Temporal Characteristics of Brown Carbon over the Central Indo-Gangetic Plain

Recent global models estimate that light absorption by brown carbon (BrC) in several regions of the world is ∼30–70% of that due to black carbon (BC). It is, therefore, important to understand its sources and characteristics on temporal and spatial scales. In this study, we conducted semicontinuous measurements of water-soluble organic carbon (WSOC) and BrC using particle-into-liquid sampler coupled with a liquid waveguide capillary cell and total organic carbon analyzer (PILS-LWCC-TOC) over Kanpur (26.5°N, 80.3°E, 142 m amsl) during a winter season (December 2015 to February 2016). In addition, mass concentrations of organic and inorganic aerosol and BC were also measured. Diurnal variability in the absorption coefficient of BrC at 365 nm (b_{abs_365}) showed higher values (35 ± 21 Mm^–1) during late evening to early morning hours and was attributed to primary emissions from biomass burning (BB) and fossil fuel burning (FFB). The b_{abs_365} reduced by more than 80% as the day progressed, which was ascribed to photo bleaching/volatilization of BrC and/or due to rising boundary layer height. Further, diurnal variability in the ratios of b_{abs_405}/b_{abs_365} and b_{abs_420}/b_{abs_365} suggests that the BrC composition was not uniform throughout a day. WSOC exhibited a strong correlation with b_{abs_365} (slope = 1.22 ± 0.007, r² = 0.70, n = 13 265, intercept = −0.69 ± 0.17), suggesting the presence of a significant but variable fraction of chromophores. Mass absorption efficiency (MAE) values of WSOC ranged from 0.003 to 5.26 m² g^–1 (1.16 ± 0.60) during the study period. Moderate correlation (r² = 0.50, slope = 1.58 ± 0.019, n = 6471) of b_{abs_365} was observed with the semivolatile oxygenated organic aerosols (SV-OOA) fraction of BB resolved from positive matrix factorization (PMF) analysis of organic mass spectral data obtained from a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS). The low-volatility OOA (LV-OOA) fraction of BB had a similar correlation to b_{abs_365} (r² = 0.54, slope = 0.38 ± 0.004, n = 6471) but appears to have a smaller contribution to the absorption.