Seasonal and diurnal variation of PM_2.5 HULIS over Xi'an in Northwest China: Optical properties, chemical functional group, and relationship with reactive oxygen species (ROS)

Humic-like substances (HULIS) in particulate matter (PM) play critical roles in the atmospheric changes in our environment. In this study, high time resolution PM_2.5 samples were collected to insight the abundances, spectroscopic characters, chemical groups, and oxidative potential of HULIS in Xi'an, China. The average mass concentrations of HULIS in term of carbon (HULIS-C) was 11.55 ± 5.85 and 8.28 ± 2.23 μg C m⁻³ in winter and summer, respectively. The diurnal variations of HULIS displayed three peaks (03:00–07:00, 10:00–14:00, and 18:00–21:00 LT) in winter, but a single peak (08:00–12:00 LT) in summer. The optical parameters show obvious difference between winter and summer. Fourier Transform infrared (FT-IR) spectra revealed that the HULIS mainly consisted of aliphatic chains, aromatic rings, and carboxylic groups. The FT-IR spectra proved that the aromatic compounds and carboxylic acids dominated the diurnal variation in winter, while carboxylic acids were responsible for that in summer. The normalized oxidative potential of HULIS exhibited an inverse trend with the HULIS-C concentration measured by the DCFH assay. The diurnal reactive oxygen species (ROS) productions from HULIS presents three valleys (03:00–07:00, 10:00–14:00, and 18:00–21:00 LT) in winter but only a single valley (08:00–12:00 LT) in summer. In addition, a positive correlation (R² ≈ 0.6, p＜0.01) between the oxidative potential and specific ultraviolet absorbance at a wavelength of 254 nm (SUVA₂₅₄) in winter suggested that the oxidative power of HULIS could be more related to the compounds with high light absorbance, high aromaticity, and molecular weight. The results of this study offer more solid knowledge on the spectral and chemical characteristics and oxidative potential of the HULIS in PM_2.5 of the typical city in northwest China.