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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 - and NH 4 + played a dominant position in PM 2.5 chemical compositions of Beijing, Tianjin, and Baoding; secondary transformation of gaseous pollutants contributed significantly to 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.)

The PM 2.5 concentration characteristics of "2+26" cities in Beijing-Tianjin-Hebei and the surrounding region during the Spring Festival and the Lantern Festival in 2021 were analyzed. The average daily concentration and hourly concentration of PM 2.5 in the three days before and after the Spring Festival and the Lantern Festival from 2016 to 2021 and the characteristics of the PM 2.5 chemical components during the Spring Festival and the Lantern Festival from 2019 to 2021 were comparatively analyzed. In addition, the impact of meteorological factors on PM 2.5 concentration from 2016 to 2021 and the key factors affecting the forecast result of Beijing during the Spring Festival in 2021 were discussed. The results showed that the combination of fireworks and firecrackers and adverse weather conditions resulted in a moderate to heavy pollution process during the Spring Festival in 2021 and a light to moderate pollution process during the Lantern Festival in "2+26" cities. The average ρ (PM 2.5 ) was 111 μg·m -3 . The hourly peak ρ (PM 2.5 ) was 156 μg·m -3 , which was the lowest in 2016 to 2021. The three days before and after the Lantern Festival in 2021, the average ρ (PM 2.5 ) of "2+26" cities was 85 μg·m -3 , and the hourly peak ρ (PM 2.5 ) was 125 μg·m -3 . During this period, the hour number of heavy and severe pollution was the lowest in 2016 to 2021. Component observation results showed that the impact of fireworks and firecrackers during the Spring Festival and the Lantern Festival in 2021 was gradually reduced, and the effect of bans and restrictions for fireworks and firecrackers was significant. The regional pollution was dominated by conventional pollution accumulation and nitrate. The concentration of organic matter in some cities during the Spring Festival was relatively high, which may be related to local industrial emissions and the impact of blasting fireworks and firecrackers. The comparative analysis of meteorological factors and PM 2.5 concentration showed that the factors such as wind speed and direction, ground pressure, and relative humidity all affected PM 2.5 concentration and the duration of the regional pollution process. The influence of oscillation of the boundary layer transport sink on diffusion conditions in the North China Plain, the location and intensity of the small high pressure in North China and the intensity of its high-altitude moving airflow, and the uncertainty of fireworks displays, as well as the low accuracy of weather forecasts caused by frequent weather changes during the alternate period of winter and spring, could affect the accuracy of air quality forecast results in the northern Beijing-Tianjin-Hebei region. In summary, although the impact of fireworks and firecrackers has been reduced in recent years, in order to improve the air quality, it is recommended to further strengthen the control of fireworks and firecrackers, especially outside the Beijing Fifth Ring Road and Beijing's surrounding cities.

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.)

Most previous studies treat regional transport of aerosols as a whole, without distinguishing the transport of secondary aerosols and that of their precursors. A new method of quantifying the transport forms of secondary inorganic aerosols (SIA) using the Nested Air Quality Prediction Modeling System was proposed. The contribution of nonlocal emissions to SIA in the receptor region was divided into three parts: (1) SIA chemically formed by nonlocal emissions in their source regions; (2) SIA chemically formed by nonlocal emissions during transport; and (3) SIA chemically formed by nonlocal emissions in the receptor region, representing transport of precursors. In the North China Plain, the transport of precursors and SIA produced during transport are the two main transport forms. Furthermore, the contribution from transport of precursors increased under polluted conditions in most cities. The results indicate that joint control of precursors is paramount for mitigating air pollution. Plain Language Summary: Uncertainties still exist about the sources of secondary inorganic aerosols (SIA), which are one of the crucial drivers of haze pollution. The spatial distribution of precursors is not exactly consistent with that of aerosols, which is puzzling to policymakers. Most existing studies only treat contribution from regional transport of nonlocal emissions as a whole but do not distinguish the transport of secondary aerosols themselves and that of their precursors. Therefore, it is necessary to quantify the contribution from transport of precursors and secondary aerosols. The Nested Air Quality Prediction Model System with an online tracer‐tagging module was used to investigate the regional sources of SIA in Beijing and surrounding cities. The regional contribution of nonlocal emissions to SIA in the receptor region was divided into three parts: (1) SIA chemically formed by nonlocal emissions in their source region; (2) SIA chemically formed by nonlocal emissions in transport pathway; and (3) SIA chemically formed by nonlocal emissions in the receptor region, indicating transport of precursors. The results showed that transport of precursors and the SIA produced during transport are two main transport forms. The results strongly suggest that the joint control of precursors is important for mitigating air pollution. Key Points: A new method of quantifying the transport forms of secondary inorganic aerosols (SIA) using an air quality model was proposedThe transported SIA in the receptor region is from conversion of nonlocal emissions in the source region, transport path, and receptor regionThe chemical conversion of nonlocal emissions after they leave their source region is crucial in the North China Plain [ABSTRACT FROM AUTHOR]

Air quality in Beijing-Tianjin-Hebei and its surrounding regions has been significantly improved during the Three-Year Action Plan for Winning the Battle Against Air Pollution from 2018 to 2020. The PM 2.5 concentration can be significantly influenced by meteorological conditions, and the effectiveness of anthropogenic emissions reduction on PM 2.5 over the North China Plain remains unclear. In this study, the Nested Air Quality Model System (NAQPMS) and observations were used to quantitatively examine the contribution of meteorological and emission variations to autumn-winter (November to February) PM 2.5 concentrations in Beijing and its surrounding regions from 2018 to 2020. The model reproduced the temporal evolution of atmospheric pollutants and components well. The results showed that the observed mean PM 2.5 concentrations during the winter were reduced by 15% and 29% in 2019 and 2020, respectively, compared with that in 2018. In 2019, the meteorological conditions over the Beijing-Tianjin-Hebei (BTH) region were poor, causing an increase of PM 2.5 by 7.6% in BTH and 11% in Beijing; therefore, the control of air pollutant emission compensated for the unfavorable influence of meteorology and emission reduction was the decisive factor for PM 2.5 reduction; In 2020, the meteorological conditions were relatively favorable, and the changes in meteorological factors and emission reduction led to a decrease in PM 2.5 concentration by 18.6% and 10.5%, respectively, compared to 2018. Emission reduction also contributed to decrease in nitrate, ammonium, organic matter, element carbon and precursors. This work confirmed the obvious environmental benefit of pollution control measures from 2018 to 2020. There was obvious regional heterogeneity in the effect of meteorology and the impact of adverse meteorological conditions should be considered during the formulation of air pollution control measures. The results can serve as a reference for the formulation of air quality objectives or the evaluation of the environmental effect of pollution control schemes. • The effect of winter meteorology and control measures in 2018 to 2020 was assessed. • Observed winter PM 2.5 reduced by 15% and 29% in 2019 and 2020, respectively, compared to 2018. • Unfavorable meteorology in 2019 partially counteracted the emission control effects. • Emission mitigation played an important role in air quality improvement. [ABSTRACT FROM AUTHOR]