Indian Journal of Marine Sciences Vol. 33(1), March 2004, pp. 56-64
Atmospheric deposition fluxes of 7Be, 210Pb and chemical species to the Arabian Sea and Bay of Bengal R Rengarajan* & M M Sarin Planetary & Geosciences Division, Physical Research Laboratory, Navrangpura, Ahmedabad 380 009, India *[E-mail:
[email protected]] Received 3 November 2003 Aerosol samples collected close to the air-sea interface, between February 1997 to February 1999, over the Arabian Sea and Bay of Bengal were analyzed to determine the atmospheric dry-deposition of Fe (dust inputs), anthropogenic constituents (NO3-, SO42-) and environmental nuclides (7Be, 210Pb). In general, aerosol 210Pb concentrations showed a good correlation with 7Be, suggesting the long-range transport of 210Pb from the continents (via upper troposphere) and similarities in the processes governing their deposition through the marine boundary layer (MBL). The relatively low 7 Be/210Pb ratios observed over the Bay of Bengal, during February 1999, are dominated by aerosol transport from the continental surface sources. The dry deposition fluxes of 210Pb and 7Be, to these two oceanic regions, average around 245 and 1860 Bq m-2 y-1, respectively. The non-sea-salt (nss) SO42- (range: 1.7 to 9.4 µg m-3) and NO3- (range: 0.6 to 4.1 µg m-3) are uncorrelated in the MBL, presumably because continental pollution sources for SO42- overwhelm the transport of NO3from crustal dust and biomass burning. The oceanic biogenic emission (DMS) constitutes a very minor source for nss-SO42-. The enhanced concentrations of aerosol NO3- and Fe observed over the Arabian Sea are attributed to dust storm activities from the adjacent desert areas (Saudi Arabia and Thar). Significant scatter in the linear regression analyses indicate that the aerosol composition along the coastal tracks is different from those transported to the open ocean. On average, dry deposition fluxes of N-NO3 and non-marine SO42- are 150 and 1225 mg m-2 y-1, respectively. In contrast, dry deposition of Fe over the Arabian Sea (255 mg m-2 y-1) far exceeds that over the Bay of Bengal (93 mg m-2 y-1). Thus, dust from desert regions appears to be a potential source of micronutrients (Fe) to the surface Arabian Sea. [Key words: Arabian Sea, Bay of Bengal, aerosol composition, atmospheric deposition fluxes chemical species, Be, Pb, nuclides] [IPC Code: Int.Cl.7 C09K3/30]
1. Introduction The concentrations of trace species such as NO3-, SO42- and Fe in the marine boundary layer (MBL) are highly variable depending on the relative contribution from major sources namely oceanic aerosols, continental dust, active volcanoes, biomass burning and anthropogenic emissions. Their transport and deposition processes are episodic and are related to emission patterns and meteorological conditions1. Nitrogen is usually considered as the limiting nutrient in the ocean. In addition, micronutrients such as Fe also play a vital role in regulating phytoplankton growth2-5. In the present day scenario, increased fluxes of these species to oceanic surface waters can have major impact on the fertilization of marine ecosystem, changing the radiative balance and climate forcing and reduced visibility due to scattering of sunlight6,7. During the past two decades, there has been a considerable interest to assess the atmospheric deposition fluxes of chemical constituents to the ocean surface and their role in contributing new nutrients to the surface waters8-11. More recently,
Lelieveld et al.7 have reported on the enhanced pollution levels over the northern Indian Ocean (toward the Intertropical Convergence Zone at about 6°S) during the period of January to March 1999 (winter monsoon). Atmospheric particulate transport of chemical species and their deposition to the surface ocean can be evaluated by simultaneous measurement of the two environmental radionuclides, 7Be and 210Pb, associated with the ambient aerosol particles10,12-15. 7 Be is a cosmogenic radionuclide produced in the atmosphere by spallation of oxygen and nitrogen by high-energy cosmic rays. About 75% of 7Be is produced in the lower stratosphere and 25% in the upper troposphere16,17. It is removed from the atmosphere by radioactive decay (half-life = 53.3 d) and by wet and dry deposition. The mean tropospheric residence time of 7Be is estimated to be between 22 and 48 days18. In contrast, airborne 210Pb (half-life = 22.3 years) is formed in the atmosphere from the radioactive decay of 222Rn, a decay product of 238U, continuously emanating from surface soils. As a
RENGARAJAN & SARIN: FLUXES OF Be, Pb AND CHEMICAL SPECIES
result, relatively high 210Pb concentration is found in continental air masses while its oceanic source is insignificant. Thus, 210Pb serves as an ideal tracer for studying the transport and deposition of continental aerosols to the surface ocean. Soon after their production, both these nuclides get attached to ambient aerosols, and their relative concentrations provide information on their transport from the continental versus oceanic and/or upper versus lower tropospheric sources. This study reports the dry deposition fluxes of N - NO3-, non-marine SO42- and Fe along with the simultaneous measurements of 7Be and 210Pb in bulk- aerosol samples collected over the Arabian Sea and the Bay of Bengal. Our main focus is to investigate the relationships among the aerosol constituents; as such studies over these tropical oceanic regions are sparse. Furthermore, our choice of sampling during winter season (February-March) provides ideal opportunity to study the atmospheric transport of a wide variety of continental aerosols (mineral dust, sulphate, soot and products of biomass burning). 2. Materials And Methods Bulk-aerosol samples from the MBL were collected using high-volume air samplers during four cruises, between February 1997 and February 1999, onboard ORV Sagar Kanya [SK] and FORV Sagar Sampada
57
[SS]. The sampling transects, located between 5°N and 20°N, are shown in Fig. 1; each track represents a time- integrated sample. During SK121 cruise, nine samples were collected at 21°N 64°E (Fig. 1) over several days while the ship was occupying a station for the deployment of free-floating sediment traps. These collections were made under favourable wind conditions to avoid contamination. Air was continuously filtered through 25 cm×20 cm Whatman® glass microfibre filters at the flow rate of 1.2 m3 min-1. During SS164 and SS172 cruises, Gelman® tissuequartz fibre filters were used. On average, filters were replaced after filtering ~2000 m3 of air; sealed in clean polyethylene bags and shipped to the laboratory for chemical analysis. In order to avoid contamination from ship’s exhaust, sampling was carried out while cruising between the stations and under favourable wind direction. In the laboratory, one-fourth of each filter was packed into a plastic vial for non-destructive analyses of 210Pb and 7Be on a HPGe well detector (Canberra Model GCW 2523) coupled to a multi-channel analyzer (Canberra Series 35+). The detector had an active volume of 120 cc; with well depth and diameter of 40 mm and 16 mm respectively. A 15 cm thick lead shielding was used to reduce the background of the detector. The gamma energies at 46.5 keV (Intensity = 4.05%) and 477.7 keV (Intensity =
Fig. 1—Cruise tracks for aerosol sampling across the Arabian Sea and Bay of Bengal during the ORV Sagar Kanya and FORV Sagar Sampada cruises. Each track represents a time-integrated sample for 8-10 hours. Samples 1-9: Cruise SK121 (February 1997); Sampling tracks 10 to 17: Cruise SS164 (March-April 1998); 18-23: Cruise SS152 (February 1997) and 24-29: Cruise SS172 (February 1999).
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10.3%) were monitored for 210Pb and 7Be respectively. The detector-calibration for 7Be was achieved from a standard source prepared by loading a known amount of 238U solution (in equilibrium with the daughter nuclides) onto a glass fibre filter in order to match the sample matrix and geometry. The detection efficiency for the photon peak at 477.7 keV was determined by interpolating those of 214Pb (295 and 352 keV) and 214Bi (609 keV). 7Be activities in the samples were corrected for decay to the time of collection. For 210Pb, the detector efficiency was also ascertained from the above standard source in which the 210Pb activity was precisely measured, via 210Po, by alpha spectrometry. In addition, several samples counted on HPGe detector were re-analyzed for 210Pb by alpha spectrometry; an excellent agreement (±10%) between these two techniques was observed. Another half-portion of the filter was soaked in 30 ml of Milli-Q water (resistivity: 18.2 MΩ cm-1) for 8-10 hours to extract water-soluble ions10. The extraction was repeated with three separate aliquots of 30 ml. The leaches were combined and the total volume was reduced to 50 ml in a Nalgene® polypropylene volumetric flask. The concentrations of Cl-, NO3- and SO42- were determined by Ion Chromatograph (Dionex Model 2000i/SP) interfaced with a peak integrator (Spectraphysics Model 4290). The calibration was done using multi-anion working standards (Cl-, NO3- range: 0 to 10 µg ml-1; SO42range: 0 to 5 µg ml-1) prepared in Milli-Q water by suitable dilution of individual stock solutions. Standards were spiked with concentrated Na2CO3/NaHCO3 solution to closely match the eluent conductivity and to eliminate the interference of water dip appearing near Cl- peak. The filter blanks for NO3were less than 1 µg/filter and for Cl- and SO42- they were 15 and 10 µg/filter respectively. The overall error (± 1σ) in the concentrations of anions was ± 5% and includes uncertainties arising from calibration and reproducibility of measurements. After the water extraction, remaining filter residue was treated with 4N quartz distilled HCl at 60° C. After ~12 hr, the acid extract was centrifuged, the clear solution was diluted to a final volume of 50 ml and Fe concentration was measured by flame AAS (Perkin-Elmer Model 4000). The acid soluble component of Fe, as extracted above, represents an upper limit on the fraction of the metal that is readily soluble in seawater 19.
3. Results and Discussion The calculation of non-sea-salt (nss) components of chemical species is essential for identifying and quantifying processes that influence the chemistry of marine aerosols20. In this study, nss-SO42- was calculated as: nss-SO42- = [total SO42-]−[0.140 × Cl-]
… (1)
where 0.140 being the mass ratio of SO42-/Cl- in surface seawater. The use of chloride as an index to derive the sea-salt contribution21 could result in overestimating the nss-SO42-. This has led to the use of Na and/or Mg as proxies to derive sea salt component of aerosols. This approach, however, could not be adopted in this study because of our choice to use the glass or quartz fibre filters that contribute higher blank levels of Na. Nevertheless, sea-salt correction for NO3-, Fe, 7Be and 210Pb is insignificant 10. For nss-SO42-, sea salt contribution was less than 20% of the total SO42-; except one sample in which the correction was as much as 36%. The concentrations of NO3-, nss-SO42-, Fe, 210Pb and 7Be in the aerosol samples are presented in Table 1. 7Be concentration varied between 2.4 to 14.5 mBq m-3 whereas that of 210Pb varied between 0.27 to 1.68 mBq m-3. The 210Pb values are similar to those measured over the Arabian Sea10 during winter of 1994/1995. The reported 22 mean annual 210Pb concentration in surface air over the continents, between 0° - 30°N latitude, is 0.56 mBq m-3. The range of mean 210Pb concentration in air at the North Pacific SEAREX network sites (5° - 20°N) has been reported 23 to be 0.13 to 0.20 mBq m-3. These concentrations have been related to their source from Asian dust. The elevated levels of 210Pb in this study could arise due to proximity of the continental sources. The activity ratio of 7Be/210Pb ranged between 2.6 and 17.7; with relatively higher values in the Arabian Sea during February 1997 and lowest ratios in the eastern transect (Sample Nos. 25-28) of the Bay of Bengal during February 1999 (Table 1, Fig. 1). The pattern of lower 7Be levels, during the latter sampling period (Table 1), is attributed to relatively low intrusion of upper tropospheric 7Be and predominant transport of aerosol constituents from the continental surface sources. For the data from the three cruises, SK121, SS164 and SS152 (Fig. 1, Table 1), a significant correlation (n=22, r=0.61) between the concentrations of 7Be and 210 Pb is observable in the MBL over the Arabian Sea
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RENGARAJAN & SARIN: FLUXES OF Be, Pb AND CHEMICAL SPECIES Table 1— Aerosol NO3-, nss-SO42-, Fe, 210Pb and 7Be concentrations over the Arabian Sea and the Bay of Bengal Code
Volume (m3)
NO3
-
µg m-3 nss-SO42-
SK121 (February 7-26, 1997)$ 1 630 4.1 2 975 1.8 3 1509 2.2 4 1089 1.6 5 1865 0.6 6 1535 2.2 7 1735 0.6 8 1377 1.8 9 1014 2.2 SS164 (March 20 - April 16, 1998) 10 3194.6 1.0 11 2559.6 1.9 12 1987.9 2.0 13 3029.0 0.8 14 5571.8 1.3 15 4386.2 0.6 16 3435.0 1.4 17 3780.7 0.6
ng m-3 Fe
(mBq m-3)
210
Pb
7
Be
Be/210Pb (A.R.)
7
Arabian Sea 7.2 3.2 9.4 6.5 6.0 5.2 1.7 3.6 3.6
-* -
0.98±0.13 0.56±0.07 0.57±0.06 0.42±0.06 0.58±0.06 0.72±0.08 0.27±0.04 0.82±0.08 0.76±0.09
14.5±0.3 7.3±0.1 8.4±0.1 7.4±0.1 8.9±0.1 7.8±0.2 3.1±0.1 6.9±0.1 7.8±0.2
14.8±2.0 13.0±1.6 14.8±1.6 17.7±2.6 15.3±1.6 10.8±1.2 11.3±1.7 8.4±0.8 10.3±1.2
3.9 4.0 4.0 2.1 3.8 3.8 3.2 3.3
837 2173 619 557 352 1175 567
0.97±0.11 0.82±0.09 0.71±0.08 0.90±0.10 1.05±0.11 0.93±0.10 0.89±0.10 0.79±0.09
8.7±0.2 11.4±0.1 4.0±0.1 7.1±0.1 7.7±0.1 6.7±0.1 9.8±0.1 7.7±0.1
9.0±1.0 13.9±1.5 5.6±0.6 7.9±0.9 7.4±0.8 7.2±0.8 11.1±1.3 9.7±1.1
Bay of Bengal SS152 (February 14-27, 1997) 18 2079.0 3.2 5.8 1.06±0.10 8.8±0.1 19 4554.0 2.0 5.1 1.19±0.10 11.8±0.1 20 2326.5 0.6 1.7 0.53±0.05 3.5±0.1 21 3539.6 0.9 1.8 0.59±0.05 3.7±0.1 22 2282.2 1.5 4.9 0.70±0.06 5.4±0.1 23 2040.7 0.7 2.7 0.88±0.08 8.8±0.2 SS172 (February 2-28, 1999) 24 3068.4 2.9 7.1 520 1.68±0.18 6.6±0.2 25 2125.2 1.2 2.1 323 0.97±0.11 3.9±0.1 26 2699.4 1.2 3.0 360 0.92±0.10 3.0±0.1 27 3336.6 1.6 2.9 214 0.73±0.08 2.4±0.1 28 2416.2 1.5 7.0 260 1.08±0.12 2.8±0.1 29 2873.4 1.9 8.0 280 1.19±0.13 5.2±0.1 *Fe concentrations were not measured on glass fibre filters used during SK121 and SS152 cruises. $ NO3-, nss-SO42- and 210Pb data are from Sarin et al.10.
and Bay of Bengal (Fig. 2). Such a trend is suggestive of a long-range transport of 210Pb via upper troposphere and its subsequent removal by ambient aerosols similar to those of 7Be. Data for SS172 cruise (Table 1), however, follows a distinctly different trend (as stated above) and is not presented in Fig. 2. A similar correlation had been reported for the atmospheric deposition fluxes of these two nuclides over the Sargasso Sea 14. Dry-deposition fluxes (Fd) of 7Be and 210Pb were calculated by multiplying their geometric mean concentrations in air with effective settling velocity (Vd): Fd = Ci × Vd
… (2)
8.3±0.8 9.9±0.8 6.6±0.6 6.2±0.6 7.7±0.7 10.0±0.9 4.0±0.4 4.0±0.5 3.3±0.4 3.3±0.4 2.6±0.3 4.3±0.5
Fig. 2—Co-variation of 7Be and 210Pb in aerosol samples (n=22). Data for SS172 cruise samples with distinctly lower 7Be/210Pb activity ratios (Table 2) are not plotted. The average 7Be/210Pb activity ratio of 7.5 for 5°-20°N latitude belt is lower by a factor of 2 compared to mid-latitude (30°-40°N) oceanic regions.
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The term Vd varies with particles size (from gravitation settling of large particles to impaction and diffusion of small particles) and is dependent on climatological and physical conditions in the troposphere, especially in the coastal environment24,25. It is important to emphasize that the dry deposition fluxes may vary by an order of magnitude due to uncertainties in Vd. Based on the annual mean 210Pb concentration of 25×10-3 dpm m-3 in the marine aerosols and its measured flux of 20 dpm m-2 d-1 (using free-floating sediment traps at 130-150 m below the surface in the Arabian Sea), Sarin et al.10 had obtained an “effective deposition velocity” (Vd) of 210Pb aerosols to be ~0.9 cm s-1. Using Vd as 0.9 cm s-1, the average dry deposition fluxes of 7Be and 210Pb over the Arabian Sea are 2155 and 215 Bq m-2 y-1 respectively; whereas the corresponding fluxes over the Bay of Bengal are 1560 and 275 Bq m-2y-1 (Table 2). It is implicit in these flux calculations that both 7Be and 210Pb reside on the similar size aerosols and that their removal mechanism is also similar. It is also relevant to emphasize that the concentrations of 7 Be and 210Pb during February-March (Table 1) are representative of the whole year. Earlier study10 had shown that 210Pb concentration in aerosol is high during winter compared to that in summer and monsoon. Therefore dry-deposition fluxes (Table 2) calculated using the winter data are an upper limit. Lal et al.27 had reported the wet deposition 210Pb flux, based on measurements in rains over sampling stations in India, ranging from 183 to 317 Bq m-2y-1 and a mean 7Be flux of 1267 ± 417 Bq m-2y-1. The annual 210Pb atmospheric flux on continents22 between 10° - 30°N latitude is 160 Bq m-2 y-1. A comparison of atmospheric dry-deposition fluxes of these
environmental nuclides among the relevant sites is presented in Table 2. In this study, we compute a mean 210Pb flux of 245 Bq m-2y-1 over the latitudinal belt of 5°-20°N. The mean flux measured for 210Pb over the Arabian Sea and the Bay of Bengal is significantly higher than those reported from the other oceanic sites (Table 2). These regional differences could arise due to increased transport of 210Pb from the continental sources, proximity to land and differences in settling velocities (0.1 to 0.3 cm s-1) used in the calculations. The NO3- concentration ranged from 0.6 to 4.1 µg m-3 and that of nss-SO42- from 1.7 and 9.4 µg m-3 (Table 1); with significant differences in the concentrations at the coastal versus open ocean tracks (Fig. 1). Their mean concentrations in the MBL of the Arabian Sea and Bay of Bengal for individual cruises are summarized in Table 3. Typical time-series measurements of 7Be, 210Pb, NO3- and nss-SO42during the SK121 cruise in the Arabian Sea are shown in the Fig. 3. In general, aerosol NO3- concentrations show an increasing trend with 210Pb for the individual cruise samples (Fig. 4A), providing strong evidence that NO3- is transported from the continental sources to the MBL by the prevailing wind fields during winter months (February-March). However, scatter in the data (Fig. 4A) represents the differences in the transport patterns of the continental aerosol species; 210 Pb is principally derived from dust sources while biomass burning could represent a major source28 of NO3-. The pattern of high NO3- values in the SK121 cruise (Table 1, Fig. 4A) could additionally be associated with episodes of high dust transport from the desert region (Saudi Arabia and Thar) to the Arabian Sea. The time-series variations in Fig. 3,
Table 2—Comparison of annual atmospheric dry-deposition fluxes of 7Be and 210Pb Site/location Arabian Sea (5-20°N) Bay of Bengal (5-20°N) Arabian Sea Versoix (46°N) Oahu (21°N) and Enewetak (11°N)£
Dry deposition flux (Bq m-2y-1) 7 210 Be Pb 2155 1560 -
215 275 120$
2087±23 1000-1200*
150±3 30-50
Reference Present study@ Present study@ Sarin et al.10 Caillet et al.26
Uematsu et al.12; Turekian et al.23 Bermuda (33°N) 1483* 68* Kim et al.14, 15 Mid-Atlantic Bight (40°N) 2167* 130* Kim et al.14, 15 @ Based on the atmospheric concentration during the period of February-March. $ Represents measured flux of 210Pb based on free-floating shallow sediment traps. £ Pacific Island stations. *Total flux (wet+dry).
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RENGARAJAN & SARIN: FLUXES OF Be, Pb AND CHEMICAL SPECIES Table 3—Mean concentrations and annual fluxes of NO3-, nss-SO42-, Fe, 7Be and 210Pb over the Arabian Sea and the Bay of Bengal Cruise #
Arabian Sea SK121 SS164 Mean Bay of Bengal SS152 SS172 Mean
na
NO3Conc. Flux µg m-3 mg m-2y-1
nss-SO42Conc. Flux µg m-3 mg m-2 y-1
7
Fe Conc. ng m-3
210
Be
Conc. Flux Flux Conc. mg m-2 y-1 mBq m-3 Bq m-2y-1 mBq m-3
Be/210Pb (A.R.)
7
Pb Flux Bq m-2y-1
9 8
1.9 1.2 1.6
180 113 150
5.2 3.5 4.4
897
255
7.3 7.9 7.6
2070 2240 2155
0.63 0.88 0.76
180 250 215
12.9 9.0 11.0
6 6
1.5 1.7 1.6
142 161 150
3.7 5.0 4.4
326
93
7.0 4.0 5.5
1985 1134 1560
0.83 1.10 0.97
235 312 275
8.1 3.6 5.9
1225b
-Not measured. a n denotes number of samples. b Represents anthropogenic SO42- flux over the Arabian Sea and Bay of Bengal; computed based on the SO42-/210Pb ratio in the aerosols as ~5 µg mBq-1 (corrected for sea salts and DMS source) and the mean annual 210Pb flux of 245 Bq m-2 y-1.
Fig. 3—Daily-integrated aerosol samples analyzed for continentderived constituents (NO3-, SO42- and 210Pb) and stratospheric produced nuclide 7Be. Relatively high concentrations observed for all constituents on 7th day of February 1997 are associated with high winds and episodic dust events.
coupled with the wind trajectories originating in desert regions further suggest that episodic continental source for NO3- can account for larger scatter in the data presented in Fig. 4A. In Eq. 2, using Vd = 0.3 cm s-1 (ref. 29) and average NO3concentration of 1.6 µg m-3 (Table 3), we have calculated dry deposition flux of NO3- to be ~150 mg m-2 y-1 (0.4 mg m-2 d-1) during February-March; quite comparable to the flux of 0.6 mg m-2 d-1 derived by Sarin et al. 10 for the Arabian Sea based on the two year (1995 and 1997) data for the same season (Table 1). Alternately, based on the annual average NO3-/210Pb ratio and the measured 210Pb deposition flux, Sarin et al. 10 had reported a value of 0.9 mg m-2 d-1 for the NO3- deposition flux to the Arabian Sea for the sampling sites between 11° and 21°N. The NO3- to nss-SO42- mass ratios of the aerosol samples over the Arabian Sea and Bay of Bengal varied from 0.1 to 0.6; significantly lower than the reported28 value, 1.44±0.19, for the AROCE station at Barbados in the Tropical North Atlantic. African biomass burning was suggested to be a significant source of high NO3- relative to nss-SO42- in the MBL at Barbados. In this study, the seasonal and spatial variations in the nss-SO42- concentrations (range: 1.7 to 9.4 µg m-3, Table 1) and observed NO3- : nss-SO42mass ratios (Fig. 4B) suggest that the transport of SO42-, to these two oceanic regions, from the continental sources overwhelms that of NO3- (derived from continental dust and biomass burning sources). The relatively high NO3- : nss-SO42- ratios (Fig. 4B) are related to the occurrence of episodic high dust events during SK121 cruise (Table 1). This phenomenon also appears to affect the relationship between NO3- and 210Pb as explained above. The large
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nss-SO42- concentration of ~0.5 µg m-3, in the clean MBL south of the ITCZ, could be attributed to oceanic source of DMS 7. Such an observation suggests that non-sea-salt marine biogenic source of SO42- makes no more than a minor contribution to the measured nss-SO42- concentrations (Table 1). After correcting for the marine biogenic component of SO42from mean values of the aerosol constituents (Table 3) during individual cruises for which both nss-SO42- and 210Pb were measured in the same samples; the non-marine SO42-/210Pb ratios range between 4 to 5 µg mBq-1. These ratios for the Arabian Sea and Bay of Bengal, measured during winter season, are significantly higher than those reported for some of the SEAREX sites 23 (0.8 to 2.3 µg mBq-1); attesting to our earlier contention that anthropogenic sources dominate the aerosol SO42- characteristics. Based on the SO42-/210Pb ratio of the continental aerosols as ~5 µg mBq-1 and the 210Pb flux computed in this study (Tables 2 and 3), it is possible to derive non-marine SO42- flux across the air-sea interface. By using such an approach, the average deposition flux of non-marine SO42- (anthropogenic) to the Arabian Sea and Bay of Bengal during winter months is 3.4 mg m-2 d-1; significantly higher than that for the continentally derived NO3- flux in this study, (0.4 mg m-2 d-1).
Fig. 4—Scatter plots: (a) NO3- concentrations show a general increasing trend with continental tracer 210Pb for the samples from same cruises; (b) NO3- and nss-SO42- are uncorrelated in the MBL; NO3-/SO42- mass ratio varies between 0.1 to 0.6; (c) The continental source of 210Pb helps to characterize the transport of anthropogenic SO42- to the MBL as evident from their covariation. The average nss-SO42-/210Pb ratio (corrected for DMS source) centers around 5 µg mBq-1; significantly higher than that observed over the SEAREX network sites.
scatter for the data in Fig. 4B also reflects that the ionic ratios in aerosol particles sampled along the coastal tracks (Fig. 1) are significantly different than those in the interior of the Arabian Sea and Bay of Bengal. The well-defined continental source of 210Pb to the atmosphere helps in tracking the sources of nss-SO42-, viz. biogenic (DMS origin) vis-à-vis anthropogenic (Fig. 4C). The data from Indian Ocean Experiment (INDOEX) have documented that aerosol
The air-sea deposition of Fe has been the focus of increased debate and interest as Fe deficiency can limit phytoplankton productivity under certain conditions 2. In this study, we have attempted to characterize the principal source of atmospheric particulate transport of Fe and its deposition to the surface Arabian Sea and Bay of Bengal. The mean values of Fe concentrations in the MBL at the Arabian Sea are about three times higher than those over the Bay of Bengal (Tables 1 and 3). This pattern can be accounted for by enhanced transport of mineral aerosols, from the desert areas, to the Arabian Sea. Dust storm activities from desert regions are enhanced during February-March due to the combined effects of low rainfall and increased occurrence of high winds associated with cold fronts30. By using the deposition velocity of mineral aerosols10,29 as 0.9 cm s-1 and mean values of Fe content in aerosols (Table 3), the dry deposition fluxes of Fe to the Arabian Sea and the Bay of Bengal are derived as 255 and 93 mg m-2 y-1 respectively (Table 3). Herut et al.25 had determined dry deposition flux of Fe at the Mediterranean coast of Israel to be ~450 mg m-2 y-1. The reported annual fluxes of Fe are considerably lower at Bermuda and
RENGARAJAN & SARIN: FLUXES OF Be, Pb AND CHEMICAL SPECIES
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4. Conclusion It has been increasingly recognized that transport pattern of continental substances and pollutants from south and southeast Asia to the Indian Ocean, during January-March, can have profound impact on the biogeochemical cycles of the atmosphere and surface ocean. To quantify the atmospheric dry deposition of selected species (NO3-, SO42- and Fe), their simultaneous measurements along with the two environmental nuclides (7Be and 210Pb) in aerosol samples from the MBL over the Arabian Sea and Bay of Bengal have been carried out. The results obtained have led to the following conclusions:
Fig. 5—Comparison of average-annual dry deposition fluxes computed for the Arabian Sea and Bay of Bengal. Flux of major crustal element, Fe, is nearly three-fold higher over the Arabian Sea relative to that over the Bay of Bengal.
mid-Atlantic coast (≤25 mg m-2 y-1) although the site at Bermuda is characterized by significant transport of Saharan dust 14. The considerably higher deposition of Fe over the Arabian Sea is attributed to its proximity to the continents and the prevailing wind directions. A synoptic view for understanding the observed dry deposition of NO3-, Fe, 7Be and 210Pb across the Arabian Sea and Bay of Bengal can be found in Fig. 5. Two of the continent-derived species, Fe and 210 Pb, show somewhat contrasting deposition trends (Fig. 5), suggesting that mineral aerosols from desert regions to the Arabian Sea are characterized by relatively high Fe/210Pb ratio (1.2 µg mBq-1) than those of finer size entering the Bay of Bengal (Fe/210Pb ≈ 0.3 µg mBq-1). It is also evident from Fig. 5 that the NO3- fluxes are uniform over these two oceanic regions and far exceed those of Fe, thus emphasizing the dominant transport of NO3- from biomass burning and industrial pollutants over the crustal source.
1. A significant correlation between 7Be and 210Pb concentrations has been observed. This suggests that atmospheric transport of 210Pb to the MBL is mediated primarily via upper troposphere. The ratio of 7Be and 210Pb fluxes, though distinctly different over the Arabian Sea and Bay of Bengal, provide a useful means in documenting the longterm dry deposition fluxes of chemical species to the open ocean; which are otherwise difficult to monitor continuously from research vessels. 2. The anthropogenic sources dominate the aerosol SO42- concentration in the MBL; whereas oceanic DMS source makes a minor contribution to the total nss-SO42-. The dry deposition of non-marine SO42- far exceeds that of NO3- transported from biomass burning and crustal dust sources. 3. Episodic high levels of Fe observed over the Arabian Sea are attributed to enhanced transport of mineral aerosols from the desert regions of Saudi Arabia and Thar (Rajasthan); thus characterizing the Arabian Sea as one of the potential oceanic sites for the high atmospheric deposition of Fe. Acknowledgement We thank the crew members of the ORV Sagar Kanya and FORV Sagar Sampada for their valuable help during the Arabian Sea and Bay of Bengal cruises. Funding support provided by the Department of Ocean Development, New Delhi is thankfully acknowledged. References 1 Prospero, J M, Barrett K, Church T, Dentener F, Duce R A, Galloway J N, Levy II H, Moody J & Quinn P, Atmospheric deposition of nutrients to the North Atlantic Basin, Biogeochem, 35 (1996) 27-73.
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