Measurement report: Changes in light absorption and molecular 1 composition of water-soluble humic-like substances during a winter 2 haze bloom-decay process in Guangzhou, China

18 Water-soluble humic-like substances (HULIS) absorb light in near-UV and visible 19 wavelengths and exert significant influence on the atmospheric environment and climate. 20 However, knowledge on HULIS evolution during haze bloom-decay process is limited. 21 Herein, PM 2.5 samples were obtained during a winter haze event in Guangzhou, China, 22 and light absorption and molecular composition of HULIS were investigated by UV-vis 23 spectrophotometry and ultrahigh-resolution mass spectrometry. Compared with HULIS 24 in clean days, the absorption coefficients (Abs 365 ) of HULIS in haze days were 25 significantly higher but the mass absorption efficiencies (MAE 365 ) were relatively lower, 26 suggesting diverse and dynamic absorption properties of HULIS during haze episodes. 27 The CHO and CHON compounds were the most abundant components in HULIS, 28 followed by CHOS, CHONS, and CHN. Haze HULIS presented comparatively higher 29 molecular weight, lower aromaticity index (AI mod ), and higher O/C w , O/N w , and O/S w 30 ratios, indicating that HULIS fractions undergo relatively higher oxidation during haze 31 days than clean days. Moreover, CHON and CHO compounds with high AI mod were the 32 major potential chromophores in HULIS and significantly contributed to HULIS light 33 absorption. It's worth noting that the proportions of these chromophores were decreased 34 during haze event, mainly owing to their higher oxidation and longer aging period during 35 haze episode. Besides, accumulated contribution of organic compounds emitted from 36 vehicles and formed from stronger reactions of bio-VOCs also diluted light-absorbing 37 compounds in haze HULIS. These findings help to understand HULIS evolution during 38 haze bloom-decay process in the subtropic region of China. 39 of the evolution of The present performed comprehensive characterization of HULIS in PM 2.5 collected during a haze event in The abundances and light absorption properties of HULIS first and and Subsequently, four HULIS samples different analyzed using both To our the present study aim to apply a combination of and molecular characterization by FT-ICR to investigate The results obtained provide novel insights into the evolution during and are important for predicting and effects of in