[Light Absorption and Fluorescence Characteristics of Atmospheric Water-soluble Organic Compounds and Humic-like...

The light absorption and fluorescence characteristics of atmospheric water-soluble organic compounds (WSOC) and humic-like substances (HULIS) during the winter season in Guangzhou were examined using UV-vis spectroscopy and excitation-emission matrix spectroscopy combined with parallel factor analysis (EEM-PARAFAC). The results showed that the SUVA₂₅₄, HIX, and MAE₃₆₅ values of HULIS were higher than those of WSOC, suggesting that the former had higher aromaticity, humification, and light-absorption capacity in winter atmospheric PM_2.5 in Guangzhou. EEM-PARAFAC analysis identified three fluorescence components, including fulvic-like acid (C1), humic-like acid (C2), and protein-like (C3) components. The total humic-like components (C1+C2) accounted for 78% and 85% for WSOC and HULIS, respectively, which indicated that humic-like fluorescence components were the major components for both WSOC and HULIS and that HULIS were enriched with the dominant humic-like fluorophores. In addition, the aromaticity, humification, light-absorbing capacity, and C2 levels of WSOC and HULIS during the haze episode were significantly higher than those in the non-haze episode. This suggested that the water-soluble organics with higher molecular weights and stronger light-absorption capacities tended to form during the haze episode. The correlations analysis revealed strong negative correlations between C1 levels of WSOC and HULIS and HIX, MAE₃₆₅, OC_sec, K⁺, SO₄^2-, and NH₄⁺. Additionally, strong positive correlations were observed between C2 levels and the same factors. These results implied that the decrease in C1 and increase in C2 might lead to increased humification and light-absorption in WSOC and HULIS, and biomass burning and secondary organic aerosols might contribute to the C2 component.