Optical source apportionment of aqueous brown carbon (BrC) on a daytime and nighttime basis in the eastern Indo-Gangetic Plain (IGP) and insights from 13 C and 15 N isotopic signatures

• Published in: The Science of the total environment Vol. 894; p. 164872
• Main Authors: Dey, Supriya, Ghosh, Pronoy, Rawat, Prashant, Choudhary, Nikki, Rai, Akansha, Meena, Rohit, Mandal, Tuhin K, Mao, Jingying, Jia, Shiguo, Rastogi, Neeraj, Sharma, Sudhir K, Sarkar, Sayantan
• Format: Journal Article
• Language: English
• Published: Netherlands 10.10.2023
• Subjects: Water-soluble organic carbon (WSOC); Stable isotopes; Positive matrix factorization (PMF); Humic-like substances (HULIS); Radiative forcing
• ISSN: 1879-1026

This study reports day-night and seasonal variations of aqueous brown carbon (BrC ) and constituent humic-like substances (HULIS) (neutral and acidic HULIS: HULIS-n and HULIS-a) from the eastern Indo-Gangetic Plain (IGP) of India during 2019-2020. This is followed by the application of the receptor model positive matrix factorization (PMF) for optical source apportionment of BrC and the use of stable isotopic ratios (δ C and δ N) to understand atmospheric processing. Nighttime BrC absorption and mass absorption efficiencies (MAE) were enhanced by 40-150 % and 50-190 %, respectively, compared to the daytime across seasons, possibly as a combined effect from daytime photobleaching, dark-phase secondary formation, and increased nighttime emissions. MAE /MAE (i.e., E /E ) ratios and Angstrom Exponents revealed that BrC and HULIS-n were relatively more aromatic and conjugated during the biomass burning-dominated periods while BrC and HULIS-a were comprised mostly of non-conjugated aliphatic structures from secondary processes during the photochemistry-dominated summer. The relative radiative forcing of BrC with respect to elemental carbon (EC) was 10-12 % in the post-monsoon and winter in the 300-400 nm range. Optical source apportionment using PMF revealed that BrC absorption at 300, 365 and 420 nm wavelengths in the eastern IGP is mostly from biomass burning (60-75 %), followed by combined marine and fossil fuel-derived sources (24-31 %), and secondary processes (up to 10 %). Source-specific MAEs at 365 nm were estimated to be the highest for the combined marine and fossil fuel source (1.34 m  g ) followed by biomass burning (0.78 m  g ) and secondary processing (0.13 m  g ). Finally, δ C and δ N isotopic analysis confirmed the importance of summertime photochemistry and wintertime NO -dominated chemistry in constraining BrC characteristics. Overall, the quantitative apportionment of BrC sources and processing reported here can be expected to lead to targeted source-specific measurements and a better understanding of BrC climate forcing in the future.