Atmospheric water-soluble constituents in fine and coarse mode aerosols from high-altitude site in Western India: Long-range transport and seasonal variability

Abstract

We present a comprehensive one-year data set on water-soluble ionic species (Na⁺, NH⁺₄, K⁺, Mg⁺, Ca²⁺, NO₃^-, SO^2-₄, and HCO₃^-) in PM_2.5 (fine) and PM_10-2.5 (coarse) aerosols from a high-altitude site (Mt. Abu, 24.6 °N, 72.7 °E, 1680 m asl) in high-dust region of western India. The water-soluble ionic composition (WSIC) varied from 1.0 to 19.5 µg m^-3 in the fine mode and constitutes 50, 39 and 31% of the aerosol mass during winter, summer and monsoon respectively, with dominant contribution from SO₄^2-, NH₄⁺ and HCO₃^-. Furthermore, a two-fold increase in the abundances of nss-SO₄^2- and NH₄⁺ and their co-variability during wintertime, relative to high-dust conditions in summer, suggest dominance of anthropogenic sources and long-range transport of combustion products (biomass burning and fossil-fuel emissions) from northern India. In the coarse mode, WSIC ranged from 0.1 to 24.8 µg m^-3 and its contribution to aerosol mass was consistently low (annual average = 21%) with predominance of Ca²⁺ and HCO₃^-, indicating contribution from carbonate rich mineral dust. The nss-SO₄^2-/NO₃^- mass ratio exhibits extreme variability during winter, with values ranging from 2.7 to 101 in PM_2.5 and 0.001 to 2.7 in coarse (PM_10-2.5) mode. The relatively high abundance of nitrate in the coarse mode, during all seasons, indicates its association with mineral dust. The temporal variability is further evident from significantly lower aerosol mass and WSIC during the SW-monsoon (July-Sept) due to efficient washout. The chemical data set also documents near quantitative neutralization of acidic species (NO₃^- and SO₄^2-) by NH₄⁺ in PM_2.5 and mineral dust in PM_10-2.5, thus, representing a dominant atmospheric chemical transformation process occurring in the high-dust semi-arid region.