dowd and Smith (1993) over the Northeast Atlantic show that in clean air masses submicron sea-salt aerosol concentration shows a strong increase with wind speed down to a dry particle radius of 50 nm. Our measurements did not include any chemical analysis of aerosols. However, because nss sulphate particles show little correlation with wind speed (O’Dowd and Smith, 1993), we have examined the correlation of total aerosol concentration of the particles of 56 to 103 nm size with wind speed in clean air masses of 48 –60jS. Similar to O’Dowd and Smith (1993), we have divided our data in four ranges of particle diameters, viz. 56 – 100, 100 – 178, 178 – 316 and 316 –1000 nm.

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Fig. 8. Total aerosol concentrations vs. wind speed for different aerosol size ranges on the outward and return journeys.

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Fig. 8 shows the variation of total aerosol concentration in each size range with wind speed. The logarithmic regression fits for wind speed, U, and particle concentrations in four size ranges on the onward and the return cruises are found as follows: CC

SD

Onward cruise logN75 = 1.87 + 0.012U logN130 = 1.93 + 0.017U logN240 = 1.17 + 0.022U logN420 + 750 = 0.75 + 0.011U

0.17 0.23 0.21 0.29

0.26 0.27 0.38 0.13

Return cruise logN75 = 1.93 – 0.011U logN130 = 1.83 + 0.006U logN240 = 1.06 + 0.005U logN420 + 750 = 0.68 + 0.003U

0.15 0.12 0.049 0.073

0.20 0.15 0.31 0.13

Little correlation with correlation coefficients of 0.17 to 0.29 is seen on the onward cruise. However, on the return cruise when wind speeds are always high (between 10 and 20 ms 1) throughout the region, the total aerosol concentration shows almost no correlation with wind speed.

5. Changes in aerosol size distribution associated with the ITCZ From the positions of the ship and the ITCZ, it can be inferred that the ship was within the ITCZ roughly from 0900 to 2100 h on March 30, 1997. Our observations in the ITCZ on the return journey show peak concentrations of up to 2  104 particles cm 3. Fig. 9 shows the aerosol size distributions observed before, during and after the passage of the ship through the ITCZ. The size distributions observed before entering the ITCZ, i.e., from 2100 h on March 29 to 0600 h on March 30, are trimodal. Curves start changing shape at 0600 h, and size distributions from 0900 to 2100 h on March 30 are of the power-law type and show higher concentrations. It is worth noting that in the ITCZ, the concentration of particles in the accumulation mode also increases by one to two orders of magnitude. Size distributions measured after 0000 h on March 31 are again of the trimodal type with much lower values of total particle concentrations. Very high negative values of N* in Fig. 7 within the ITCZ show that the nucleation mode particles have a short ageing time of a few days or less and, thus, most likely have been transported in the MBL from the FT. In the ITCZ, the nucleation mode particles can grow by cloud processes and are transported down by mesoscale subsidence.

6. The extension of continental aerosols over the ocean The extension of continental aerosols to a distance of a few hundreds of kilometers over oceans has often been reported near landmasses (e.g., Hoppel and Frick, 1990; Deshpande

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Fig. 9. Aerosol size distributions on March 29 – 31, 1997 before, during and after the passage of the ship through the ITCZ during the return cruise.

and Kamra, 1995). Such extensions of continental aerosols over ocean while approaching Port Louis, Mauritius and Durban, South Africa from the open sea or departing from there are very well signatured in our measurements on the return journey. Further, our data very

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well demonstrate the effect of the surface wind direction on the extension of air pollutants from the continent to ocean. Fig. 10 shows the changes in the size distribution when the ship was approaching, departing or berthed at Durban from 0530 to 2205 h on March 20, 1997. While approaching Durban, size distributions at 0000 and 0300 h are typical of those observed over oceans. Concentrations in the accumulation mode (130 < d < 1000 nm) and nucleation mode (13 < d < 130 nm) are comparable. The size distributions from 0600 to 1200 h and at 2100 h on March 20 measured at Durban harbor are monomodal and show comparatively higher concentrations of particles of all sizes. The increase in each size range is up to one to two orders of magnitude as compared to the concentrations observed over the open ocean. After departure from Durban, the concentration at 0000 h on March 21, 1997 is still high, but the size distribution becomes bimodal. Aerosol concentrations in each size range decrease and size distributions systematically attain the oceanic bimodal nature at 0300 h on March 21, 1997. Similar results are obtained when approaching or departing from Port Louis, Mauritius on both the onward and the return journeys. While approaching the Indian coastline on the return journey, total aerosol concentration has, on the average, a positive gradient north of the Equator. As discussed earlier, the minimum in size distributions at 422 nm gradually disappears as one moves north of the Equator. One important difference between the observations made while approaching

Fig. 10. Aerosol size distributions while approaching, departing or berthed at (a) Durban during 0530 to 2205 UT on March 20, 1997, and (b) Mauritius during 0815 to 1315 UT on March 27, 1997.

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Fig. 11. NCEP reanalysis plots of surface winds for 0000 UT on (a) December 16, 1996 and (b) March 23, 1997. The ship position is shown by the circle.

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the Indian coastline, on the one hand, and approaching or going away from the coastlines of South Africa and Mauritius, on the other, is that while the transport of continental aerosols is observed to extend even up to the Equator in the former case, the aerosol transport is confined to a distance of only about 200 km in the later cases. This difference can be readily explained in terms of the prevailing wind directions in these areas. For example, the NCEP reanalysis data of surface winds for December 16, 1996 and March 23, 1997 is shown in Fig. 11. The northern Indian Ocean is dominated by the northeasterly winds transporting pollutants from the Indian subcontinent. On the other hand, the areas west of the South African or Mauritius coastlines are dominated by the westerly winds bringing pristine air of the Southern Hemisphere which confine the continental aerosols close to the coastlines.

7. Discussion A ubiquitous feature in most of our measurements, especially over the northern Indian Ocean, the ITCZ and near the coast of Antarctica, is the high concentration of the nucleation mode particles. These particles are rarely found in the background MBL air masses over the open oceans (Hegg et al., 1990, 1993; Clarke, 1993; Hoppel et al., 1994; Covert et al., 1996; Clarke et al., 1998). Concentrations of precursor gases in such regions are generally too low to promote new particle production. Raes (1995) proposed that entrainment from the FT is a source for nucleation mode particles in the MBL. Experimental data of Covert et al. (1996) and recent measurements under the Atlantic Stratocumulus Transition Experiment (ASTEX) (Clarke et al., 1997) and the Aerosol Characterization Experiment (ACE) (Raes et al., 2000; Bates et al., 2000; Vab Dingenen et al., 2000) also show that the nucleation or ultrafine mode is generally found only in those regions where new particles are mixed down to the lower MBL from the FT. The northern Indian Ocean experiences large-scale subsidence during the northern winter as the winter Hadley cell in this region extends from 30jN to 10jS (e.g., Newell et al., 1972). The downward motion in this region may be accompanied with the transport of the nucleation mode and ultrafine particles formed by the gas-to-particle processes in the FT in the outflow regions of convective cells. These new particles grow by condensation and coagulation in the large-scale Hadley circulation and are eventually brought down to the MBL (Covert et al., 1996). In the MBL, these particles grow by homogeneous and heterogeneous processes. Hoppel et al. (1994) shows that most of the particles in the accumulation mode are produced by cloud processes that convert nucleation mode particles to significantly larger sizes by cloud processes. These particles are also removed by coagulation, cloud droplet nucleation and precipitation scavenging processes. The relative efficiencies of the formation and removal processes determine the concentrations of the nucleation and accumulation mode particles. Superimposed on this is the large concentration of continental aerosols being transported from North to South in this region. This may explain the large concentrations and variability of the nucleation mode particles in this region. The differences observed in our measurements during the onward journey in December – January and the return journey in March – April can be explained on the basis of the seasonal difference of airflow in this region. In the month of December, the northeast

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monsoon has just set in and the surface winds over the Indian Ocean are weak. As a result, there is not much of persistent transport of pollutants from the continent to the Indian Ocean before this period and the area is relatively clean. However, during the period of return journey in March –April, the area has already experienced persistent northeasterly winds transporting continental pollutants to the Indian Ocean region for a few months of the northeast monsoon season and, therefore, the area is already significantly polluted. Thus, during the January to April period, the environment over the Indian Ocean undergoes a transition from a relatively clean to a fairly polluted one. The results of model simulation study of sulphate aerosol transport by Pham et al. (1996), which are generally consistent with the Advanced Very High Resolution Radiometer (AVHRR) retrieved aerosol optical depths, also show little transport of sulphate aerosol over the Indian Ocean in January and a significant transport by April. Our measurements of size distribution of submicron aerosols offer the first data set over such a large latitudinal range over these longitudes to study the changes in size distribution as aerosols travel southward from the Indian subcontinent. A north – south gradient of aerosol concentration observed over the northern Indian Ocean strongly supports the continent-to-ocean transport of aerosols. This polluted air of the Northern Hemisphere meets and mixes with the pristine air of the Southern Hemisphere in the ITCZ. As the ITCZ is located south of the Equator in this season, the polluted air from the Northern Hemisphere penetrates well into the Southern Hemisphere and carries its pollutants up to the southern limit of the ITCZ. These pollutants may be convected up with the deep convective activity in the ITCZ and get distributed over larger area with zonal winds at higher altitudes. During the periods of no or weak convective activity in the ITCZ, the pollutants carried by winds from the Northern Hemisphere may accumulate in the ITCZ at lower altitudes. Now, there is strong evidence of interhemispheric transport effected by eddies that wrap around the ITCZ (Krishnamurti et al., 1998). These eddies transport the clean pristine air of the Southern Hemisphere to the Indian Ocean in the Northern Hemisphere and the polluted continental air to the Southern Hemisphere. Thus, these eddies may cause accumulation of air pollutants in some regions and form some pockets of high aerosol concentrations. Observation of some peaks in aerosol concentration in the ITCZ in our measurements supports the formation of such pockets of very high aerosol concentrations. Observation of such a peak in total aerosol concentration at about 5jS, which lies within the ITCZ in the months of March – April, has also been reported by Rhoads et al. (1997). The period from January to April is the dry season over the northern Indian Ocean region. Thus, the wet removal of continental aerosols over this region of Indian Ocean is less likely. This may also contribute to the large aerosol concentrations over the northern Indian Ocean. The northeastern winds over the Indian Ocean transport with them not only the aerosol particles, but also the trace gases from the landmasses of India and Arabia (Rhoads et al., 1997; Naja et al., 1999; de Gouw et al., 2001). In addition, some trace gases are also generated over the sea. With the abundance of solar radiation available in the tropics, the production of new particles by the gas-to-particle conversion processes is expected to be quite large in this area. Our observations of very large concentrations of small particles in the nucleation mode support such a hypothesis. However, these large concentrations can

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also be attributed to an insertion of dust, which has been transported over a long distance with favorable winds from the arid regions of continent. Recent measurements of Krishnamurti et al. (1998) during the INDOEX experiment support the transport of dust from Arabia to the Indian Ocean from the back trajectory analysis of airmass. The transportation of continental aerosols and trace gases beyond the southern limit of the ITCZ is almost negligible. As a consequence, the size distribution curve for the 25 – 20jS belt in Fig. 5 shows comparatively less concentration of the particles in the nucleation mode. However, the gas-to-particle conversion of photooxidation products of the dimethyl sulphide (DMS) emitted by the oceans is a source of the submicron particles which, over the remote oceans, primarily consist of non-sea-salt sulphate particles. The nucleation mode particles generated in this process can grow by condensation of trace gases and the cloud processes in nonraining clouds (Hoppel et al., 1985, 1986). Strong surface winds in the roaring forties generate large number of particles by the breaking waves on the sea surface. Although wavebreaking and bursting of whitecap bubbles dominantly contribute to the coarse mode, O’Dowd and Smith (1993) have shown that this process can generate aerosol down to sizes of 50-nm radius. The curves for the 40– 30jS belt in Fig. 5, show not only large concentrations of particles in each size range, but also the change in their size distributions from trimodal to bimodal with comparatively much higher concentrations of ‘large’ particles. The region of the 60j –40jS belt is dominated with the mid-latitude westerlies and is almost free from the continental air, and the wind direction is persistent for several days. Thus, total aerosol concentration is comparatively steady and the size distribution is trimodal. The region of the Antarctic water (south of 60jS) is often frequented by lowpressure systems circulating around the Antarctic continent associated with the Antarctic Convergence Zone. The peaks in the aerosol concentration observed in this region may be due to the gas-to-particle conversion of precursor gases under the strong westerly winds. Direct production of sea-salt particles from the sea surface is an additional source of the accumulation and coarse mode particles (O’Dowd and Smith, 1993; Bates et al., 1998). Enhanced concentrations of the particles in these modes in the Southern Hemisphere, especially in the roaring forties and fifties and during the weather fronts, are likely to have a contribution from this source.

Acknowledgements The authors express their gratitude to the Department of Ocean Development for supporting these studies at Antarctica. The meteorological data provided by the India Meteorological Department is thankfully acknowledged.

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