Black and brown carbon as aerosol climate-forcing agents in the eastern Indo-Gangetic Plain: insights from bulk and chromophoric analysis

Rana, Archita (2024) Black and brown carbon as aerosol climate-forcing agents in the eastern Indo-Gangetic Plain: insights from bulk and chromophoric analysis. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Archita Rana (17IP020)) 17IP020.pdf - Submitted Version Restricted to Repository staff only Download (10MB)

Abstract

Aerosol black carbon (BC), brown carbon (BrC) and the latter’s constituents such as nitroaromatic compounds (NACs) are significant climate-forcing agents. This thesis employed a coupled chemical-optical approach to characterize BC and methanol-soluble BrC (BrCme) chromophores in the eastern Indo-Gangetic Plain (IGP). The averaged BC mass was 7.8±4.7 μg m⁻³ with abundance during the winter. Regional transport of BC-rich smoke plumes was observed, and shortwave forcing by BC was estimated to be 37±22 W m⁻². The annual average babs_me_365 was 22±13 Mm⁻¹, peaking in the winter and post-monsoon. Profiles of mass absorption efficiency (MAE), refractive index (k) and Angstrom exponent (AE) indicated a mix of moderately- and weakly-absorbing BrC, with BrCme contributing 20-26% to radiative forcing relative to BC between 300-400 nm. MAE250nm/MAE365nm ratios and humification index revealed more aromatic and conjugated chromophores during the biomass burning periods whereas high biological index in summer indicated marine influence. EEM-PARAFAC analysis identified four types of fluorophores: two humic-like (HULIS) and two protein-like (PRLIS). FTIR further confirmed the presence of phenol-like species during biomass burning seasons while ¹H NMR spectra exhibited lowest relative aromaticity but highest saturated aliphatic structures and oxygenated compounds in the summer. NACs (mean: 185±94 ng m⁻³) were dominated by nitrosalicylic acids (NSAs), followed by nitrophenols (NPs) and nitrocatechol (NCs). The contribution of NACs to organic aerosol mass was only 0.42±0.23%, but their contribution to BrC absorption for 300–450 nm was higher by an order to magnitude (8±4%). Positive matrix factorization (PMF) quantified three NAC sources: fossil fuels (49%), secondary formation (40%), and biomass burning (11%). At a molecular level, CHON was the most dominant group (42%), followed by CHO (34%), with <10% contribution from organosulfates and nitroxy-organosulfates. Overall, this study demonstrated the importance of IGP emissions in constraining BC and BrC characteristics in the eastern IGP corridor.

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