Aerosol acidity and secondary organic aerosol formation during wintertime over urban environment in Western India

Abstract

Atmospheric mass concentration of ambient PM_2.5, its chemical composition and aerosol acidity have been studied during the wintertime from an urban site in a semi-arid region of western India. The concentration of PM_2.5 ranged from 32 to 106 µg m^-3 during the 30-day sampling period; and carbonaceous species (EC, OC) and water-soluble inorganic constituents (WSIC) account for ~58% and ~29% of the mass, respectively. The contribution of SO₄^2- to PM_2.5 is significant (Average: 17.5%) and that of NO₃^- is minor (Average: ~2%). The ratio of water soluble to particulate organic carbon (WSOC/OC) varies from 0.26 to 0.52 (Average: 0.41) and provides evidence for the significant role of secondary organic aerosol (SOA) in an urban environment. Measured concentration of [H⁺]_air, used as an index of aerosol acidity, varies from 0.25 to 11 µmol m^-3 and exhibits linear increase with the secondary organic carbon (SOC). Our field-based data provide the direct evidence for enhanced SOA formation at high level of acidity, consistent with laboratory studies.