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The influence of the underlying surface on the atmospheric environment over rivers and lakes is not fully understood. To improve our understanding, this study targeted the Twain-Hu Basin (THB) in central China, with a unique underlying surface comprising a dense "water network" over rivers and lakes. In this study, the Weather Research and Forecasting Model with Chemistry (WRF-Chem) was used to simulate the impact of this dense "water network" on a wintertime heavy PM2.5 pollution event in the THB. On this basis, the regulating effects of density and area of the lake groups, with centralized big lakes (CBLs) and discrete small lakes (DSLs), on PM2.5 concentrations over the underlying surface of the dense "water network" in the THB were clarified, and the relative contributions of thermal factors and water vapor factors in the atmospheric boundary layer to the variation of PM2.5 concentrations were evaluated. The results show that the underlying surface of dense "water networks" in the THB generally decreases the PM2.5 concentrations, but the influences of different lake-group types are not uniform in spatial distribution. The CBLs can reduce the PM2.5 concentrations over the lake and its surroundings by 4.90–17.68% during the day and night. The ability of DSLs in reducing PM2.5 pollution is relatively weak, with the reversed contribution between −5.63% and 1.56%. Thermal factors and water vapor–related factors are the key meteorological drivers affecting the variation of PM2.5 concentrations over the underlying surface of dense "water networks". The warming and humidification effects of such underlying surfaces contribute positively and negatively to the "purification" of air pollution, respectively. The relative contributions of thermal factors and water vapor–related factors are 52.48% and 43.91% for CBLs and 65.96% and 27.31% for DSLs, respectively. The "purification" effect of the underlying surface with a dense "water network" in the THB on regional air pollution highlights the importance of environmental protection of inland rivers and lakes in regional environmental governance. In further studies on the atmospheric environment, long-term studies are necessary, including fine measurements in terms of meteorology and the environment and more comprehensive simulations under different scenarios. [ABSTRACT FROM AUTHOR]

Vertical wind shear (VWS) significantly impacts the vertical mixing of air pollutants and leads to changes in near-surface air pollutants. We focused on Changsha (CS) and Jingmen (JM), the upstream and downstream urban sites of a receptor region in central China, to explore the impact of VWS on surface PM2.5 changes using 5-year wintertime observations and simulations from 2016–2020. The surface PM2.5 concentration was lower in CS with higher anthropogenic PM2.5 emissions than in JM, and the correlation between wind speed and PM2.5 was negative for clean conditions and positive for polluted conditions in both two sites. The difference in the correlation pattern of surface PM2.5 and VWS between CS and JM might be due to the different influences of regional PM2.5 transport and boundary layer dynamics. In downstream CS, the weak wind and VWS in the height of 1–2 km stabilized the ABL under polluted conditions, and strong northerly wind accompanied by enhanced VWS above 2 km favored the long-range transport of air pollutants. In upstream JM, local circulation and long-range PM2.5 transport co-determined the positive correlation between VWS and PM2.5 concentrations. Prevailed northerly wind disrupted the local circulation and enhanced the surface PM2.5 concentrations under polluted conditions, which tend to be an indicator of regional transport of air pollutants. The potential contribution source maps calculated from WRF-FLEXPART simulations also confirmed the more significant contribution of regional PM2.5 transport to the PM2.5 pollution in upstream region JM. By comparing the vertical profiles of meteorological parameters for typical transport- and local-type pollution days, the northerly wind prevailed throughout the ABL with stronger wind speed and VWS in transport-type pollution days, favoring the vertical mixing of transported air pollutants, in sharp contrast to the weak wind conditions in local-type pollution days. This study provided the evidence that PM2.5 pollution in the Twain-Hu Basin was affected by long-distance transport with different features at upstream and downstream sites, improving the understanding of the air pollutant source–receptor relationship in air quality changes with regional transport of air pollutants. [ABSTRACT FROM AUTHOR]

Regional transport of air pollutants is modulated by large‐scale synoptic circulation, which has been poorly understood for heavy air pollution in the downstream receptor region. In the present study of 5‐year (2015–2019) observation, we targeted the Twain‐Hu Basin (THB), a region of heavy PM2.5 pollution over central China, to investigate the regulation of synoptic circulation governing regional PM2.5 transport for heavy air pollution. It was found that regional transport of PM2.5 predominated 65.2% of the heavy pollution events (HPEs) over the THB based on the statistics of observational environment and meteorology. By employing the FLEXPART‐WRF model, the regional transport of PM2.5 from upwind areas in central and eastern China (CEC) to receptor region in the THB was identified with three prominent pathways in the northerly, northeasterly, and easterly directions respectively. We then used T‐mode principal component analysis in conjunction with the K‐means cluster method to categorize synoptic circulations, and it was recognized that three regional PM2.5 transport pathways for the HPEs over central China were governed respectively by three patterns of synoptic circulation over CEC with (a) weak high air pressure to the north (13.3%), (b) strong high air pressure to the northeast (46.7%), and (c) weak high air pressure to the east (40.0%). This study reveals a significant modulation of large‐scale synoptic circulation for regional transport of air pollutants in environmental change. Plain Language Summary: Compared to emission source regions (e.g., North China Plain), the modulation of synoptic circulation on regional transport of PM2.5 in heavy air pollution over the downwind receptor region has been poorly understood. In this study, therefore, we investigated the heavy air pollution events (HPEs) in the Twain‐Hu Basin (THB) over central China–a critical regional transport hub–from 2015 to 2019, by using the model FLEXPART‐WRF and the T‐mode principal component analysis (T‐PCA) combined with the K‐means cluster method. The majority of the HPEs in THB were triggered by regional transport of PM2.5 due to the southward invasion of cold air mass from upwind regions driven by East Asian winter monsoonal winds. Based on FLEXPART‐WRF simulations, regional PM2.5 transport was characterized along three routes in the northerly, northeasterly and easterly directions with the averaged contribution of 57.9% to PM2.5 concentrations for HPEs in the THB, which is closely related to evolution of synoptic circulation. By utilizing T‐PCA combined with the K‐means cluster method, three synoptic circulation types were identified to build three regional PM2.5 transport pathways, revealing an important regulation of large‐scale atmospheric circulations on regional air pollutant transport causing HPEs over central China. Key Points: Heavy air pollution events induced by regional PM2.5 transport presents an increasing trend over the past 5 years in central ChinaThree regional transport pathways are identified to central China in the northerly, northeasterly, and easterly directions respectivelySynoptic circulation modulates regional transport in air quality change with the large contribution to air pollutants over central China [ABSTRACT FROM AUTHOR]