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

Özet

Atmospheric PM1 (particulate matter with aerodynamic diameter ≤ 1 μm) samples have been collected duringfoggy (n = 17) and non-foggy nights (n = 19) in wintertime at Kanpur in central Indo-Gangetic Plain (IGP) toassess light absorption characteristics and direct radiative forcing of water-extractable brown carbon (BrC). Wehave observed a significant enhancement (two-tailed t-test: t = 2.2; at significance level: p < 0.05) in theabsorption coefficient of water-extractable BrC at 365 nm (babs-BrC-365) from non-foggy (Avg.: 53.5 Mm−1) tofoggy episodes (69.3 Mm−1). Enhancement in mass absorption efficiency (MAE) of BrC (1.8 m2/g C) duringfoggy episodes is consistent with that of babs-BrC-365. Absorption Ångström exponent (AAE) remained similar (2.8)during foggy and non-foggy episodes. Significantly lower value of AAE (2.8) at Kanpur compared to other placesin IGP (∼5) highlights more light absorbing potential of atmospheric BrC over central IGP. Furthermore, MAE ofEC 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 ∼15% and 20%, respectively during foggyevents. These observations are also reflected by an increase (t = 11.1; p < 0.05) in direct radiative forcing ofwater-extractable BrC (relative to EC) in the atmosphere: from 23.7 ± 10.8% during non-foggy to54.3 ± 16.5% during foggy episodes. Differences in chemical composition, loading, absorption properties anddirect radiative forcing (DRF) of carbonaceous aerosols during non-foggy and foggy episodes indicate predominantinfluence of fog-processing.