Optical properties, molecular characterizations, and oxidative potentials of different polarity levels of water-soluble organic matters in winter PM2.5 in six China's megacities

Humic-like substances (HULIS)accounted for a great fraction of water-soluble organic matter (WSOM) in PM2.5,whichefficiently absorb ultraviolet(UV) radiation andposeclimate and health impacts. Inthis study, themolecular structure,optical properties, and oxidative potential (OP) of acid- and neutral-HULIS (denoted as HULIS-a, and HULIS-n, respec-tively), andhigh-polarity WSOM (HP-WSOM)were investigated in winter PM2.5collected at six China's megacities. Forboth carbon levels and optical absorption coefficients (babs_365), HULIS-a/HULIS-n/HP-WSOM showed significant spa-tial differences. For each city, the carbon levels and babs_365follow a similar order of HULIS-n>HULIS-a>HP-WSOM.Besides, the babs_365of HULIS-n and HULIS-a showed the same order of Harbin>Beijing approximate to Wuhan>Xi'an>Guang-zhou>Chengdu, while HP-WSOM exhibited an order of Wuhan>Chengdu>Xi'an>Harbin>Beijing>Guangzhou.Both HULIS-a and HULIS-n were abundant in aromatic and aliphatic compounds, whereas HP-WSOM was dominatedby a carboxylic acid group. The OP (in unit of nmol H2O2 mu g-1C) followed the order of HP-WSOM>HULIS-a>HULIS-n in all the cities. The OPs of HULIS-a, HULIS-n, and HP-WSOM in Harbin and Beijing were much higher than those of other cities, attributing to the high contribution from biomass burning. Highly positive correlations between reactive oxygen species (ROS) of HULIS-a and MAE365were obtained in Chengdu, Wuhan, and Harbin, but ROS of HULIS-n had stronger correlation with MAE365in Harbin, Chengdu, and Xi'an

Automated summary

The spatial distribution and optical properties of HULIS and HP-WSOM in winter PM2.5 were investigated in six megacities in China. HULIS-a and HULIS-n were mainly composed of aromatic and aliphatic compounds, while HP-WSOM was dominated by carboxylic acid groups. Additionally, there were strong positive correlations between reactive oxygen species (ROS) of HULIS-a and HULIS-n and MAE365 in certain cities.