Polarity-based chemical characterization of humic-like substances (HULIS) in the Indo-Gangetic Plain (IGP) and linkage with optical signatures of aqueous brown carbon

Humic-like substances (HULIS) are complex macromolecular components of light-absorbing water-soluble organic carbon (brown carbon; BrC_aq) consisting of polyacidic and monocyclic/polycyclic structures. HULIS contribute to climate forcing via strong absorption at UV and near-visible wavelengths, and by facilitating the formation of cloud condensation nuclei. Consequently, characterization of the HULIS fractions of BrC is critical to develop a robust understanding of its sources, atmospheric processing and optical effects. To address this issue, we report HULIS fractionation from bulk BrC_aq by pH-based multi-step solid phase extraction (hydrophobic fraction at pH=7: neutral HULIS (H-n); hydrophilic fraction at pH=2: acidic HULIS (H-a)) and corresponding optical (UV-Vis and 3D-fluorescence) and chemical properties (¹H NMR and FT-IR) on a diurnal and seasonal basis. The study was conducted on PM_2.5 samples collected in the eastern part of the Indo-Gangetic Plain (IGP), which is dominantly affected by marine airmasses mixed with fossil fuel emissions during summer, and biomass burning emissions in the IGP outflow during post-monsoon and winter.

Distinct diurnal and seasonal variations were observed for optical and chemical signatures of bulk BrC_aq and HULIS fractions. At 365 nm, daytime absorption of BrC_aq, H-n and H-a was 3-13 times higher in post-monsoon and 2-8 times higher in winter as compared to summer while that for nighttime samples were enhanced by factors of 2-7 and 3-13, respectively. These possibly point towards greater emission intensity and/or enhanced dark-phase formation of soluble chromophores of BrC_aq and H-n during nighttime and an enhanced presence of conjugated aromatics during post-monsoon and winter. On the other hand, enrichment of H-a during daytime might be due to increased polarity of oxidation products via photochemical processing. Excitation-emission matrix fluorescence coupled with the parallel factor analysis (PARAFAC) model identified two types of HULIS (condensation and degradation) and combinations of tryptophan and tyrosine type protein-like substances (PRLIS) in BrC_aq across seasons, with higher PRLIS abundance (78%) in summer associated with marine biogenic emissions and higher HULIS abundance in post-monsoon (62%) and winter (67%) linked to regional biomass burning. H-n showed greater humification during nighttime as compared to day (0.74-1.92 vs. 0.67-1.15) while H-a was more humified during daytime (0.62-1.54 vs. 0.57-1.34), suggesting that the degree of unsaturation in H-a was associated with the atmospheric ageing process. Aliphatics (H-C) were the most abundant group among BrC_aq, H-n and H-a, accounting for about 35-51% of total organic hydrogen throughout the seasons. An increased contribution of H-Ar was observed in H-a (8-24%), followed by H-n (2-19%) and BrC_aq (7-14%) suggesting dominance of conjugated aromatics in the HULIS fractions. In BrC_aq, aliphatic groups (O-H, N-H, C-H) were dominant in summer, consistent with transported fossil fuel emissions, while biomass burning-derived aromatic signatures (phenolic O-H, C=O, C=C) were prominent during post-monsoon and winter. In contrast, HULIS fractions showed aliphatic signatures of O-H and –COOH^- throughout the seasons. Overall, these findings provide the first insights into the source- and atmospheric processing-dependent chromophoric composition of BrC over the IGP.