Molecular characteristics of aqueous brown carbon in the eastern Indo-Gangetic Plains: Insights from a high resolution mass spectrometry approach

Light-absorbing organic aerosols, often referred to as brown carbon (BrC), significantly contribute to atmospheric brown clouds and are a major climate forcing agent over South Asia. In addition to direct emissions in the form of fine mode aerosol, BrC forms secondarily in the atmosphere via homogeneous and heterogeneous chemical reactions involving anthropogenic and biogenic precursor gases, thereby enhancing the complexity of its molecular composition. Detailed molecular characterization and identification of potential BrC chromophores are essential to facilitate a proper understanding of BrC source profiles, atmospheric transformation processes and resultant climate effects. However, owing to its relatively short atmospheric lifetime and large spatial variability, molecular-level characterization of BrC aerosol is challenging. Here we report the first insights into molecular profile of aqueous BrC in the Indian subcontinent, specifically, the eastern Indo-Gangetic Plains (IGP) with a distinct heterogeneity of BrC sources, on a diurnal and seasonal basis. To this end, daytime and nighttime PM_2.5 samples collected during 2019-2020 at a rural receptor location in the eastern IGP were extracted for the aqueous BrC fraction and subsequently analyzed using high-performance liquid chromatography coupled with a diode array detector and a time-of-flight mass spectrometer (HPLC-DAD-ToF-ESI-MS).

In total, around 3000 chemical formulas of organic compounds were determined in the positive and negative modes, which were classified into four major groups: CHO, CHON, CHONS and CHOS. In the negative mode, CHO- (36-48%) was the most abundant group, followed by CHON- (23-31%) and S-containing groups (CHONS- (7-19%) and CHOS- (11-32%)) whereas CHON+ (47-58%) showed the highest abundance in the positive mode followed by CHO+ (21-29%) and S-containing groups (CHONS+ (11-18%) and CHOS+ (3-11%)). The reaction between ammonia and carbonyls could lead to the formation of abundant CHON+ compounds with reduced N-containing groups (averaged O/C: 0.2, H/C: 1.7), while CHON- consisted of oxidized N-containing groups (averaged O/C: 0.5, H/C: 1.1). Daytime samples were enriched with CHO- and CHOS- compounds as compared to nighttime samples throughout the seasons, potentially suggesting photochemical formation of these multifunctional compounds. On the contrary, N-containing compounds such as CHON- and CHONS- showed higher abundance during nighttime, suggesting the importance of dark-phase NO₃^- chemistry. The higher double-bond equivalent (DBE) value of the CHON- group in post-monsoon and winter (7-8) indicated the presence of unsaturated compounds possibly emitted from agricultural residue burning or via secondary formation through NO_x reactions. In contrast, the higher DBE value of the CHOS- group during summer (~6) suggested the emission of S-containing compounds from diesel vehicles, coal combustion or secondary formation via photochemical reaction pathways. The enrichment of water-soluble nitroaromatic chromophores (C₆H₅NO₃, C₇H₇NO₄, C₉H₇NO₄, etc.) during post-monsoon and winter was consistent with the dominant presence of a biomass burning source, echoing our previous findings based on multiple independent lines of evidence. Overall, these results provide the first insights into the linkage between BrC chemical and optical properties in the Indian context.