| Turbulent diffusion and horizontal transport play an important role in the variation of aerosol heavy pollution during the emission stabilization periods.However,there is no aerosol turbulent diffusion term in the current mesoscale meteorological-chemical model,and previous work on the variation mechanism of aerosol heavy pollution process has lacked combining long-term boundary layer vertical structure observations with mesoscale models including aerosol turbulent diffusion term,especially in Anhui region,where the topography is more complex and the pollution process is more susceptible to regional transport,and it is more complicated to understand the mechanism of heavy pollution formation and variation.In this study,we aim at the above-mentioned important issues concerning the mechanism of aerosol heavy pollution change,and the aerosol heavy pollution processes in Anhui Province,a typical region in central China,are selected as the research object.Through the theoretical analysis of turbulent diffusion and horizontal transport and the application of the mesoscale model,we reveal the evolution characteristics of the heavy pollution process and the formation and variation mechanisms in Anhui region,and compare it with the mechanism of heavy pollution change in Beijing region,which is located in the Beijing-Tianjin-Hebei region.Anhui Province is adjacent to the North China Plain(NCP)region(China’s most serious aerosol polluted region)and the Yangtze River Delta(YRD)region(China’s fastest growing economy region and also heavily polluted region),and the special geographical location of this region has an important influence on the transport and diffusion of pollutants.Although there have been many related studies on the evolution of pollution characteristics,most of these studies are limited to short-time individual cases,the mechanism of heavy pollution evolution during heavy pollution episodes(HPEs)in this region is not clear.Meanwhile,the contributions of emissions and meteorological conditions during HPEs in this region are of particular interest to the scientific and technical community,government and the public.In addition,the turbulent transport of momentum-heat-particle during HPEs has some variability,and it is unreasonable to treat the vertical mixing of particles with the heat turbulent diffusion term in the traditional boundary layer parameterization scheme.In this study,the observational analysis of long-term mesoscale boundary layer structure evolution and numerical simulation are closely combined to analyze the variation mechanism of heavy pollution in Anhui from different scales,and to reveal the influence of turbulent diffusion and horizontal transport in the process of heavy pollution by combining high-resolution turbulence observation data and mesoscale model simulation.And by comparing two methods for calculating turbulent diffusion coefficients in the stable boundary layer,the turbulent diffusion coefficient of particle,which were not considered in the mesoscale meteorological-chemical model in the past,are established and embedded in the mesoscale model for online evaluation and validation.The main findings include:(1)In terms of macro-scale circulation situation,Anhui region is mainly controlled by two main types of synoptic patterns: prevailing northwest wind and northeast wind.Pollutants from upstream are first transported to northern Anhui under the influence of prevailing winds.Due to the influence of terrain obstruction,pollutants are not completely transported to southern Anhui,resulting in regional differences in pollution characteristics in Anhui.However,the shift of wind direction is the key factor for the explosive growth of pollutant concentration in southern Anhui.(2)From the perspective of mesoscale boundary layer structure variation,the heavy pollution processes in Anhui region usually include three types:(i)processes that include two stages of heavy pollution transport and cumulation,similar to the Beijing region;(ii)processes that include heavy pollution transport and cumulation stages,followed by transport stage,different from the Beijing region;and(iii)processes characterized by explosive growth of pollutant concentrations due to short-term regional transport,which is also a unique pollution characteristic of the Anhui region.(3)In terms of turbulence characteristics at micro-scale,the turbulence characteristics show obvious differences in the transport and cumulation stages during HPEs,and the turbulence characteristics do not change linearly with the pollutant concentration increases linearly.The higher pollutant concentration,the smaller degree of variation of turbulence characteristics,showing that weak turbulence corresponds to the cumulation type of aerosol pollution.(4)Northern and southern Anhui are mainly influenced by meteorological conditions,while central Anhui is mainly affected by emissions.During the explosive growth that exists the cumulation stage of HPEs,about 68% of the explosive increase in pollutant concentrations can be considered as a feedback effect of unfavorable meteorological conditions on pollutant concentrations.In addition,the relative contributions of local emissions and regional transport were quantified by sensitivity experiments in model,accounting for about 47% and 43%,respectively.(5)There is dissimilarity in the turbulent transport of momentum-heat-particle during HPEs.The increase in the proportion of inverse gradient transport of momentum fluxes at night during HPEs,may be due to the appearance of turbulent barrier effect.The presence of pollutants alters the thermal structure of the boundary layer,which in turn affects the transport of heat fluxes.Although particle is also a scalar,the particle flux does not show a similar daily variation as heat due to the different sources/sinks.Therefore,turbulent transport processes of different variables need to be considered separately.(6)Comparing the two methods(the classical Monin-Obukhov similarity theory and the mixing-length theory)of calculating turbulent diffusion coefficients in the stable boundary layer(SBL),it is found that the uncertainty of calculating turbulent diffusion coefficients by the mixing-length theory is smaller.Therefore,the turbulent diffusion coefficient of particle is established based on the mixing-length theory and embedded into the mesoscale model for online evaluation and validation.The newly established turbulent diffusion coefficient of particle in the model has the following advantages:(i)it can be used to calculate the vertical mixing process of pollutants alone and does not affect the vertical mixing process of other parameters;(ii)it is more applicable to the SBL by explicit local gradient calculation to characterize the vertical mixing process of pollutants;(iii)it is more efficient in calculation and can be more easily applied to the GRAPES＿CUACE model developed by China in the future.(7)The newly established turbulent diffusion coefficient of particle can improve the overestimated PM2.5 concentrations at night in eastern China,and not worsen the underestimated PM2.5 concentrations at night along the mountains in northern China.Meanwhile,the overestimated PM2.5 concentrations are improved and mixing more uniformly in the vertical direction,resulting in an increase in the upper PM2.5 concentrations and a decrease in the near-surface PM2.5 concentrations.However,for complex terrain areas,the changes in turbulent diffusion are less sensitive to changes in pollutant concentrations,and the effects of other processes(e.g.,advective transport)are more significant. |
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