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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 (SUVA 254) 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. [Display omitted] • Oxidative potential of HULIS decreased with the increase of HULIS-C concentration. • Aromatic compounds and carboxylic acids dominated the diurnal variation in winter. • Carboxylic acids were responsible for the diurnal variation in summer. • The oxidative power of winter HULIS was related to high light-absorbance and high-aromaticity compounds. [ABSTRACT FROM AUTHOR]

In the context of the outbreak of coronavirus disease 2019 (COVID-19), strict lockdown policies were implemented to control nonessential human activities in Xi'an, northwest China, which greatly limited the spread of the pandemic and affected air quality. Compared with pre-lockdown, the air quality index and concentrations of PM2.5, PM10, SO2, and CO during the lockdown reduced, but the reductions were not very significant. NO2 levels exhibited the largest decrease (52%) during lockdown, owing to the remarkable decreased motor vehicle emissions. The highest K+ and lowest Ca2+ concentrations in PM2.5 samples could be attributed to the increase in household biomass fuel consumption in suburbs and rural areas around Xi'an and the decrease in human physical activities in Xi'an (e.g., human travel, vehicle emissions, construction activities), respectively, during the lockdown period. Secondary chemical reactions in the atmosphere increased in the lockdown period, as evidenced by the increased O3 level (increased by 160%) and OC/EC ratios in PM2.5 (increased by 26%), compared with pre-lockdown levels. The results, based on a natural experiment in this study, can be used as a reference for studying the formation and source of air pollution in Xi'an and provide evidence for establishing future long-term air pollution control policies. [ABSTRACT FROM AUTHOR]

There is growing evidence suggesting the enhancement of brown carbon (BrC) in severe haze episodes. In this study, hourly measurements of BrC in PM 2.5 were conducted in Xi'an, a typical city in northwestern China during winter and summer. The absorption coefficient for methanol exacts at 365 nm (b abs365, methanol , which is typically used as a proxy of methanol-soluble BrC) in the winter sampling period was over 7 times than that in summer. The mass absorption cross-section for methanol extracts (MAC 365, methanol , normalized by b abs365, methanol to organic carbon, OC) in winter sampling period was nearly 1.5 times of that in the summer. During the winter haze days, the average b abs 365, methanol peaked at midnight and the lowest values in the morning, in contrast to high levels in afternoon and low levels at night in non haze days. Unlike the diurnal patterns in winter, summer b abs365, methanol diurnal variation presented high midday and low afternoon levels in haze days. However, in non haze days, the pattern showed high morning levels and night low levels. Haze and non haze variations of chemical species levels, b abs365, methanol , and MAC 365, methanol during winter and summer sampling time showed that the effects of atmospheric aging were complex and could either enhance or reduce light absorption of BrC. Source apportionment based on positive matrix factorization receptor model and multiple linear regressions showed that primary emission was an important contributor to BrC emissions during the winter sampling period, whereas secondary formation played an important role in summer. Daily variations of b abs365,methanol , MAC 365,methanol , OC, OC/EC during winter and summer. Image 1 • High time-resolved BrC observation during winter and summer was conducted. • Atmospheric aging can either enhance or reduce light absorption by BrC. • Primary emission and secondary formation were important contributors to BrC during winter and summer, respectively. [ABSTRACT FROM AUTHOR]