Sources and atmospheric processing of brown carbon and HULIS in the Indo-Gangetic Plain: Insights from compositional analysis

We present here spectroscopic compositional analysis of brown carbon (BrC) and humic-like substances (HULIS) in the Indian context under varying conditions of source emissions and atmospheric processing. To this end, we study bulk water-soluble organic matter (WSOM), neutral- and acidic-HULIS (HULIS-n and HULIS-a), and high-polarity (HP)-WSOM collected in the eastern Indo-Gangetic Plain (IGP) with respect to UVeVis, fluorescence, FT-IR, H-1 NMR and C-13 characteristics under three aerosol regimes: photochemistry-dominated summer, aged biomass burning (BB)-dominated post-monsoon, and fresh BB-dominated winter. Absorption coefficients (b(abs)_365 nm; Mm(-1)) of WSOM and HULIS fractions increase by a factor of 2-9 during winter as compared to summer, with HULIS-n dominating total HULIS+HPWSOM absorption (73-81%). Fluorophores in HULIS-n appear to contain near-similar levels of aromatic and unsaturated aliphatic conjugation across seasons, while HULIS-a exhibits distinctively smaller-chain structures in summer and post-monsoon. FT-IR spectra reveals, among others, strong signatures of aromatic phenols in winter WSOM suggesting a BB-related origin. H-1 NMR-based source attribution coupled with back trajectory analysis indicate the presence of secondary and BB-related organic aerosol (SOA and BBOA) in the post-monsoon and winter, and marine-derived OA (MOA) in the summer, which is supported by C-13 measurements. Overall, these observations uncover a complex interplay of emissions and atmospheric processing of carbonaceous aerosols in the IGP. (C) 2020 Elsevier Ltd. All rights reserved.