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

Abstract

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 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 (Abs365) of HULIS on haze days were significantly higher but the mass absorption efficiencies (MAE365) 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 (AImod); and higher O/Cw, O/Nw, and O/Sw ratios, indicating that HULIS fractions undergo relatively high oxidation during haze days compared to 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 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.

Measurement report: Characteristics of nitrogen-containing organics in PM2.5 in Urumqi, northwest China: differential impacts of combustion of fresh and old-age biomass materials

“…The CHO species in ESI− had higher OSC (-1.85 to 1.1) than those in ESI+ (Figures 2c and 2d), which was consistent with a recent study conducted in Guangzhou, China (Zou et al, 2023). The predominant subgroups of CHO in ESI− were BBOA and semivolatile oxidized OA (SV-OOA), which was different from the observation in Shanghai (dominated by SV-OOA and low-volatility oxidized OA) (Wang et al, 2017a).…”

Distribution, chemical and molecular composition of high and low-molecular-weight humic-like substances in ambient aerosols

“…Hundreds of peaks can be observed in the spectra ranging from m/z 100 to 450 for all samples, with most ions being abundant within the m/z 150-350 range. These spectrum characteristics are similar to those previously reported for HULIS in ambient aerosols and BB emissions (He et al, 2023;Song et al, 2022;Sun et al, 2021;Wang et al, 2019;Zhang et al, 2021;Zou et al, 2023). HULISs feature a high concentration of both CHON (30-58%) and CHO (21-38%).…”

“…The notably higher content of CHO in summer HULISs are likely due to a wide distribution of biogenic VOC-derived SOAs during the summer season (Li et al, 2022;Sun et al, 2023). CHON content in winter HULISs is generally higher than in summer ones, potentially due to more significant contributions from direct BB, as well as secondary nitrogen-related chemical processes during the winter season (He et al, 2023;Zhang et al, 2021;Zou et al, 2023). This finding is supported by the greater number of fire spots (Fig.…”

“…The intensity-weighted molecular parameters (Xw) of MW, H/C, O/C, DBE, and AI values were calculated according to the equation ( 6) (He et al, 2023;Zhang et al, 2021;Zou et al, 2023):…”

“…These studies have provided insights into the overall structural characteristics of complex HULIS, including their abundances, chemical and optical characteristics (Huo et al, 2021;Mukherjee et al, 2020;Win et al, 2018;Zhang et al, 2022b;Zheng et al, 2013). In recent years, high-resolution mass spectrometry (HRMS) techniques, such as Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) and orbitrap HRMS, in combination with electrospray ionization (ESI), have emerged as powerful tools for elucidating the molecular-level characteristics of HULIS (Lin et al, 2012;Sun et al, 2021;Wang et al, 2019;Zou et al, 2023). By utilizing HRMS, researchers have gained deeper insights into the complexity and chemical heterogeneity of HULIS at the molecular level.…”