A Comparative Study Of The Influence Of Dry And Wet Southwest Vortex On Air Quality In The Sichuan Basin In Winter And Spring

In recent years,the economy of Sichuan-Chongqing region is developing rapidly,and the air pollution problem is becoming more and more serious,which attracts the attention of the national and local governments.Among them,the topography of Sichuan Basin is complex,its unique geographical situation,weather and climate conditions make pollutants difficultly to spread,resulting in frequent pollution events.As an important weather system in southwest China,the effect of southwest vortex on the air pollution in the basin needs to be studied.Therefore,firstly,the paper showed the statistical analysis to the characteristic of air pollution in Sichuan basin.Secondly,the correlation between the meteorological factors of typical cities in Sichuan Basin,the maximum mixed layer thickness,ventilation coefficient and the corresponding concentration of six pollutants had been studied.Finally,the statistical method was used to study the statistical classification of southwest vortex weather system in winter and spring,and the influence on the difference of air quality in the basin and the corresponding meteorological conditions were analyzed.The main conclusions are as follows:?1?The AQI in Sichuan Basin has been decreasing from southwest to northeast in recent years,and the main pollutant are PM_2.5,O₃,PM₁₀.The concentration of PM_2.5,PM₁₀,SO₂ in Chengdu,Yibin,Dazhou and Chongqing decreased year by year,but the concentration of PM_2.5,PM₁₀0 exceeded the national secondary concentration limit,while the concentrations of NO₂,CO and O₃ changed slightly.The annual variation of the concentration of PM_2.5,PM₁₀,SO₂,NO₂,CO are high in winter and low in summer,while the concentration of O₃ is lower in winter and3 times higher in summer.The diurnal variation of PM_2.5.5 and PM₁₀,NO₂,CO concentrations is double-peak,while the concentrations of O₃ and SO₂ presented single peak type.?2?The maximum mixing layer thickness and ventilation coefficient of Chengdu,Yibin,Dazhou and Chongqing were higher in winter and lower in summer,while the inverse temperature thickness,inverse temperature intensity and stable energy were opposite,all of which are lower in winter and higher in summer.The correlation analysis showed that the maximum mixed layer thickness was negatively correlated with the concentration of PM_2.5,PM₁₀,CO and positively correlated with the concentration of O₃.There was a significant negative correlation between ventilation coefficient and pollutant concentration?except NO₂?.Inverse temperature thickness,stable energy and pollutant concentration were positively correlated to inhibit pollutant diffusion,precipitation,air pressure and pollutant concentration had a negative correlation,which was conducive to the diffusion or wet removal of pollutants.On the whole,the correlation between pollution meteorological parameters and pollutant concentration was the most significant in winter.?3?There were 104 southwest vortices in winter and spring from 2014 to 2018,and the number of southwest vortices occurring in winter was less than that in spring.The southwestern vortices with precipitation below 1 mm and without precipitation are defined as dry vortices.The southwestern vortices with precipitation greater than or equal to 1 mm are called wet vortices.The number of dry vortex activity in winter was about 2 times that in spring,and the pollution was the most serious,the wet vortex activity occured mainly in spring;the air quality of dry vortex was mainly polluted in winter and spring,and most cities' air quality were excellent under the control of wet vortex.The dry vortex had a great contribution to the heavy pollution in the Sichuan Basin in winter.?4?The above-mentioned wet vortices are divided into wet vortex?and wet vortex?,and the precipitation in wet vortex?is less than 10 mm,and the precipitation in wet vortex?is above moderate rain.The concentration of PM_2.5,PM₁₀,SO₂ in winter dry vortex was about 2 times that of wet vortex?and 3 times that of wet vortex?.The concentration of pollutants under the control of dry vortex and wet vortex?in winter were obviously higher than that in spring.The concentration of PM_2.5.5 in winter vortex was the highest and the pollution was the most serious,and its concentration exceeded the national secondary concentration limit of 26?g/m³.Under the control of wet vortex?,the pollution was the least.?5?Under the control of dry vortex,wet vortex?and wet vortex?system,there were obvious differences in meteorological factors and pollution meteorological parameters of Sichuan Basin in winter and spring.The order of wind speed,relative humidity and ventilation coefficient was as follows:dry vortex<wet vortex?<wet vortex?;the order of the thickness of inverse temperature,stable energy was:dry vortex>wet vortex?>wet vortex?.The wind speed of dry vortex in winter and spring is small,the relative humidity is low,the ventilation coefficient is weak,the energy of stability is high,the inverse temperature is obvious,the weather is steady and steady,and the pollutants accumulate continuously and produce heavy pollution.In the wet vortex system,the atmospheric stability is low and the diffusion ability is enhanced.Precipitation also plays a role in the wet removal of pollutants,and with the increase of the precipitation order,the removal effect of the pollutants is more obvious,so the wet vortex is beneficial to the diffusion of pollution or the wet removal of pollutants,the air quality is good.?6?Through the comparison of dry and wet vortex cases,it was found that the dry vortex system was concentrated in the middle layer,the upper layer converges and sinks,and the vertical movement near the ground was restrained,and the inversion temperature was formed in the middle layer,so that the accumulation of pollutants was not easy to diffuse and the pollution was aggravated.The wet vortex activity was mainly in the near-surface layer,and the cyclonic circulation develops vigorously in the vertical direction,which was conducive to the diffusion of pollutants.At the same time,the production of precipitation had a wet scavenging effect on pollutants,so that the pollution could be alleviated.