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A year-long study on water soluble ionic constituents of PM 10 was conducted at mid-Brahmaputra plain during 2012-2013. Water soluble ions associated with PM 10 viz. SO 4 2-, NO 3 -, Cl -, F -, NH 4 + , K + , Ca 2+ , Na + and Mg 2+ were analyzed using Ion chromatograph (Metrohm 882 Compact IC Professional) for three representative sites. Marked differences were observed regarding PM 10 loading and concentrations of major ions in three different sites, with maximum PM 10 (71.1±56 µg/m 3 ) at urban site. Among anions, dominance of SO 4 2and Clwere observed and Na + , NH 4 + and K + were found to be predominant cations. An explicit seasonal variation of ionic constituents was observed during the study period with maximum abundance of ions during winter season, and SO 4 2was the most abundant ion during all the seasons. Pre-monsoon season showed influence of crustal input with high mass concentrations of Ca 2+ and Na + in PM 10 .The equivalent ion balance reveals the fact that except winter season, all other seasons experienced alkaline nature of particulates in the atmosphere. Influence of anthropogenic activities to the ionic constitutions of PM 10 was revealed by calculating ionic ratios and enrichment factor. HYSPLIT trajectory analysis showed influence of long-range transport of pollutants to this region.

Episodic meji burning (a festive biomass burning) is a significant local contributor of PM 10 in the mid-Brahmaputra Valley. The PM 10 samples of meji burning period were chemically characterized for elements, ions, carbon and the USEPA's 16 priority Polycyclic Aromatic Hydrocarbons (PAHs). This paper presents the concentrations of various attributes of PM 10 during meji festivities. As such, linear relationship between EC, OC and other chemical species, and diagnostic ratios of selected PAHs indicated strong influence of biomass burning on PM 10 load and its properties.

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