Measurement report: Changes in light absorption and molecular composition of water-soluble humic-like substances during a winter haze bloom-decay process in Guangzhou, China

Water-soluble humic-like substances (HULIS) absorb light in near-UV and visible wavelengths and exert significant influence on the atmospheric environment and climate. However, knowledge on HULIS evolution during haze bloom-decay process is limited. Herein, PMinline-formula_2.5 samples were obtained during a winter haze event in Guangzhou, China, and the light absorption and molecular composition of HULIS were investigated by UV–Vis spectrophotometry and ultrahigh-resolution mass spectrometry. Compared with HULIS on clean days, the absorption coefficients (Absinline-formula₃₆₅) of HULIS on haze days were significantly higher but the mass absorption efficiencies (MAEinline-formula₃₆₅) were relatively low, suggesting diverse and dynamic absorption properties of HULIS during haze episodes. The CHO and CHON compounds were the most abundant components in HULIS, followed by CHOS, CHONS, and CHN. Haze HULIS presented comparatively high molecular weight; a lower aromaticity index (AIinline-formula_mod); and higher inline-formula $M5inlinescrollmathml{chem\mathrm{normal\; O}/\mathrm{normal\; C}}_{\mathrm{normal\; w}}$ 30pt14ptsvg-formulamathimg6283b47d7c5e8aac2b99d371c1a87d55 acp-23-963-2023-ie00001.svg30pt14ptacp-23-963-2023-ie00001.png , inline-formula $M6inlinescrollmathml{chem\mathrm{normal\; O}/\mathrm{normal\; N}}_{\mathrm{normal\; w}}$ 30pt14ptsvg-formulamathimge4462014b81ef0fc9528f3456b92d140 acp-23-963-2023-ie00002.svg30pt14ptacp-23-963-2023-ie00002.png , and inline-formula $M7inlinescrollmathml{chem\mathrm{normal\; O}/\mathrm{normal\; S}}_{\mathrm{normal\; w}}$ 30pt14ptsvg-formulamathimg06171f34bc7f294815c69cf47e694235 acp-23-963-2023-ie00003.svg30pt14ptacp-23-963-2023-ie00003.png ratios, indicating that HULIS fractions undergo relatively high oxidation during haze days compared to clean days. Moreover, CHON and CHO compounds with high AIinline-formula_mod were the major potential chromophores in HULIS and significantly contributed to HULIS light absorption. It is worth noting that the proportions of these chromophores decreased during haze events, mainly owing to their higher oxidation during haze episodes. Besides, accumulated contribution of organic compounds emitted from vehicles and formed from reactions of biogenic volatile organic compounds (bio-VOCs) also diluted light-absorbing compounds in haze HULIS. These findings help us to understand HULIS evolution during haze bloom-decay processes in the subtropic region of China.