Spatial distribution of aerosol concentrations over the Arabian Sea and the Indian Ocean during IFP of INDOEX

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1 Spatial distribution of aerosol concentrations over the Arabian Sea and the Indian Ocean during IFP of INDOEX K. Parameswaran*, Prabha R. Nair, Rekha Rajan and D. Balasubrahamanyam Space Physics Laboratory, Vikram Sarabhai Space Centre, Thiruvananthapuram , India During the intense field phase of the INDOEX, a high volume sampler was sailed on cruise to collect aerosol samples from different oceanic environments. The variation of aerosol mass concentration (m) with latitude and longitude was studied. The aerosol concentration near the West Coast is ~ 40 µg/m 3, which is almost twice the amount that was observed in this region during FFP-98. This is in agreement with the data obtained from surface measurement at Trivandrum. An enhancement in m was observed ~ 5 N associated with the convergence of air stream coming over Arabian Sea, Bay of Bengal and the eastern parts. During the cruise period, the ITCZ located around 4 S is rather broad and less dynamic. A significant decrease in m was observed in this region. A significant enhancement in m was observed around 20 S during the up leg along the longitudinal sector E. During the return leg an enhancement in m is observed in the northern hemisphere, with increasing latitude (in the Arabian Sea sector). The aerosol loading increases further north of Goa towards the Bombay coast. The overall spatial pattern of m during IFP-99 compares favourably with that observed during FFP-98. *For correspondence. ( spl_vssc@vssc.org) THE Intense Field Phase (IFP-99) of Indian Ocean Experiment (INDOEX) was scheduled during the period January to March The cruise track is organized based on the observations 1 made during the FFP-98. During the cruise period intense aerosol observations are also carried out at different locations in the country to study the characteristics of aerosol system prevailing over the continent. For the present study the aerosol mass concentration (m) obtained using a high volume sampler (HVS) is used to examine the spatial variation in aerosol loading in the oceanic environment. However, at this stage no detailed analysis incorporating data from other experiments is attempted because they are yet to be archived. The present paper deals with the first cut results obtained from HVS sailed on the IFP-99 cruise and interpretations for the observed features on the basis of wind maps provided by National Centre for Medium Range Weather Forecasting Station (NCMRWF). INDOEX cruise and aerosol measurements In connection with the main campaign of the INDOEX programme, the IFP-99 cruise of ORV Sagar Kanya was scheduled for 51 days during the period January to March 1999, when the prevailing tropospheric wind over the continent is northeasterly. The main emphasis of this cruise experiment was to study the aerosol loading near the continent during the winter season, its gradient in the far oceanic regions of Indian Ocean, transport across Inter Tropical Convergence Zone (ITCZ) and to study the characteristics of southern hemispheric oceanic aerosols. To understand the continental influence on the oceanic environments two longitudinal scans, one in the southern hemispheric Indian Ocean south of ITCZ and another in the Arabian Sea, were also planned during this cruise experiment. The cruise track was finalized based on these objectives. The scientific cruise started from Goa on 20 January After sailing along the West Coast of the Indian continent for four days, the vessel parted off the southern tip of the Indian peninsula on 25 January 1999 and entered the Indian Ocean. The ship sailed directly towards the southern hemisphere along 77 E longitude up to 20 S. From this point it had a longitudinal scan from 77 E to 59 E to reach Mauritius around 11 February 1999, first cutting along the longitude to reach ~ 62 E and then taking a direct path along this longitude from 17 S to 15 N to reach the middle Arabian Sea region, where it had another longitudinal cut from 62 E to 72 E (around 15 N latitude). After completing this main phase, the ship reached 14.5 N, 72.3 E, on 5 March 1999 and had a secondary cruise along the West Coast up to 10 N, 74.7 E and back to 17 N, 68.6 E before returning to Goa on 11 March Different aerosol, boundary layer and oceanic experiments were conducted on this cruise and the HVS is one among them. This is a single stage sampler (HANDI-VOL G 2000), which measures the mass loading of total suspended particles (TSPs). The sampling rate of HVS was ~ 20 CFM calibrated in laboratory before taking for the CURRENT SCIENCE (SUPPLEMENT), VOL. 80, 10 APRIL

2 cruise. Quartz fibre substrates were used for collecting the particles. About 60 blank substrates are preconditioned (by first drying in an oven and desiccating) and weighed using a microbalance (sensitivity 2 µg). These blank substrates were taken in self-sealing polythene envelopes. The sampler was located on the ship deck (~ 10 m above the sea level) towards the front end of the ship, so as to sample the marine air from the front, uncontaminated by the ship exhaust. For each sampling, a fresh collection substrate from the polythene envelope was mounted on the sampler and aerosol samples collected for about 6 to 7 h continuously. After completing the collection, the aerosol-laden substrate was immediately transferred to the same envelope and stored in the plastic container. Aerosol samples were collected on almost all the days except during the portcall when it was mandatory to stop all the experiments. About 50 aerosol samples were collected from different regions in the oceanic environment along the cruise track. At the end of the cruise, the aerosol samples were brought to the laboratory and desiccated before weighing. By weighing the aerosol-laden substrates using the same microbalance the mass of aerosol particles collected on each substrate was estimated by subtracting the respective tare weights. The mass concentration (m) was estimated by dividing the mass of aerosols collected by the flow rate of the sampler. Figure 1 shows a plot of aerosol mass concentration measured at different locations along the cruise track. The length of the vertical arrow is proportional to the mass concentration at a particular location where mean position is represented by circles. On an average one sample was collected over a latitude/longitude region of ~ 1.7 (representing approximately 200 km). The corresponding date on which each sample is collected is marked adjacent to the mean position of the ship. The starting point at Goa corresponds to 20 January 1999 and the remaining dates follow. The cruise track can be obtained by joining these points by straight lines. As the ship had a secondary cruise near the Indian continent in March 1999, after completing the main cruise, the points near the continent are highly clustered making it difficult to distinguish. Hence these data are presented in a separate frame. Figure 1 a shows the mass concentrations on the main phase of the cruise from 20 January 1999 to 5 March 1999, which covered two latitudinal and two longitudinal scans. Figure 1 b, which is drawn in an expanded scale, presents the mass concentration measured during the latter part of the campaign, from 3 March 1999 to 11 March Figure 1 a shows the variation of mass concentration along the latitude from 15 N to 20 S and longitude from 62 E to 77 E and Figure 1 b shows the same along the West Coast region. Figure 2 shows the mean vector wind (wind speed and direction) recorded on the ship during the sampling period. Figure 2 a shows the wind vectors during the former part of IFP-99 cruise and Figure 2 b the same during the latter part (near the West Coast). The length of the arrow represents the wind speed and the arrowhead represents the direction. Aerosol data and observation results The aerosol mass concentration near Goa was ~ 40 µg/m 3. As the ship moves south along the coastal belt, m a b Figure 1. A plot of aerosol mass concentration observed during the IFP-99 cruise (a) from 20 January 1999 to 5 March 1999 and (b) during 4 March 1999 to 11 March 1999 when the cruise sailed up and down in the EZ of Indian continent. 162 CURRENT SCIENCE (SUPPLEMENT), VOL. 80, 10 APRIL 2001

3 decreases and reaches a minimum of ~ 19 µg/m 3 around 11.5 N. The aerosol concentration remains more or less uniform till the ship reaches close to equator (from 11.5 N to 0.5 N), except for an increase (to ~ 55 µg/m 3 ) around 5 N between Male and Sri Lanka. The wind speed in this region, in general, is less than 5 m/s and shows large variability from point to point. Figure 2 a shows that on 25 January 1999 when the ship is around 5 N latitude the wind is very strong and is directed from the continent. The wind vector diagram on different days in the IFP-99 cruise region has been provided 2 by NCMRWF. These maps on a few selected days are presented in Figure 3. These wind vectors are at 925 hpa level and the date is indicated above each frame. The first frame on 26 January 1999 shows that air streams originating from the north and advecting over the Arabian Sea and the Bay of Bengal as well as those originating from the east (Indonesia and other continents) reach the cruise track around 5 N. This localized convergence of air streams from different directions causes an enhancement of aerosol concentration in this region. The aerosol concentration at a point just above this location (8.3 N) was only 41 µg/m 3 and showed a decreasing trend from 11 N onwards. In the absence of this convergence, at 5 N latitude the concentration would have decreased below 40 µg/m 3. The convergence caused an additional contribution of ~ 15 µg/m 3 in this region, which might have been contributed by the air stream coming from the east. South of the equator the aerosol mass concentration decreases significantly reaching a minimum (10 µg/m 3 ) around 4 S. Wind field maps in Figure 3 show that the ITCZ during this period is located around this latitude. Significant decrease in m observed in this region was associated with this meteorological phenomenon. This observation is in agreement with that reported 1 during the first field phase of INDOEX in February March In this region the wind vector measured on the ship deck (Figure 2 a) shows a rapid rotation by 90 and is directed across longitude (from west to east). South of ITCZ, m shows a steady increase reaching a value ~ 30 µg/m 3 at (16.5 N, 77 E) on 3 February As shown by the wind map on 2 February 1999 in Figure 3 b, in this region, the cruise track encounters southern hemispheric air along with air stream originating from the Australian continent. Increase in m observed at this location can be attributed to this continental influence. In this context it will be worth examining the results 1 obtained during FFP-98 in which increase in m south of ITCZ was rather small. During FFP-98 the cruise track was significantly shifted (~ 61 E longitude) from the Australian continent. The pronounced enhancement observed in the present cruise south of ITCZ can hence be attributed to the continental influence in southern hemispheric air. The IFP-99 cruise had its first longitudinal scan around 20 S in the southern hemisphere from 77 E to 59 E. From Figures 2 and 3 it can be seen that the winds are very strong and are directed almost from southeast. During the first three samplings of this longitudinal scan, because of the particular nature of the wind direction, the HVS faced a slight contamination of fumes originating from the ship exhaust. The recorded mass concentration during this period was ~ µg/m 3. Because of this contamination these values are discarded for the present study. Even a b Figure 2. A plot of mean wind vector during each sampling presented in Figure 1; (a) from 20 January 1999 to 5 March 1999 and (b) during 4 March 1999 to 11 March 1999, when the cruise made measurement in the West Coast region. CURRENT SCIENCE (SUPPLEMENT), VOL. 80, 10 APRIL

4 after discarding these points one is left with 5 values for m, lying almost at the same latitude in this longitude sector, which shows a decrease from 77 E to 59 E (as the ship reaches Mauritius). It would be worth in this context to note that the wind speeds are significantly large in this region. Strong winds over the ocean surface can lead to localized production of aerosols from the ocean surface (sea spray 3 ). But from Figure 3 it can be seen that though the wind speed remains generally high throughout the longitude sector 77 E to 59 E, m shows a decrease as the ship sails towards Mauritius. The observed longitudinal gradient in this latitude can hence be attributed to the continental influence. The mass concentration near Mauritius on 11 February 1999 was ~ 8 µg/m 3, which is much lower than that observed on 11 March 1998 (17 µg/m 3 ) during 1 FFP-98. This difference may be attributed to the difference in the prevailing meteorological conditions in this region. This value of m is almost same as that observed within the ITCZ. The average mass concentration near to Mauritius (from 10 to 18 February 1999) was ~ 10 µg/m 3. On the return leg of the IFP-99 cruise, as the ship first sails north-east from Mauritius, m shows a small increase and reaches a steady value of ~ 21 µg/m 3 around 63 E longitude. The third frame in Figure 3 shows the wind map for 23 February The ITCZ is located almost at the same region (~ 4 S) throughout the period when the cruise was in the southern hemisphere. Unlike the case of FFP-98, during IFP-99 the ITCZ is rather broad and less dynamic. This feature is quite clear from Figure 3. The value of m within ITCZ (on 22 and 24 February 1999) was ~ 7 µg/m 3. North of ITCZ, m increases steadily with increase in latitude reaching 40 µg/m 3 in the middle Arabian Sea (12.3 N, 61 E). The mass concentration increases from 7 µg/m 3 at ~ 3 S to 40 µg/m 3 ~ 12 N indicating a latitudinal gradient of 2.5 µg/m 3 / lat. This amounts to an e-fold increase along a distance ~ 1100 km. The second longitudinal scan in the northern hemisphere was around 15 N, from 60.5 E to 73 E. The mass concentration increases from 26.8 µg/m 3 at 60.6 E to 42.5 µg/m 3 at 72.3 E (near to Goa) with a longitudinal gradient of 1.3 µg/m 3 / lat. This amounts to an e-fold decrease of m from the coast with a scaling distance of ~ 2500 km. From Figure 1 b it can be seen that m decreases from 14 N to 11 N along the West Coast. The mean wind recorded on cruise (Figure 2 b) shows that they are, in general, strong and directed either from northwest or west, towards the mainland. The wind map in Figure 3 c (for 9 March 1999) shows a clockwise circulating wind system centred on the West Asian region. This wind system can transport aerosol-laden air from West Asian regions (and from the north-westerly part of Central India) towards the West Coast, on to the southern tip of the Indian peninsula, where the wind is rather weak, leading to an Figure 3. Wind maps on a few selected days during IFP CURRENT SCIENCE (SUPPLEMENT), VOL. 80, 10 APRIL 2001

5 accumulation of particles in this region. The mass concentration near the coast (from Figure 1 b) was in the range 33 to 47 µg/m 3, with a minimum around 10 N. North of Goa m increases up to 47 µg/m 3 around 16 N, 71.2 E. Discussion In connection with the INDOEX programme measurements on aerosol, mass concentrations have been carried out at ~ 10 m above the sea surface over the Arabian Sea and the Indian Ocean regions to study the advection of continental aerosols in oceanic environments. Both the FFP-98 and IFP-99 cruises scanned the same latitude region during the winter season. The cruise track for FFP- 98 and IFP-99 was slightly different. The IFP-99 cruise covered a larger area in the same latitude region. The cruise track for IFP-99 was almost rectangular with two latitudinal scans and two longitudinal scans. In addition to this, towards the end of IFP-99 main phase, measurements were carried out near the West Coast. The aerosol data available along a closed loop on this cruise can be used to generate a contour map of aerosol mass concentration within the constraint that the data are available only along the periphery of this rectangular region. Figure 4 shows the contour map generated using the data presented in Figures 1 a and b. This map provides an illustrative picture of aerosol pockets and the nature of advection. A high aerosol concentration was seen near the Indian subcontinent (5 N to 15 N latitude, 72 E to 80 E longitude). Another small high was also observed around the same latitude in the longitude sector 50 E to 60 E. From Figure 3 it can be seen that an anticyclone located over the West- Asian regions, which can disperse the aerosols (divergence). A tentative low is seen around E longitude. The penetration of aerosols across West Coast is confined to longitudes east of this region. Due west of this region, the aerosol loading appears to be contributed by advection from northwest and northwestern regions of Central India. It would be worth in this context to examine the conclusion 1 arrived based on aerosol loading and surface streamlines 4 during FFP-98, which also indicated an advection of arid aerosols in these months from West Asian and northwestern parts of Central India over the Arabian Sea. A depletion of m was observed around 4 S associated with the Inter Tropical Convergence Cell. A significant increase in m was observed around 20 S in the E Figure 4. Contour plot of m during IFP-99 of the Indian Ocean experiments, generated from the measurements shown in Figure 1. longitude region (unexplored during FFP-98), which can be attributed to the advection of continental aerosols from the Australian region. Thus, except for some small deviations, the overall spatial distribution of m during FFP-98 and IFP-99 show large similarities. Aerosol loading observed near the Trivandrum coast during IFP-99 was significantly larger than that observed during FFP-98, which was indicated in surface measurement 5 at Trivandrum also. 1. Parameswaran, K., Prabha R. Nair, Rekha Rajan and Venkata Ramana, M., Curr. Sci., 1999, 76, Iyengar, G. R., Ramesh, K. J., Paliwal, R. K. and Madan, O. P., Structural characteristics of the inter-tropical convergence zone over the equatorial Indian Ocean during the INDOEX-99 field phase experiment, National Workshop on Scientific results from IFP-99, INDOEX-India program, July Patterson, E., Kaing, M., Delany, C. S., Wartburg, A. C., Lesli, A. C. and Huebert, B. J., J. Geophys. Res., 1980, 85, Jha, B. and Krishnamoorthi, T. N., Real time meteorological re-analysis atlas during pre-indoex field phase 1998, FSU Report #98-08, Department of Meteorology, Florida State University, Tallahassee, FL , USA, August Parameswaran, K., Prabha R. Nair and Rekha Rajan, Curr. Sci. (Suppl.), 2001, 80 (this issue). CURRENT SCIENCE (SUPPLEMENT), VOL. 80, 10 APRIL