Wintertime PM_2.5 and PM₁₀ carbonaceous and inorganic constituents from urban site in western India

Abstract

Daily variability in the chemical composition of atmospheric PM_2.5 and PM₁₀ has been studied from an urban site (Ahmedabad) in western India over a span of 30 days during winter. The PM_2.5 and PM₁₀ mass concentrations ranged from 32 to 106 µ g m^{- 3} and 121 to 327 µ g m^{- 3,} respectively. On average, PM_2.5 constitutes ~ 33% of PM₁₀, indicating dominance of coarse mode aerosols in the urban atmosphere. The particulate EC and OC show higher abundances in PM_2.5 (average: 3.0 ± 0.9 and 18.3 ± 5.9 µ g m^{- 3} respectively) whereas those in PM₁₀ are 4.4 ± 2.4 and 29.8 ± 11.2 µ g m^{- 3} respectively. A linear increasing trend and representative OC/EC ratio of 6.2 indicate their primary source from biomass burning emissions. The water-soluble organic carbon (WSOC: 4.0-14.7 µ g m^{- 3}) and its linear relationship with K⁺ (0.6-1.7 µ g m^{- 3}) in PM_2.5 further support biomass burning emissions as a dominant source for carbonaceous aerosol. Among water-soluble inorganic species, SO₄^2- is the most abundant (range: 3.2-22.5 µ g m^{- 3}); almost all of it occurs in fine mode (PM_2.5) and exhibits near-quantitative neutralization with NH₄⁺ (r = 0.98, slope: 1.3). The water-soluble Ca²⁺ and Mg²⁺ mainly abundant in the coarse mode, suggest significant contribution from mineral dust. Documenting large temporal variability in the chemical composition of coarse and fine mode aerosol is essential in order to assess the changing regional emission scenario over mega-cities and their down-wind transport.