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

摘要

Atmospheric PM1 (particulate matter with aerodynamic diameter??a?¤??1????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 (babs-BrC-365) from non-foggy (Avg.: 53.5??Mma??1) to foggy episodes (69.3??Mma??1). Enhancement in mass absorption efficiency (MAE) of BrC (1.8??m2/g C) during foggy episodes is consistent with that of babs-BrC-365. Absorption ??ngstr??m 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 (a??5) highlights more light absorbing potential of atmospheric BrC over central IGP. Furthermore, MAE of EC at 660??nm during foggy period (8.5??m2/g) is relatively high as compared to that during the non-foggy episode (7.0??m2/g). The MAE of BrC and EC exhibited enhancement by a??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.