Aerosol-cloud-precipitation interactions remain a major source of uncertainty in atmospheric science, especially in complex terrain where topography can significantly modulate these effects. This study investigates aerosol impacts on precipitation systems migrating from mountains to plains in Northern China during early autumn 2021 using coupled meteorology-chemistry simulations validated against comprehensive observational datasets. While synoptic forcing remains the primary driver of precipitation initiation, aerosols exert a net invigorating effect on peak precipitation events over the plains. This enhancement shows pronounced spatial heterogeneity and a distinct diurnal cycle, with peak impacts typically occurring during the afternoon when convective instability is highest, and persisting into the evening. Microphysical analysis identifies cloud condensation nuclei activation as the dominant pathway, leading to latent heat release from enhanced ice-phase processes that exceeds radiative effects and strengthens updrafts. Crucially, topography modulates these interactions through terrain blocking, which concentrates pollutants in the plains, and a “mountain chimney effect” that transports aerosols to elevated layers via daytime upslope flows and downslope advection from highland sources. These terrain-mediated processes create favorable conditions for aerosol-induced invigoration as precipitation systems propagate toward the North China Plain. Our findings underscore the vital role of three-dimensional topographic modulation in aerosol-cloud interactions and highlight the need to resolve terrain-driven aerosol transport in regional weather and climate models.

Severe wintertime PM2.5 pollution persists in the Urumqi-Changji-Shihezi urban agglomeration, China's largest arid-region urban cluster, despite emission controls that have improved air quality in eastern China. Using air quality monitoring data, radiosonde observations, and chemical transport model simulations for two winters (2022-2024), we investigated the combined roles of synoptic-scale circulation and basin terrain in modulating pollutant accumulation. Results show that PM2.5 pollution exhibits strong spatial synchrony among Urumqi, Changji, and Shihezi, indicating dominant regional meteorological control beyond local emissions. Stagnant westerly circulation with warm advections aloft fosters persistent cold-air pool within the Junggar Basin, producing strong thermal inversions, shallow boundary layers, and PM2.5 accumulation in the urban agglomeration. In contrast, Siberian cold surges and northwesterly flows channel cold air through low-elevation gaps on the basin's north and west margins, weakening stratification, enhancing surface winds via horizontal and partial downward momentum transport, and deepening the boundary layer, which promotes pollutant dispersion. These findings highlight the critical interplay between large-scale forcing and complex terrain in controlling air quality in arid basins and provide a scientific basis for improved forecasting and targeted mitigation strategies.

Tangshan, a major industrial and agricultural center in northern China, frequently experiences significant PM2.5 pollution events during winter, impacting its large population. These pollution episodes are influenced by multi–scale meteorological processes, though the complex mechanisms remain not fully understood. This study integrates surface PM2.5 concentration data, ground-based and upper–air meteorological observations, and ERA5 reanalysis data from 2015 to 2019 to explore the interactions between local planetary boundary layer (PBL) structures and large-scale atmospheric processes driving PM2.5 pollution in Tangshan. The results indicate that seasonal variations in PM2.5 pollution levels are closely linked to changes in PBL thermal stability. During winter, day–to–day increases in PM2.5 concentrations are often tied to atmospheric warming above 1500 m, as enhanced thermal inversions and reduced PBL heights lead to pollutant accumulation. Regionally, this aloft warming is driven by a high-pressure system at 850 hPa over the southern North China Plain, accompanied by prevailing southwesterly winds. Additionally, southwesterly winds within the PBL can transport pollutants from the adjacent Beijing–Tianjin–Hebei region to Tangshan, worsening pollution. Simulations from the chemical transport model indicate that regional pollutant transport can contribute to approximately half of the near-surface PM2.5 concentration under the unfavorable synoptic conditions. These findings underscore the importance of multi-scale meteorology in predicting and mitigating severe wintertime PM2.5 pollution in Tangshan and surrounding regions.

Urban surfaces exert profound influences on local wind patterns, turbulence dynamics, and the dispersion of air pollutants, underscoring the critical need for a thorough understanding of these processes in the realms of urban planning, design, construction, and air quality management. The advent of advanced computational capabilities has propelled the computational fluid dynamics model (CFD) into becoming a mature and widely adopted tool to investigate microscale meteorological phenomena in urban settings. This review provides a comprehensive overview of the current state of CFD-based microscale meteorological simulations, offering insights into their applications, influential factors, and challenges. Significant variables such as the aspect ratio of street canyons, building geometries, ambient wind directions, atmospheric boundary layer stabilities, and street tree configurations play crucial roles in influencing microscale physical processes and the dispersion of air pollutants. The integration of CFD with mesoscale meteorological models and cutting-edge machine learning techniques empowers high-resolution, precise simulations of urban meteorology, establishing a robust scientific basis for sustainable urban development, the mitigation of air pollution, and emergency response planning for hazardous substances. Nonetheless, the broader application of CFD in this domain introduces challenges in grid optimization, enhancing integration with mesoscale models, addressing data limitations, and simulating diverse weather conditions.

The layout of urban buildings shows significantheterogeneity, which leads to the significant spatial inhomogeneity of thewind field in and over the canopy of urban street canyons. However, most ofthe current urban canopy models do not fully consider the heterogeneity ofthe urban canopy. Large discrepancies thus exist between the wind speedssimulated by the current urban canopy models and those observed in thestreet canyon. In this study, a parameterization scheme for wind fields,Inhomogeneous Wind Scheme for Urban Street (IWSUS), is developed to bettercharacterize the heterogeneity of the urban canopy. We use a computationalfluid dynamics method to generate the IWSUS scheme and compare it withobservations of the wind profile and turbulent flux in and over the streetcanyon for validation. In IWSUS, the wind speed vertical profiles at sixrepresentative positions located in a typical street canyon (i.e., thewindward or leeward side of a long straight street or the inflow or outflowend) are parameterized separately. The wind profile by IWSUS thus can betterdescribe the horizontal heterogeneity of the urban near-surface wind field,e.g., the dynamic drag effect of buildings in the lower atmospheric layer overthe urbanized land use. The validation based on observations shows that theperformance of simulation results by IWSUS is better than that bythe exponential-logarithmic (exp-log) law widely used in the current urbanschemes. We consider typical building arrangement and specific streetorientations in IWSUS for wind field simulations, which can better match thedistribution characteristics of street canyons around the observation pointin the street canyon. The averaged wind profiles and turbulence energyfluxes in the model grids of urban areas by IWSUS are also nearer to theobservations than those by the exp-log law. The normalized mean errors (NMEs)between the simulated and the observed vertical average wind speed are49.0 % for IWSUS and 56.1 % for exp-log law in the range from the groundto 4 times the average height of the buildings and 70 % for IWSUS and285.8 % for exp-log law in the street canyon (range from the ground tobuilding top). This study proves that the accuracy of simulations of land surface processes and near-ground meteorological processes over the urban canopy can be improved by fully considering the heterogeneity of the urban canopy layout structures and the inhomogeneity of wind field distributions in andover the street canyon. IWSUS is expected to be coupled with mesoscaleatmospheric models to improve the accuracy of the wind field, land surfaceenergy budget, meteorological and atmospheric chemistry simulations.

Both extreme heat and heavy air pollution can cause adverse health impacts on urban inhabitants. To understand heat stress and its relationship with boundary layer structure and air pollution in Beijing, this study analyzed surface meteorological observations, radiosonde measurements, and ground-level PM2.5 and O3 concentrations in summer from 2015 to 2019, in conjunction with simulated air quality and MERRA-2 data. We measured the heat stress using a heat index that combines temperature and humidity to quantify the sensible temperature as perceived by humans, and found that high heat risk in Beijing was often associated with a low boundary layer height and poor air quality. To reveal the underlying physical mechanism involved, we objectively classified the synoptic conditions in North China. The typical synoptic pattern associated with the coupling of heat and pollution in Beijing was found to feature a southeast-to-north pressure gradient at the 700-hPa level, leading to westward warm advection above planetary boundary layer (PBL) and southward movement of warm, humid, and polluted air masses within the PBL towards Beijing. The elevated warm advection can enhance the capping of thermal inversion over the PBL and suppress the PBL's development and the vertical dispersion of pollutants. With mountains to the north and west, pollutants and heat can be trapped in a limited volume in Beijing, increasing the health risk from heat and pollution. These findings on the meteorological mechanisms of the coupling between heat and pollution in Beijing may have important implications for limiting the current health risk and preparing for any projected changes in it in the future.

Heavy PM2.5 (particulate matter with an aerodynamics diameter less than 2.5 μm) pollution frequently happens in Wuhan under unfavorable meteorological conditions. To comprehensively understand the complex impact of both regional-scale synoptic forcing and local-scale processes within the planetary boundary layer (PBL) on air quality in Wuhan, this study analyzed long-term PM2.5 concentration measurement, near-surface and upper-air meteorological observations from March 2015 to February 2019, in combination with the Modern-Era Retrospective Analysis for Research and Applications version 2 (MERRA-2). We found that in winter the day-to-day change of PM2.5 pollution level in Wuhan was governed by the synoptic warm/cold advection, in addition to the high emissions of pollutants. The synoptic condition can largely determine both the vertical development of PBL and horizontal transport of pollutants. When a high-pressure system is located to the north of Wuhan at the 900-hPa level, the induced warm advection above 1,000 m can enhance the thermal stability of lower troposphere and inhibit the development of daytime PBL, leading to a decreased dispersion volume for pollutants. Meanwhile, within the PBL the pollutants emitted from Henan, Shandong, and Anhui provinces can be transported to Wuhan, further worsening the pollution. Our results highlight the importance of coordinated pollution controls in Central China and adjacent north regions under the unfavorable synoptic condition.

The CFD simulation results of parallel scenerios.

基于昆明市1965-2016年逐日降水资料,计算昆明市汛期(5-10月)的长周期旱涝急转指数、短周期旱涝急转指数和旱涝急转强度,采用小波分析、M-K突变检验、灰色关联度等方法,分析了昆明市的旱涝急转特征及其与城市化的相关性.结果表明:在1965-2016年,昆明市长周期旱涝急转指数LD-FAI(Long-term drought-flood abrupt alternations index)呈-0.066·(10a)-1 的下降趋势,反映出昆明市的旱转涝事件减少、涝转旱事件增多的状况,并且昆明市汛期长周期旱涝急转指数LDFAI序列出现2个突变点,但突变并不明显;昆明市长周期旱涝急转指数存在18年的主周期变化;从短周期旱涝急转指数SD-FAI(Short-term drought-flood abrupt alternations index)来看,昆明市5-7月和8-9月旱转涝事件增多、涝转旱事件减少,而7-8月和9-10月表现出相反的趋势;从旱涝急转与城市化的相关性来看,在分辨率为0.5时,城市化指标的6个因子对旱涝急转产生不同程度的影响,所有指标关联度都在0.6以上,表明城市化指标与旱涝急转显著关联,与旱涝急转强度关联度最大是烟粉尘排放量,最小的是非农业经济总产值,关联度分别为0.91和0.63.

The rapid development in the economy during past decades has caused serious air pollution issues in China with high concentrations of PM2.5 and O3, particularly in the densely populous cities. To integrate PM2.5 and O3 controls, it is necessary to understand the impacts of meteorology on both pollutants. Thereby, the complex linkages between planetary boundary layer (PBL), synoptic forcing, regional transport, and heavy pollution in Beijing and Shanghai during summer were investigated using long-term measurements, simulations, and reanalysis. Influenced by the unfavorable meteorological conditions, PM2.5 pollution and O3 pollution often simultaneously occurred. In Beijing, the heavy concurring pollutions usually happened on the days with shallow afternoon PBL and southerly/southwesterly prevailing winds. Within the PBL, the pollutants emitted from the southern plains can be transported to Beijing and accumulated on the windward side of the mountains. At the top of PBL, the synoptic southerly warm advections can strengthen the elevated thermal inversion layer and suppress the development of PBL, leading to worse pollution. Contrarily, the heavy pollutions in Shanghai usually occurred on the days with deep afternoon PBL and southwesterly warm advections within the PBL. Although the warm advections were more favorable to the PBL development than the movements of cool marine air mass, the input of pollutants from the southwest can overweigh this advantage, resulting in poor air quality in Shanghai. The occurrence of heavy pollution or clean condition in Shanghai was primarily determined by the synoptic forcing rather than the local PBL structure. This comparative study indicates that the relationship between PBL height and pollution level is changeable and complicated, which needs to be elucidated from the synoptic perspective.

Partly due to the lack of fine-resolution measurements of the planetary boundary layer (PBL), the impacts of PBL on the aerosol pollution in the densely populous Shandong province were not well understood. On the basis of long-term PM2.5 measurements, fine-resolution radiosonde data, and meteorological reanalysis from April 2016 to March 2019, the aerosol pollution in Jinan and Qingdao and its complex relationships with the multi-scale meteorological conditions were investigated in this study. During an annual cycle, prominent seasonal variations of PM2.5 concentrations can be observed in both cities, with heaviest pollution in the heating season and relatively low concentrations in summer. Significant positive correlationwas found between the monthly PM2.5 concentrations and thermal stability of the lower troposphere, indicating that the seasonal shifts of PBL play an important role in regulating the variations of aerosol pollution, in addition to the seasonal changes in the emissions. In the heating season, influenced by unfavorable synoptic patterns, heavy pollution often simultaneously happened in Jinan and Qingdao. Utilizing an objective synoptic classification approach with reanalysis data, two dominant synoptic types led to heavy pollution in Jinan and Qingdao were identified, which were featured by 900-hPa warm advections from the west or southwest with weaker prevailing winds. These synoptic types not only strengthened the elevated thermal inversion and inhibited the vertical dilution of pollutants locally, but also caused the regional transports of pollutants to Jinan and Qingdao from high-emission upstream regions, such as the Beijing-Tianjin-Hebei region, Henan province and Jiangsu province. Therefore, to prevent heavy pollution in Jinan and Qingdao, regional joint measures should be implemented with full consideration of synoptic impact.

Severe haze events occur frequently in China, characterized by exceedingly high concentration of fine particulate matter (smaller than 2.5 mu m, or PM2.5). These extremes are caused by synthetic physical-chemical processes, including emissions, chemical formation, planetary boundary layer processes, regional circulation, weather and climate. These processes are multi-scales, ranging from nanometers to thousands of kilometers. The complex interplays among these processes make it more difficult to understand the formation of severe haze events, which also influences model development and weather forecast. Here, we review the contributions of dominant mechanisms in severe haze events, especially their roles in temporal oscillations of PM2.5 concentrations, and discuss the interplays among these atmospheric multi-scale processes. Previous studies indicate that: (1) Secondary aerosols become the dominant components in aerosols. Heterogeneous aqueous reactions play important roles in gas-to-particle conversion, especially in the late haze period; (2) PM2.5 owns extensive temporal oscillations (on daily, weekly, to monthly timescales), which are caused by multi-scale processes; (3) high aerosols in China have already influenced photochemistry, boundary layer, weather and climate processes. The complex interplays among aerosols and the above processes make it more difficult to unravel the causes, mechanisms, and trends for haze pollution. In the future, the following issues should be considered: cooperative observations of aerosols, gas pollutants, photochemistry, and meteorological variables should be strengthened, especially their vertical distribution in the troposphere; multidisciplinary research should be strengthened, especially among atmospheric physics, chemistry, weather and climate; and model simulations related to the interplays among aerosols and atmospheric physical and chemical processes should be strengthened.

Rapid urbanization and industrialization have led to deterioration of air quality in the Beijing–Tianjin–Hebei (BTH) region due to high loadings of PM2.5. Heavy aerosol pollution frequently occurs in winter, in close relation to the planetary boundary layer (PBL) meteorology. To unravel the physical processes that influence PBL structure and aerosol pollution in BTH, this study combined long-term observational data analyses, synoptic pattern classification, and meteorology–chemistry coupled simulations. During the winter of 2017 and 2018, Beijing and Tangshan often experienced heavy PM2.5 pollution simultaneously, accompanied by strong thermal inversion aloft. These concurrences of pollution in different cities were primarily regulated by the large-scale synoptic conditions. Using principal component analysis with geopotential height fields at the 850 hPa level during winter, two typical synoptic patterns associated with heavy pollution in BTH were identified. One pattern is characterized by a southeast-to-north pressure gradient across BTH, and the other is associated with high pressure in eastern China. Both synoptic types feature warmer air temperature at 1000 m a.g.l., which could suppress the development of the PBL. Under these unfavorable synoptic conditions, aerosols can modulate PBL structure through the radiative effect, which was examined using numerical simulations. The aerosol radiative effect can significantly lower the daytime boundary layer height through cooling the surface layer and heating the upper part of the PBL, leading to the deterioration of air quality. This PBL–aerosol feedback is sensitive to the aerosol vertical structure, which is more effective when the synoptic pattern can distribute more aerosols to the upper PBL.

Basing on the nearly 50-year precipitation data of 4 major city (Kunming, Qujing, Yuxi, Chuxiong) in urban agglomeration of central Yunnan, this paper analyzes the variation of precipitation in annual, seasonal, and monthly time scale by using methods including the statistical analysis, the wavelet analysis, accumulative leveling method, and the grey relational analysis. The result shows that the average precipitation in past 50 years is 928.1 mm. The trend of precipitation is declining, and a mutation appears in 2009. Besides, precipitation has a dry and wet season in these region, and the wet season is between May and October but the dry season is between November and April of next year, There are more precipitation in the rainy season and less precipitation in dry season. The maximum of precipitation (184.0 mm) appears in July, but the minimum of precipitation (13.8 mm) appears in December. According to the wavelet analysis, the change of annual precipitation is about 22 years, the summer precipitation has a 19-day increase-decrease cycle with more precipitation and smaller variation, and the winter precipitation has a 20-day increase-decrease cycle with less precipitation and larger variation. The grey relational analysis show that the temperature and the specific humidity are dominant natural factors, and the population and the city size are dominant human factors. Besides, the natural factors play greater role than the artificial factors.

根据1993-2016年昆明市24 a太阳辐射及其他相关资料,采用回归分析、Mann-Kendall突变检验、小波分析和灰色关联度等方法,分析昆明市地面太阳辐射变化特征,并对其影响因子进行了分析.结果 表明,近24a来昆明市地面接收年太阳辐射量在4620.21～6221.65MJ/m2之间变化,最大值出现在2016年,最小值出现在2008年,平均每年的太阳辐射为5598.06 MJ/m2,且整体上呈现波动式上升趋势.昆明市干湿季分明,干季(11月一次年4月)地面得到的太阳辐射量较多,为2825.22MJ/m2,雨季(5-10月)较少,为2774.41MJ/m2;一年中地面接收太阳辐射量最多月份在4月,其值为604.75MJ/m2,最少的是12月,其值为361.01MJ/m2,相差243.74MJ/m2.地面接收的太阳辐射有一个突变点为2014年,但不明显.年小波分析表明,近24 a昆明市各年地面接收的太阳辐射量主周期为15a;季节小波分析发现,干季比雨季振幅大,其主周期都为19d.近24 a昆明市地面太阳辐射受自然和人为因子共同影响,其中自然因子对太阳辐射影响最大的是降水量,人为因子中对太阳辐射影响最大的是工业总产值,且自然因子的作用大于人为因子.

China suffers from high levels of PM2.5 pollution, which is often exacerbated by unfavorable planetary boundary layer (PBL) structures. Partly due to a lack of appropriate observations, the PBL-aerosol linkages in China are not clearly understood. Thus, we investigated these linkages from a national perspective using sounding data collected from 2014 to 2017. Correlation analyses revealed a significant anti-correlation between monthly boundary layer height (BLH) and aerosol pollution that was ubiquitous across China, indicating the important role of the PBL in regulating the seasonal variations of pollution in China. Besides, the day-to-day variations in pollution were modulated by the daily variabilities in the PBL structure. During winter, highly polluted days in most of the Chinese cities studied were associated with a low BLH, strong thermal stability, and weak PBL winds. In the North China Plain and Northeast China, the wintertime heavy pollution was often related to southerly winds and moister PBL. This study has important implications for understanding the crucial role that the PBL plays in modulating aerosol pollution in China.

The day-to-day variations in the planetary boundary layer (PBL) structure and air quality are closely governed by large-scale synoptic forcings. Partly due to the lack of long-term PBL observations during the winter in Beijing, the complex relationships between the large-scale synoptic patterns, local PBL structures/processes, and PM2.5 pollution have not been fully understood. Thus, this study systematically investigated these linkages by combining aerosol measurements, surface meteorological observations, radiosonde data, reanalysis, long-term three-dimensional meteorological simulations, and idealized meteorology-chemistry coupled simulations. Based on the validated long-term simulation results, the boundary layer height (BLH) in Beijing during two winters from 2013 to 2015 was calculated and compared with PM2.5 measurements. A significant anti-correlation was found between the daily BLH and PM2.5 concentration in Beijing, indicating the importance of the PBL structure on the variations in the aerosol pollution levels. Those days with low BLHs are often accompanied by a strong elevated thermal inversion layer. Based on the daily 900-hPa geopotential height fields, seven synoptic patterns were identified using an objective approach, in which two types were found to be associated with heavy PM2.5 pollution in Beijing. One pattern was characterized by weak northwesterly prevailing winds and a strong elevated thermal inversion layer over Beijing, and the local emissions of aerosols played a decisive role in the formation of heavy pollution. The other pattern was associated with southerly prevailing winds, which could transport the pollutants emitted from southern cities to Beijing. According to the meteorology-chemistry coupled simulations, southerly regional transportation can contribute approximately 56% of the PM2.5 in Beijing. The results of this study have important implications for understanding the crucial roles that multiscale meteorological factors play in modulating the aerosol pollution in Beijing during the winter.

Beijing experiences frequent PM2.5 pollution, which is influenced by the planetary boundary layer (PBL) structure/process. Partly due to a lack of appropriate observations, the impacts of PBL on PM2.5 pollution are not yet fully understood. Combining wind-profiler data, radiosonde measurements, near-surface meteorological observations, aerosol measurements, and three-dimensional simulations, this study investigated the influence of PBL structure and the low-level jet (LLJ) on the pollution in Beijing from 19 to 20 September 2015. The evolution of the LLJ was generally well simulated by the model, although the wind speed within the PBL was overestimated. Being influenced by the large-scale southerly prevailing winds, the aerosols emitted from the southern polluted regions could be easily transported to Beijing, contributing to ~68% of the PM2.5 measured in Beijing on 20 September. The relative contribution of external transport of PM2.5 to Beijing was high in the afternoon (≥80%), which was related to the strong southerly PBL winds and the presence of thermally-induced upslope winds. On 20 September, the LLJ in Beijing demonstrated a prominent diurnal variation, which was predominant in the morning and after sunset. The occurrence of the LLJ could enhance the dilution capacity in Beijing to some extent, which favors the dilution of pollutants at a local scale. This study has important implications for better understanding the complexity of PBL structure/process associated with PM2.5 pollution in Beijing.

Purpose of Review During the past decades, the number and size of megacities have been growing dramatically in China. Most of Chinese megacities are suffering from heavy PM 2.5 pollution. In the pollution formation, the planetary boundary layer (PBL) plays an important role. This review is aimed at presenting the current state of understanding of the PBL-PM 2.5 interaction in megacities, as well as to identify the main gaps in current knowledge and further research needs. Recent Findings The PBL is critical to the formation of urban PM 2.5 pollution at multiple temporal scales, ranging from diurnal change to seasonal variation. For the essential PBL structure/process in pollution, the coastal megacities have different concerns from the mountainous or land-locked megacities. In the coastal cities, the recirculation induced by sea-land breeze can accumulate pollutants, whereas in the valley/basin, the blocking effects of terrains can lead to stagnant conditions and thermal inversion. Within a megacity, although the urbanization-induced land use change can cause thermodynamic perturbations and facilitate the development of PBL, the increases in emissions outweigh this impact, resulting in a net increase of aerosol concentration. Moreover, the aerosol radiative effects can modify the PBL by heating the upper layers and reducing the surface heat flux, suppressing the PBL and exacerbating the pollution. Summary This review presented the PBL-PM 2.5 interaction in 13 Chinese megacities with various geographic conditions and elucidated the critical influencing processes. To further understand the complicated interactions, long-term observations of meteorology and aerosol properties with multi-layers in the PBL need to be implemented.

The northeastern China frequently experiences severe aerosol pollution in winter under unfavorable meteorological conditions. How and to what extent the meteorological factors affect the air quality there are not yet clearly understood. Thus, this study investigated the impacts of synoptic patterns on the aerosol transport and planetary boundary layer (PBL) structure in Shenyang from 1 to 3 December 2016, using surface observations, sounding measurements, satellite data, and three-dimensional simulations. Results showed that the aerosol pollution occurred in Shenyang was not only related to the local emissions, but also contributed by trans-boundary transport of aerosols from the Beiijng-Tianjin-Hebei (BTH) region. In the presence of the westerly and southwesterly synoptic winds, the aerosols emitted from BTH could be brought to Shenyang. From December 2 to 3, the aerosols emitted from BTH accounted for ∼20% of near-surface PM2.5 in Shenyang. In addition, the large-scale synoptic forcings could affect the vertical mixing of pollutants through modulating the PBL structure in Shenyang. The westerly and southwesterly synoptic winds not only brought the aerosols but also the warmer air masses from the southwest regions to Shenyang. The strong warm advections above PBL could enhance the already existing thermal inversion layers capping over PBL in Shenyang, leading to the suppressions of PBL. Both the trans-boundary transport of aerosols and the suppressions of PBL caused by the large-scale synoptic forcings should be partly responsible for the poor air quality in Shenyang, in addition to the high pollutant emissions. The present study revealed the physical mechanisms underlying the aerosol pollution in Shenyang, which has important implications for better forecasting and controlling the aerosols pollution.