Chemical characteristics of PM₁₀ aerosols and airmass trajectories over Bay of Bengal and Arabian sea during ICARB

Abstract

For the first time, chemical characterization of PM₁₀ aerosols was attempted over the Bay of Bengal (BoB) and Arabian Sea (AS) during the ICARB campaign. Dominance of SO₄^2-, NH₄⁺ and NO₃^- was noticed over both the regions which indicated the presence of ammonium sulphate and ammonium nitrate as major water soluble particles playing a very important role in the radiation budget. It was observed that all the chemical constituents had higher concentrations over Bay of Bengal as compared to Arabian Sea. Higher concentrations were observed near the Indian coast showing influence of landmass indicating that gaseous pollutants like SO₂, NH₃ and NO _x are transported over to the sea regions which consequently contribute to higher SO₄^2-, NH₄⁺ and NO₃^- aerosols respectively. The most polluted region over BoB was 13°-19°N and 70°-90°E while it was near 11°N and 75°E over AS. Although the concentrations were higher over Bay of Bengal for all the chemical constituents of PM₁₀ aerosols, per cent non-sea salt (nss) fraction (with respect to Na) was higher over Arabian Sea. Very low Ca²⁺ concentration was observed at Arabian Sea which led to higher atmospheric acidity as compared to BoB. Nss SO₄^2-alone contributed 48% of total water soluble fraction over BoB as well as AS. Ratios SO₄^2- /NO₃^- over both the regions (7.8 and 9 over BoB and AS respectively) were very high as compared to reported values at land sites like Allahabad (0.63) and Kanpur (0.66) which may be due to very low NO.3 over sea regions as compared to land sites. Air trajectory analysis showed four classes: (i) airmass passing through Indian land, (ii) from oceanic region, (iii) northern Arabian Sea and Middle East and (iv) African continent. The highest nss SO₄^2- was observed during airmasses coming from the Indian land side while lowest concentrations were observed when the air was coming from oceanic regions. Moderate concentrations of nss SO^2- ₄ were observed when air was seen moving from the Middle East and African continent. The pH of rainwater was observed to be in the range of 5.9-6.5 which is lower than the values reported over land sites. Similar feature was reported over the Indian Ocean during INDOEX indicating that marine atmosphere had more free acidity than land atmosphere.