LAUSR

Abstract The present study outlines the mechanism and brings out the chemistry involved in winter haze formation characterized by reduced visibility and severe respiratory problems especially in rural areas. It deals with the study of aerosol chemical composition and meteorological parameters in two consecutive winter seasons (2015/16 and 2016/17). Elevated PM2.5 mass concentrations were recorded (average = 231.5 ± 12.9 and 257.1 ± 21.9 μg m−3 in 2015/16 and 2016/17, respectively) during haze (polluted) days which was more than twice the PM concentrations during non-haze days (average = 101.6 ± 20.2 and 110.2 ± 25.4 μg m−3 in 2015/16 and 2016/17, respectively). During non-haze period, the contribution of carbonaceous aerosols (CAs) and secondary inorganic aerosols (SIAs) was 40% and 29% of PM2.5 but when haze formed both CAs and SIAs increased. During haze (polluted) episodes, high OC/EC ratios (>3.0) indicated secondary organic aerosol formation and high NH4+/SO42− molar ratios (>2.0) indicated the formation of (NH4)2SO4 and NH4HSO4 as major NH4+ salts. The results obtained with E-AIM Model simulations showed good agreement with measured values. E Aim Model IV was used to estimate liquid water content (LWC) and formation of inorganic salts. LWC (863 μg m−3; in situ hydrogen ion concentration, [H+]ins = 1.2 nmol m−3, and pH: 2.76) was high during haze (polluted) days as compared to non-haze days (750 μg m−3; [H+] ins = 2.4 nmol m−3, and pH:1.89) indicating enhanced hygroscopic growth of the ionic solids during haze (polluted) as compared to non-haze period. These results further corroborate the occurrence and predominance of heterogeneous reactions (under highly hygroscopic conditions) during haze (polluted) days in contrast to non-haze days when gas-phase as well as heterogeneous reactions both may occur.