Chemical characterization of ambient aerosol collected during the southwest monsoon and intermonsoon seasons over the Arabian Sea: Anions and cations

Abstract

Ambient aerosol samples were collected over the northern Indian Ocean during two 1 month-long research cruises (German R/V Meteor) that took place during the intermonsoon (May) and SW monsoon (July/August) of 1995. A high volume and two small volume collectors were used to collect samples, which were subsequently analyzed for ferrous iron, 32 elements, and anions and cations. The present paper focuses on the bulk aerosol material, the ions, while utilizing some of the trace metal data that were presented in more detail in our previous paper [Siefert et al., 1999]. Data are analyzed and interpreted with the aid of principal component and multiple linear regression analyses. Intermonsoon samples were strongly influenced by continental material, both of crustal and anthropogenic origin. The crustal component (24.5±13% of the total suspended particulate mass (TSP), 6.0±4.4 μg m^(−3)) contained 3.2% gypsum (CaSO_4). While more than half of the TSP (21.2±9.6 μg m^(−3)) during the SW monsoon was sea-salt-derived due to the strong winds prevailing during this season, only 1.7±1.1% (0.7±0.4 μg m^(−3)) was found to be of crustal origin. Sulfate (SO_4^(2−)) sources were determined and quantified with linear regression analyses utilizing specific tracers for the independent variables. Lead (Pb) was found to be a more reliable surrogate for anthropogenic SO_4^(2−) compared to nitrate (NO_3^−) during the relatively polluted intermonsoon. Soluble calcium (Ca^(2+)) served as the tracer for gypsum, and methane sulfonate (MSA) served as the tracer for biogenically derived SO_4^(2−) during both seasons. On the basis of this analysis, 75% of the non-sea-salt sulfate (NSS-SO_4^(2−)) (0.8±0.2 μg m^(−3), representing ∼2.4% of TSP) was found to be of biogenic origin during the SW monsoon with the remaining 25% of anthropogenic origin. During the intermonsoon, NSS-SO_4^(2−) accounted for 2.1±1.2 μg m^(−3) (∼9.2% of TSP) and had a composition that was 65% anthropogenic, 21% biogenic, and 14% gypsum-derived. Linear regression analyses revealed that the bio-SO_4^(2−)/MSA weight ratios appear to be consistent with the temperature dependence proposed by Hynes et al. [1986]. In this case the yield of SO_4^(2−) increased relative to MSA with an increase in temperature. Three samples during the SW monsoon, near the coast of Oman, showed lower temperatures, due to coastal upwelling, than the rest of the samples; at 24°C the bio-SO_4^(2−)/MSA weight ratio was 6.8±0.5. The remainder of the SW monsoon samples were collected at an average temperature of 27.2°C, for which the bio-SO_4^(2−)/MSA weight ratio was 13.5±4.4. At an average temperature of 28.9°C during the intermonsoon, sampling gave a ratio of 17.7±4.8. These observations indicate a temperature dependence factor between 24° and 29°C of 2.2 (i.e., a 2.2 increase in the ratio of bio-SO_4^(2−)/MSA with every degree temperature increase). Cl− deficits determined during both seasons appear to indicate that different mechanisms may govern the observed depletion of Cl− in each season.

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