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

Atmospheric mass concentration of ambient PM2.5, 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 PM2.5 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 SO42− to PM2.5 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.