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.

Recent ground-based measurements reveal that model-based observations are underestimating absorption properties and direct radiative forcing (DRF) due to carbonaceous species by a factor of 2-3 over South and East Asia. Thus, to better constraint these parameters associated with carbonaceous species, seasonal variability records through ground-based measurements are very essential. In this context, we report herein the absorption properties of water-extractable brown carbon (BrC), elemental carbon (EC), and BrC + EC and DRF of BrC and BrC + EC (relative to EC) over the Indo-Gangetic Plain (IGP) during a weak monsoon season [July-September 2015; influenced by El Nino and PDO (Pacific Decadal Oscillation)]. PM2.5 (particulate matter with aerodynamic diameter <= 2.5 mu m) samples (n = 31) have been assessed from central IGP location at Kanpur. Absorption coefficient at 365 nm (b(abs-365)) of BrC, EC, and BrC + EC centers at 3.6, 8.1, and 11.4 Mm(-1), respectively. Strong linear regression correlation (R = 0.8) of b(abs-365) of BrC with sea-salt and mineral dust corrected potassium (K-BB(+)) indicates biomass burning as the predominant source of BrC over the region in this study. Synergistic effect in b(abs) of BrC + EC (relative to that of EC) increases conspicuously (enhanced by a factor varying from 1.05 to 1.21) with an increase in wavelength from UV (365 nm) to visible region (660 nm). DRF (relative to EC) of BrC during monsoon season ranges from 3.9 to 23.8 (13.0 +/- 5.0) %, whereas total DRF (BrC + EC) ranges from 111.0 to 148.2 (126.6 +/- 10.0) %. Individual contribution of BrC and EC to total DRF has been estimated as 10 and 79.5%, respectively. The remaining contribution (10.5%) to total DRF has been attributed to synergism in absorption properties (i.e., b(abs)) of BrC + EC.

Black carbon (BC) aerosols are one of the most uncertain drivers of global climate change. The prevailing view is that BC mass concentrations are low in rural areas where industrialization and vehicular emissions are at a minimum. As part of a national research program called the Ganga Basin Ground Based Experiment-2014 under the Cloud Aerosol Interaction and Precipitation Enhancement Experiment (CAIPEEX) Phase-III of Ministry of Earth Sciences, Government of India, the continuous measurements of BC and particulate matter (PM) mass concentrations, were conducted in a rural environment in the highly-polluted Indo-Gangetic Plain region during 16th June to 15th August (monsoon period), 2014. The mean mass concentration of BC was 4.03 (+/- 0.85) mu g m(-3) with a daily variability between 2.4 and 5.64 mu g m(-3), however, the mean mass PM concentrations [near ultrafine (PM1.0), fine (PM2.5) and inhalable (PM1.0)] were 29.1(+/- 16.2), 34.7 (+/- 19.9) and 43.7 (+/- 283) mu g m(-3), respectively. The contribution of BC in PM1.0 was approximately 13%, which is one of the highest being recorded. Diurnally, the BC mass concentrations were highest (mean: 5.89 mu g m(-3)) between 20:00 to 22:00 local time (LT) due to the burning of biofuels/biomass such as wood, dung, straw and crop residue mixed with dung by the local residents for cooking purposes. The atmospheric direct radiative forcing values due to the composite and BC aerosols were determined to be +78.3, +44.9, and +45.0 W m(-2) and +42.2, +35.4 and +34.3 W m(-2) during the months ofJune, July and August, respectively. The corresponding atmospheric heating rates (AHR) for composite and BC aerosols were 2.21,1.26 and 1.26; and 1.19, 0.99 and 0.96 K day(-1) for the month ofJune, July and August, respectively, with a mean of 1.57 and 1.05 K day(-1) which was 33% lower AHR (BC) than for the composite particles during the study period. This high AHR underscores the importance of absorbing aerosols such as BC contributed by residential cooking using biofuels in India. Our study demonstrates the need for immediate, effective regulations and policies that mitigate the emission of BC particles from domestic cooking in rural areas of India. (C) 2016 Elsevier B.V. All rights reserved.