Terrain-driven Three-dimensional Evolutions Of PM_2.5 And O₃ With The Related Mechanisms Over The Sichuan Basin

Sichuan Basin（SCB）is a key polluted region in China.Owing to its special deep-basin terrain geographically surrounded by the adjoining Tibetan Plateau（TP）,Yunnan-Guizhou Plateau and mountains,special changes in the atmospheric environment and meteorology exist over the SCB.In particular,there is a lack of systematic and scientific cognition on the three-dimensional evolution and seasonal variation of topographic forcing effects on atmospheric compound pollutants(PM_2.5 and O₃)in the SCB.Here,this research focuses on the terrain-forcing 3D variation of PM_2.5 and O₃ in the SCB by using multi-source observation data and WRF-Chem simulation to analyze the spatial and temporal variations of PM_2.5 and O₃ in recent years;investigate the impacts of terrain-driven atmospheric processes on seasonal variations of PM_2.5 and O₃ with meteorological mechanisms;reveal the stratosphere-troposphere-boundary transport of O₃ with the quantitative contribution for SCB’s air quality and terrain-driven mechanisms.The major conclusions are as follows:（1）Spatial and temporal evolution of air pollutants PM_2.5 and O₃ in the Sichuan BasinAccording to ground-based environmental observations,regional air pollution in the SCB is mainly characterized by seasonal shifts between O₃ pollution in the warm season and PM_2.5 pollution in the cold season.The aerosol optical depth(AOD_550nm)in the SCB experienced an increasing tendency at a rate of+0.052 yr^-1 during 2001–2006;thereafter,it decreased speedy up to–0.058 yr^-1 over 2013-2020,whereas the interannual variation in?ngstr?m exponent(AE_470–660nm)presented a persistently increasing trend during 2001–2020,with a rate of+0.014 yr^-1.An improved atmospheric environment but an enhanced fine particle contribution to regional aerosols in the SCB were observed,representing its great meanings for precise environmental management in the future.Over the polluted SCB region,the dominant aerosol types were biomass burning/urban industrial and mixed-type aerosols with proportions of80.7%–87.5%in regional aerosols.A significant decline in polluted anthropogenic aerosols was observed below 3 km over the SCB,resulting in the regional aerosol extinction coefficients at 532 nm(EC_532nm)declining by–0.22 km^-1 from 2013 to 2020.Notably,the natural aerosol enhancement with decreases in aerosol radiative forcing within the atmosphere was found in the SCB after 2013.This indicates that an attenuated aerosol heating rate in the atmosphere,caused by aerosol variation,could alter the atmospheric thermal structure over the SCB for air pollutants accumulation in regional changes of environment and climate in recent years.（2）Vertical“sandwich”structure of PM_2.5 over SCB and the impact of terrain-forcing circulationsCombining with the observed facts and WRF-Chem model,it was found that the vertical structure of PM_2.5 over SCB was characterized by a remarkable hollow sandwiched by high PM_2.5 layers at heights of 1.5–3 km and a highly polluted near-surface layer.The southwesterlies over the Tibetan Plateau and Yunan-Guizhou Plateau resulted in a lee vortex over the SCB,which helped form and maintain heavy PM_2.5pollution.The basin PM_2.5 was lifted into the free troposphere and transported outside of the SCB.At the bottom of the SCB,high PM_2.5 concentrations were mostly located in the northwestern and southern regions.Due to the blocking effect of the plateau terrain on the northeasterly winds,PM_2.5 gradually increased from northeast to southwest in the basin.In the lower free troposphere,the high PM_2.5 centers were distributed over the northwestern and southwestern SCB areas,as well as the central SCB region.For this event,the regional emissions from SCB contributed 75.4–94.6%to the surface PM_2.5 concentrations in SCB.The SCB emissions were the major source of PM_2.5 over the surrounding plateaus,with contribution rates of 72.7%,during the dissipation stage of heavy air pollution over SCB.Due to the impacts of plateau-basin topography on East Asian monsoon and westerly interaction,such a special 3D PM_2.5structure was triggered in the SCB,which contributes a critical influence to atmospheric environmental changes in Southwest China.（3）Impact of deep basin terrain on seasonal PM_2.5 distribution over SCB and the meteorological mechanismIt is characterized that the three-dimensional distribution of topography-induced PM_2.5 concentrations over the SCB with the seasonal shift of regional PM_2.5 averages from approximately 30μg m^-3 in summer to 90μg m^-3 in winter at the surface layer and from summertime 10μg m^-3 to wintertime 30μg m^-3 in the lower free troposphere.Such basin-forced PM_2.5 changes presented the vertically monotonical declines concentrated within the lower troposphere below 3.6 km in spring,2.3 km in summer,2.6 km in autumn and 4.8 km in winter.In the SCB,atmospheric thermal structure in the lower troposphere could build a vertical convergence layer between the boundary layer and free troposphere,acting as a lid inhibiting air diffusion,which was regulated by the terrain effects on interactions of westerlies and Asian monsoons,especially the wintertime strong warm lid deteriorating air pollution in the SCB.Impacts of deep basin aggravated PM_2.5 accumulation within the SCB and transport toward the surrounding plateaus,contributing approximately 50–90%to PM_2.5 levels over the regions of the eastern Tibetan Plateau（ETP）and northern Yunnan-Guizhou Plateau.Furthermore,warm and humid air conditions within the basin effectively promoted the ratios of sulfur oxidation and nitrogen oxidation for producing the secondary PM_2.5 in the atmospheric environment.（4）Terrain-induced changes in O₃ seasonality and atmospheric physical and chemical processes over SCBThe basin terrain exerts the reversed impacts on atmospheric environment changes by aggravating summertime and alleviating wintertime near-surface O₃ levels with the relative contributions oscillating seasonally between–40%and 40%in SCB.A seasonal shift of vertical O₃ structures is dominated by summertime positive and wintertime negative changes in the lower troposphere induced by basin terrain.In addition,a negative central of O₃ seasonally oscillates from the heights of 4.0-4.5 km in summer to～3 km during winter.Basin terrain significantly contributes to near-surface O₃ changes by altering atmospheric physical and chemical processes of O₃.The terrain-induced horizontal and vertical advection in O₃ transport makes a large contribution to near-surface O₃ changes with a remarkable enhancement at night.The existence of basin terrain enhances the spring and summer O₃ production and inhibits the autumn and winter O₃ production by meteorological modulation and aerosol-radiation forcing,respectively.The terrain effects of deep basin intensify horizontal O₃transport between the western SCB region and ETP in the warm seasons driven while the vertical O₃exchange to the near-surface O₃ levels is enhanced by the nocturnal O₃-rich layer in the SCB region.（5）The contributions of stratospheric O₃ intrusion to air quality changes over SCB with the terrain-driven mechanismsStratospheric O₃ intrusion（SOI）in the TP and its surrounding areas leads to an increase of 3-8 ppb of the near-surface O₃ concentrations in the SCB,contributing11.9%-17.4%to regional environmental changes.The results reveal the important impacts of stratosphere-troposphere-boundary transport of O₃ with terrain forcing between TP and SCB on air quality over the SCB region.Both the TP’s thermal forcing on the South Asian High and the dynamic forcing of TP on the subtropical westerly jet can trigger SOI events.Deep SOI was generated by deepening westerly trough and ridge（strong atmospheric convective activity in plateau）in spring（summer）to 400-600 h Pa while shallow SOI（300-500 h Pa）was ascribed to terrain-driven westerly jet on tropopause disturbance in autumn and winter.Furthermore,TP’s leeward slope effects of the Tibetan Plateau result in downdrafts in the middle troposphere of SCB,which drives high concentrations of O₃ from stratospheric intrusion down to the lower troposphere over SCB.With the terrain effects of SCB on westerlies and Asian monsoon interactions,entrainment process at the top atmospheric boundary drives O₃to enter the boundary layer of SCB,and then changes regional atmospheric environmental changes by vertical mixing.