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

Supriya Dey; Pronoy Ghosh; Prashant Rawat; Nikki Choudhary; Akansha Rai; Rohit Meena; Tuhin K Mandal; Jingying Mao; Shiguo Jia; Neeraj Rastogi; Sudhir K Sharma; Sayantan Sarkar

doi:10.1016/j.scitotenv.2023.164872

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

Sci Total Environ. 2023 Oct 10:894:164872. doi: 10.1016/j.scitotenv.2023.164872. Epub 2023 Jun 19.

Authors

Affiliations

¹ School of Civil and Environmental Engineering, Indian Institute of Technology (IIT) Mandi, Kamand, Himachal Pradesh 175075, India.
² CSIR-National Physical Laboratory (CSIR-NPL), Dr. K.S. Krishnan Road, New Delhi 110012, India.
³ Geosciences Division, Physical Research Laboratory, Ahmedabad, Gujarat 380009, India.
⁴ Institute for Environmental and Climate Research, Jinan University, Guangzhou 510632, PR China.
⁵ Guangdong Province Key Laboratory for Climate Change and Natural Disaster Studies, Sun Yat-sen University, Guangzhou 510275, PR China; School of Atmospheric Sciences, Sun Yat-sen University, Guangzhou 510275, PR China.
⁶ School of Civil and Environmental Engineering, Indian Institute of Technology (IIT) Mandi, Kamand, Himachal Pradesh 175075, India. Electronic address: sayantan@iitmandi.ac.in.

PMID: 37343876
DOI: 10.1016/j.scitotenv.2023.164872

Abstract

This study reports day-night and seasonal variations of aqueous brown carbon (BrC_aq) 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_aq and the use of stable isotopic ratios (δ¹³C and δ¹⁵N) to understand atmospheric processing. Nighttime BrC_aq 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_{250 nm}/MAE_{365 nm} (i.e., E₂/E₃) ratios and Angstrom Exponents revealed that BrC_aq and HULIS-n were relatively more aromatic and conjugated during the biomass burning-dominated periods while BrC_aq 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_aq 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_aq 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 m² g^-1) followed by biomass burning (0.78 m² g^-1) and secondary processing (0.13 m² g^-1). Finally, δ¹³C and δ¹⁵N isotopic analysis confirmed the importance of summertime photochemistry and wintertime NO₃^--dominated chemistry in constraining BrC characteristics. Overall, the quantitative apportionment of BrC_aq 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.

Keywords: Humic-like substances (HULIS); Positive matrix factorization (PMF); Radiative forcing; Stable isotopes; Water-soluble organic carbon (WSOC).