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

Ambient aerosol samples were collected over the northern Indian Ocean durin g two 1 month-long research cruises (German R/V Meteor) that took place dur ing the intermonsoon (May) and SW monsoon (July/August) of 1995. A high vol ume and two small volume collectors were used to collect samples, which wer e subsequently analyzed for ferrous iron, 32 elements, and anions and catio ns. 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 el al., 1999]. Data are analyzed and interpr eted with the aid of principal component and multiple linear regression ana lyses. Intermonsoon samples were strongly influenced by continental materia l, both of crustal and anthropogenic origin. The crustal component (24.5 +/ - 13% of the total suspended (-3)) contained 3.2% gypsum (CaSO4). While mor e particulate mass (TSP), 6.0 +/- 4.4 mu g m(-3) than half of the TSP (21.2 +/- 9.6 mu g 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 mu g m(-3)) was found to be of crustal origin. Sulfate (SO42-) sources were determined and quantified with linear regression analyses utilizing specif ic tracers for the independent variables. Lead (Pb) was found to be a more reliable surrogate for anthropogenic SO42- compared to nitrate (NOS) during the relatively polluted intermonsoon. Soluble calcium (Ca2+) as the tracer for gypsum, and methane sulfonate (MSA) served as the tracer for biogenica lly derived SO42- during both seasons. On the basis of this analysis, 75% o f the non-sea-salt sulfate (NSS-SO42-) (0.8 +/- 0.2 mu g m(-3), representin g similar to 2.4% of TSP) was found to be of biogenic origin during the SW monsoon with the remaining 25% of anthropogenic origin. During the intermon soon, NSS-SO42- accounted for 2.1 +/- 1.2 mu g m(-3) (similar to 9.2% of TS P) and had a composition that was 65% anthropogenic, 21% biogenic, and 14% gypsum-derived. Linear regression analyses revealed that the bio-SO42-/MSA weight ratios appear to be consistent with the temperature dependence propo sed by Hynes et al. [1986]. In this case the yield of SO42- increased relat ive to MSA with an increase in temperature. Three samples during the SW mon soon, near the coast of Oman, showed lower temperatures, due to coastal upw elling, than the rest of the samples; at 24 degrees C the bio-SO42-/MSA wei ght ratio was 6.8 +/- 0.5. The remainder of the SW monsoon samples were col lected at an average temperature of 27.2 degrees C, for which the bio-SO42- /MSA weight ratio was 13.5 +/- 4.4. At an average temperature of 28.9"C dur ing the intermonsoon, sampling gave a ratio of 17.7 +/-. 4.8. These observa tions indicate a temperature dependence factor between 24 degrees and 29 de grees C of 2.2 (i.e., a 2.2 increase in the ratio of bio-SO42-/MSA with eve ry degree temperature increase). Cl- deficits determined during both season s appear to indicate that different mechanisms may govern the observed depl etion of Cl- in each season.