Light absorption characteristics of brown carbon during foggy and non-foggy episodes over the Indo-Gangetic Plain

Atmospheric PM1 (particulate matter with aerodynamic diameter <= 1 mu m) samples have been collected during foggy (n = 17) and non-foggy nights (n = 19) in wintertime at Kanpur in central Indo-Gangetic Plain (IGP) to assess light absorption characteristics and direct radiative forcing of water-extractable brown carbon (BrC). We have observed a significant enhancement (two-tailed t-test: t = 2.2; at significance level: p < 0.05) in the absorption coefficient of water-extractable BrC at 365 nm (b(abs-BrC-365)) from non-foggy (Avg.: 53.5 Mm(-1)) to foggy episodes (69.3 Mm(-1)). Enhancement in mass absorption efficiency (MAE) of BrC (1.8 m(2)/g C) during foggy episodes is consistent with that of b(abs-BrC-365). Absorption Angstrom exponent (AAE) remained similar (2.8) during foggy and non-foggy episodes. Significantly lower value of AAE (2.8) at Kanpur compared to other places in IGP ( similar to 5) highlights more light absorbing potential of atmospheric BrC over central IGP. Furthermore, MAE of EC at 660 nm during foggy period (8.5 m(2)/g) is relatively high as compared to that during the non-foggy episode (7.0 m(2)/g). The MAE of BrC and EC exhibited enhancement by similar to 15% and 20%, respectively during foggy events. These observations are also reflected by an increase (t = 11.1; p < 0.05) in direct radiative forcing of water-extractable BrC (relative to EC) in the atmosphere: from 23.7 +/- 10.8% during non-foggy to 54.3 +/- 16.5% during foggy episodes. Differences in chemical composition, loading, absorption properties and direct radiative forcing (DRF) of carbonaceous aerosols during non-foggy and foggy episodes indicate predominant influence of fog-processing.