Spatial and Seasonal Variations of Water Soluble Ions in PM 10 of Mid-Brahmaputra Plain of Assam Valley

A year-long study on water soluble ionic constituents of PM₁₀ was conducted at mid-Brahmaputra plain during 2012-2013. Water soluble ions associated with PM₁₀ viz. SO₄^2-, NO₃^-, Cl^-, F^-, NH₄⁺, K⁺, Ca²⁺, Na⁺ and Mg²⁺ were analyzed using Ion chromatograph (Metrohm 882 Compact IC Professional) for three representative sites. Marked differences were observed regarding PM₁₀ loading and concentrations of major ions in three different sites, with maximum PM₁₀ (71.1 ± 56 μg/m³) at urban site. Among anions, dominance of SO₄^2- and Cl^- were observed and Na⁺, NH₄⁺ 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 SO₄^2- was the most abundant ion during all the seasons. Pre-monsoon season showed influence of crustal input with high mass concentrations of Ca²⁺ and Na⁺ in PM₁₀. 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 PM₁₀ was revealed by calculating ionic ratios and enrichment factor. HYSPLIT trajectory analysis showed influence of long-range transport of pollutants to this region.