Abstract
Humic-like substances (HULIS) are a major component of brown carbon and consequently play a major role in climate change. In this study, 70 PM2.5 samples were collected from Xi'an in winter 2019 and summer 2020. Neutral HULIS (HULIS-n), acidic HULIS (HULIS-a), and high-polarity water-soluble organic compounds (HP–WSOC) were analyzed to determine their carbon concentrations and measure their ultraviolet–visible absorption and infrared spectra. Of the three components, HULIS-n had the highest carbon content in both winter (3.29 ± 1.45 μg m−3) and in summer (1.38 ± 1.10 μg m−3). The semiquantitative results for the functional groups revealed that HP-WSOC was rich in carboxylic acids and had high aromaticity in winter, whereas HULIS-n was rich in carboxylic acids in summer. Moreover, HULIS-a was richer in nitrate esters and saturated aliphatic hydrocarbons in summer than in winter. The results for specific ultraviolet absorbance (SUVA) and E250/E365 revealed that HULIS had higher molecular weight and aromaticity in winter than in summer. HULIS-n dominated in the total light absorption of HULIS + HP-WSOC in both winter (73.08%) and summer (48.57%). Overall, the results on the carbon content, optical properties, and functional groups of WSOCs with differing polarity can improve understanding of environmental and climatic effects.
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