A detailed time‐resolved chemical characterization of ambient nonrefractory submicron aerosols (NR‐PM₁) was conducted for the first time in India. The measurements were performed during the winter (November 2011 to January 2012) in a heavily polluted city of Kanpur, which is situated in the Indo‐Gangetic Plain. Real‐time measurements provided new insights into the sources and evolution of organic aerosols (OA) that could not be obtained using previously deployed filter‐based measurements at this site. The average NR‐PM₁ loading was very high (>100 µg/m³) throughout the study, with OA contributing approximately 70% of the total aerosol mass. Source apportionment of the OA using positive matrix factorization revealed large contributions from fresh and aged biomass burning OA throughout the entire study period. A back trajectory analysis showed that the polluted air masses were affected by local sources and distant source regions where the burning of paddy residues occurs annually during winter. Several fog episodes were encountered during the study, and the OA composition varied between foggy and nonfoggy periods, with higher oxygen to carbon (O/C) ratios during the foggy periods. The evolution of OA and their elemental ratios (O:C and H:C) were investigated for the possible effects of fog processing.

Size‐resolved fog water samples were collected in two consecutive winters at Kanpur, a heavily polluted urban area of India. Samples were analyzed by an aerosol mass spectrometer after drying and directly in other instruments. Residues of fine fog droplets (diameter: 4–16 µm) are found to be more enriched with oxidized (oxygen to carbon ratio, O/C = 0.88) and low volatility organics than residues of coarse (diameter > 22 µm) and medium size (diameter: 16–22 µm) droplets with O/C of 0.68 and 0.74, respectively. These O/C ratios are much higher than those observed for background ambient organic aerosols, indicating efficient oxidation in fog water. Accompanying box model simulations reveal that longer residence times, together with high aqueous OH concentrations in fine droplets, can explain these trends. High aqueous OH concentrations in smaller droplets are caused by their highest surface‐volume ratio and high Fe and Cu concentrations, allowing more uptake of gas phase OH and enhanced Fenton reaction rates, respectively. Although some volatile organic species may have escaped during droplet evaporation, these findings indicate that aqueous processing of dissolved organics varies with droplet size. Therefore, large (regional, global)‐scale models need to consider the variable reaction rates, together with metal‐catalyzed radical formation throughout droplet populations for accurately predicting aqueous secondary organic aerosol formation.