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Imidazoles (IMs) are potential contributors to brown carbon; they may notably contribute to climate radiative forcing. However, only a few studies have assessed the mixing state, seasonal and spatial distributions of IMs, and influencing factors for IM formation in urban aerosols. In this study, two single-particle aerosol mass spectrometers were employed to investigate the IM-containing particles in the urban areas of Beijing and Guangzhou, China. IM-containing particles were identified in the size range (d va ) of 0.2-2.0 μm, accounting for 0.7-21.7 % of all the detected particles. The number fractions of IM-containing particles in both cities were the lowest in winter and the highest in spring, probably owing to the difference in the abundance of precursors and the particle acidity. Majority of (60-80 % by number) the IM-containing particles were mixed with organic carbon (OC), with the lowest fractions found in summer. Although the number fractions of IM-containing particles in Beijing were generally higher (~1.5-3 times) than those in Guangzhou, the mixing states of the IM-containing particles at these two sites were only slightly different. Potassium-rich (K-rich) and potassium-sodium (KNa) particles were rarely found in Guangzhou; they accounted for ~15 % of the IM-containing particles in Beijing. Additionally, our results indicate that particles with higher acidity are favorable for IM formation. These findings help improving our knowledge of the mixing state, seasonal variation, and spatial distribution of IMs in urban aerosols, and the insights in influencing factors into IM formation provide valuable information for future studies of the atmospheric chemical processes associated with IMs., Competing Interests: Declaration of competing interest The authors declare that they have no conflict of interest., (Copyright © 2022. Published by Elsevier B.V.)

A non-parametric ensemble model was proposed to estimate the long-term (2015-2019) particle surface area concentrations (S A ) over China for the first time on basis of a vilification dataset of measured particle number size distribution. This ensemble model showed excellent cross-validation R 2 value (CV R 2 = 0.83) as well as a relatively low root-mean-square error (RMSE = 195.0 μm 2 /cm 3 ). No matter in which year, considerable spatial heterogeneity of S A was found over China with higher S A in Beijing-Tianjin-Hebei (BTH), Yangtze River Delta (YRD), and Middle Lower Reaches of Yangtze River (MLYR). From 2015 to 2019, S A significantly decreased in representative city clusters. The reduction rates were 140.1 μm 2 ·cm -3 ·a -1 in BTH, 110.7 μm 2 ·cm -3 ·a -1 in Pearl River Delta (PRD), 105.2 μm 2 ·cm -3 ·a -1 in YRD, and 92.4 μm 2 ·cm -3 ·a -1 in Sichuan Basin (SCB), respectively. Even though such quick reduction, high S A (ranged from ~800 μm 2 /cm 3 to ~1750 μm 2 /cm 3 ) during the heavy pollution period (PM 2.5 > 75 μg/m 3 ) still existed in the above-mentioned city clusters and may provide rich reaction vessels for multiphase chemistry. A dichotomy of enhanced annual 4th maximum daily 8-h average O 3 concentrations (4MDA8 O 3 ) and decreased S A during summertime was found in Shanghai, a representative city of YRD. In Chengdu (SCB), increased 4MDA8 O 3 concentration was associated with a synchronous increase of S A from 2017 to 2019. Differently, 4MDA8 O 3 concentrations enhanced in Beijing (BTH) and Guangzhou (PRD), while not significant for S A before 2018. This work will greatly deepen our understanding of the historical variation and spatial distributions of S A over China., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier B.V. All rights reserved.)

To investigate the characteristics of particulate matter with an aerodynamic diameter less than 2.5 μm (PM 2.5 ) and its chemical compositions in the Beijing-Tianjin-Hebei (BTH) region of China during the novel coronavirus disease (COVID-19) lockdown, the ground-based data of PM 2.5 , trace gases, water-soluble inorganic ions, and organic and elemental carbon were analyzed in three typical cities (Beijing, Tianjin, and Baoding) in the BTH region of China from 5-15 February 2020. The PM 2.5 source apportionment was established by combining the weather research and forecasting model and comprehensive air quality model with extensions (WRF-CAMx). The results showed that the maximum daily PM 2.5 concentration reached the heavy pollution level (>150 μg/m 3 ) in the above three cities. The sum concentration of SO 4 2- , NO 3 - chemical compositions of Beijing, Tianjin, and Baoding; secondary transformation of gaseous pollutants contributed significantly to PM 4 + played a dominant position in PM 2.5 in the BTH region; the contribution of transportation source decreased significantly than previous reports in Beijing, Tianjin, and Baoding during the COVID-19 lockdown; but the contribution of industrial and residential emission sources increased significantly with the increase of PM 2.5 generation, and the secondary transformation was enhanced as the increased PM 2.5 concentrations. The results of WRF-CAMx showed obviously inter-transport of PM 2.5 in the BTH region; the contribution of transportation source decreased significantly than previous reports in Beijing, Tianjin, and Baoding during the COVID-19 lockdown; but the contribution of industrial and residential emission sources increased significantly with the increase of PM 2.5 concentration, and industry emission sources contributed the most to PM 2.5 concentrations. Therefore, control policies should be devoted to reducing industrial emissions and regional joint control strategies to mitigate haze pollution., (Copyright © 2021. Published by Elsevier B.V.)

This paper aimed to analyze the composition and pollution sources of particulate matter (PM) in the Beijing-Tianjin-Hebei region and its surrounding areas (henceforth the BTH region) during the heating season to support the mitigation and control of regional air pollution. Manual monitoring data from the China National Environmental Monitoring Network for Atmospheric PM in the BTH region were collected and analyzed during the 2016 and 2018 heating seasons. The positive definite matrix factor analysis (PMF) model was used to analyze the PM sources in BTH cities during the heating season. The main PM components were organic matter (OM), nitrate (NO 3 - ), sulfate (SO 4 2- ) and ammonium salt (NH 4 + ). Direct emission sources have decreased since 2016, indicating the effectiveness of governmental controls on these sources; however, secondary pollution showed an increasing trend, suggesting control measures should be strengthened. Daily regional average concentrations of OM, SO 4 2- , NH 4 + , elemental carbon (EC), chloride (Cl - ) and trace elements all showed similar trends. When air quality worsened, the concentrations of the main PM components increased, but trends of change varied among components. In 2018, concentrations of OM and chloride were highest in the Taihang Mountains, and NO 3 concentrations were highest in Anyang, Hebi, Jiaozuo and Xinxiang. The SO 4 2- concentration was highest in the southern section of the Taihang Mountains. The NH 4 + and EC concentrations were generally highest in the central and southern regions. The concentration of crustal substances was highest in some cities in the north and central parts of the BTH region. In the 2018 heating season, the pollution level of five transmission channels showed an increasing trend in the Northwest, Southeast, Yanshan, South and Taihang Mountain channels. These findings provide a scientific basis for the continued management of atmospheric PM pollution., (Copyright © 2021 Elsevier Ltd. All rights reserved.)