HOSTED BY Contents lists available at ScienceDirect Atmospheric Pollution Research journal homepage: www.elsevier.com/locate/apr Light absorption characteristics of brown carbon during foggy and non- foggy episodes over the Indo-Gangetic Plain Vikram Choudhary a,b , Prashant Rajput a,* , Dharmendra Kumar Singh a,c , Amit Kumar Singh a , Tarun Gupta a,b a Department of Civil Engineering and APTL at Center for Environmental Science and Engineering (CESE), Indian Institute of Technology Kanpur, Kanpur 208 016, India b Environmental Engineering and Management Program, Indian Institute of Technology Kanpur, Kanpur 208 016, India c NUS Environmental Research Institute (NERI), National University of Singapore, #02-01, T-Lab Building 5A Engineering Drive 1, Singapore 117 411, Singapore ARTICLE INFO Keywords: Fog-processing Brown carbon Mass absorption efficiency Absorption Ångström exponent Direct radiative forcing IGP ABSTRACT Atmospheric PM 1 (particulate matter with aerodynamic diameter ≤ 1 μm) 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 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 places in IGP (∼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 ∼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 pre- dominant influence of fog-processing. 1. Introduction Carbonaceous aerosols constitute a dominant fraction of fine-parti- culate matter (Artaxo et al., 2002; Rajput et al., 2018; Rengarajan et al., 2007). It is broadly comprised of black carbon and a large suit of or- ganic aerosols (Gustafsson et al., 2009; Pavuluri et al., 2011; Rajput et al., 2014b; Singh and Gupta, 2015, 2016). Black carbon exhibits strong absorption in visible solar spectrum (Ram and Sarin, 2009; Ram et al., 2012; Singh et al., 2014). Some of the organic species show solar absorption behavior in UV and near-blue region (300–400 nm) (Andreae and Gelencser, 2006; Chen and Bond, 2010; Cheng et al., 2011, 2016; Choudhary et al., 2017; Hecobian et al., 2010). The light absorbing component of organic aerosols is widely referred to as brown carbon (BrC). Several studies on BrC characterization in ambient aerosols have attributed its major sources to primary emission from biomass burning, fossil-fuel combustion and also from secondary transformations (Hoffer et al., 2004; Lack et al., 2013; Lukács et al., 2007; Zheng et al., 2013). Experiments on biomass burning emission characterization during LBA – SMOCC (Large scale Biosphere atmo- sphere experiment in Amazonia – SMOke aerosols, Clouds, rainfall and Climate) have shown that absorption behavior of humic like substances (HULIS) look similar to that of organic carbon extractable in water (water-soluble organic carbon: WSOC) (Hoffer et al., 2004, 2006). Furthermore, these studies have revealed that BrC derived from bio- mass burning emission is predominantly comprised of HULIS (Kumar et al., 2017). In this context, it is important to mention that during wintertime the Indo-Gangetic Plain (IGP) experiences massive emis- sions of carbonaceous aerosols from biomass burning (Rajput et al., 2014b; Ramanathan and Carmichael, 2008). High content of moisture resulting from westerly disturbances, low ambient temperature and shallower planetary boundary layer height causes prolonged fog and haze episodes over this region during wintertime (Chakraborty et al., https://doi.org/10.1016/j.apr.2017.11.012 Received 2 October 2017; Received in revised form 21 November 2017; Accepted 25 November 2017 Peer review under responsibility of Turkish National Committee for Air Pollution Research and Control. * Corresponding author. E-mail address: prajput@iitk.ac.in (P. Rajput). Atmospheric Pollution Research xxx (xxxx) xxx–xxx 1309-1042/ © 2017 Turkish National Committee for Air Pollution Research and Control. Production and hosting by Elsevier B.V. All rights reserved Please cite this article as: Choudhary, V., Atmospheric Pollution Research (2017), https://doi.org/10.1016/j.apr.2017.11.012