Reply on RC1

Abstract

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, PM2.5 samples were obtained during a winter haze event in Guangzhou, China, and light absorption and molecular composition of HULIS were investigated by UV-vis spectrophotometry and ultrahigh-resolution mass spectrometry. Compared with HULIS in clean days, the absorption coefficients (Abs365) of HULIS in haze days were significantly higher but the mass absorption efficiencies (MAE365) were relatively lower, 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 higher molecular weight, lower aromaticity index (AImod), and higher O/Cw, O/Nw, and O/Sw ratios, indicating that HULIS fractions undergo relatively higher oxidation during haze days than clean days. Moreover, CHON and CHO compounds with high AImod were the major potential chromophores in HULIS and significantly contributed to HULIS light absorption. It's worth noting that the proportions of these chromophores were decreased during haze event, mainly owing to their higher oxidation and longer aging period during haze episode. Besides, accumulated contribution of organic compounds emitted from vehicles and formed from stronger reactions of bio-VOCs also diluted light-absorbing compounds in haze HULIS. These findings help to understand HULIS evolution during haze bloom-decay process in the subtropic region of China.