| The planetary boundary layer(PBL),the place where the vertical exchange of material and energy between the earth surface and the free atmosphere happens is important in the study of air pollution.Planetary boundary layer height(PBLH)is an important parameter to measure the turbulent mixing and vertical diffusion in boundary layer.The diffusion of pollutant is inhibited in stable boundary layer.Moreover,the temperature inversion inside the boundary layer will also weaken the vertical movement of airflow,inhibit the spread of pollutants,and aggravate air pollution.Thus,it is necessary to study the boundary layer height,the temperature inversion in boundary layer and the relationship between them and ground pollutant concentration.In this paper,the planetary boundary layer height was calculated using the sounding data of the Yangtze river delta region,and the result was verified by the simulated boundary layer height from WRF3.9 model.According to the vertical profile of temperature,the temperature inversion in the boundary layer is analyzed,including the temperature inversion frequency,temperature inversion intensity,temperature inversion layer thickness,base height of temperature inversion.In addition,seven wintertime circulation patterns affecting the Yangtze river delta region were obtained in cost733 software.The boundary layer height,temperature inversion and other parameters as well as pollution conditions were analyzed under different circulation patterns.The results show that the boundary layer height in the Yangtze river delta region has certain spatial and temporal distribution.At 8:00BJT,the mean boundary layer height was high in summer,followed by spring,winter,and summer.At 08:00 and 20:00BJT,the overall boundary layer height in the Yangtze river delta region decreased from the eastern coast to the western region.It was found that PM2.5 concentrations was negatively correlated with boundary layer height.Serious air pollution is more likely to happen in lower boundary layer height condition.In the Yangtze river delta region,the frequency of temperature inversion in boundary layer reached 51%.The frequency of temperature inversion in each month is statistically presented as "V" shape.Low temperature inversions are more likely to occur in autumn and winter.Temperature inversions are more likely to occur at 8:00BJT.The thickness of the inversion layer is mainly distributed between 60 and 180 meters.The probability of temperature inversion intensity is mainly distributed between 0.5 and 1.5 K/100 m.The intensity of temperature inversion is negatively correlated with the base height of temperature inversion.The frequency of temperature inversion is much higher in winter.According to the potential height of 925 hPa,the main circulation patterns in seven Yangtze river delta areas in winter were obtained using the obliquely rotated Principal Components in T-mode(PCT)method,including T1: western high pressure(33.63%);T2: southwest high(22.35%);T3: northwest high(12.83%);T4: northwest low(6.86%);T5: northern high(9.96%);T6: southern high(6.19%);T7: eastern high(8.19%).Three typical types(T1,T2,T6)are associated with high PM2.5 concentrations.Air masses from northwest China have brought pollutants to Shanghai,causing severe pollution.The low boundary layer height under the conditions of high pressure in southwest and south China inhibits the vertical diffusion of pollutants,resulting in the further increase of PM2.5 concentrations.Under the high pressure in the southwest and south region,the near-surface temperature inversion,coupled with the stable boundary layer,avoids the diffusion of pollutants and causes serious air pollution. |
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