Characteristics Of Atmospheric Boundary Layer Under Polluted Conditions Over Beijing

The Beijing-Tianjin-Hebei metroplex region is one of the most severely polluted areas in China.Frequent pollution episodes have seriously affected the resultant human health and the sustainable development of the city.The atmospheric boundary layer is the main place for the transmission,accumulation,and diffusion of atmospheric pollutants.The boundary layer height(BLH),turbulence intensity,radiation budget,energy distribution and other thermodynamic factors directly affect the pollution diffusion.Based on surface PM_2.5 observations,multi-layer(15 levels)vertical meteorological observations from a 325 m meteorological tower,and Doppler wind lidar,we investigated the evolution of PM_2.5 pollution episodes(PPEs)in Beijing and the characteristics of changes in meteorological elements in different stages of PPEs,the characteristics of radiation budget and energy distribution near the ground in the urban boundary layer under different pollution conditions,and the inversion of the urban BLH suitable for Beijing during heavy pollution in winter.The main conclusions are as follows:From 2004 to 2016,there are 456 PPEs in Beijing.Meanwhile,PPEs are more frequent and severe in autumn and winter.We find that average PM_2.5 concentration during PPEs(141.2?g m^-3)is twice the 13-year average(70.3?g m^-3).The changes of winds have an important role in the occurrence and dissipation of pollution episodes.For vertical wind,southwesterly winds prevail from 80 m to 320 m in the Before stage and bring pollutants to Beijing.While,strong northerly winds prevail in the DSs and After stage,effectively removing pollutants.Owing to different thermal dynamic conditions,PPEs with stable atmospheric stratification occur most frequently at sunset or one hour before sunset,particularly at 1800 LST.In general,after the pollution episodes,the wind speed(WS)and BLH are lower,relative humidity(RH)is greater,and accompanied by strong stability.The characteristics of near-surface radiation balance and energy budget in December 2015were comparatively investigated.Relative to clean days,both downward shortwave radiation(DSR)and upward shortwave radiation(USR)dropped during the daytime,while downward longwave radiation(DLR)and upward longwave radiation(ULR)enhanced during nighttime on heavily polluted days.With evaluated height,the drop magnitudes of DSR and USR decreased,while the enhancement magnitude of DLR(ULR)decreased(increased).The combined effects of four radiation components significantly induced the reduction in net radiation(R_n)on polluted days,leading to the near-surface energy budget change.During daytime,compared to the clean episodes,the sensible heat flux(H)was reduced more than R_nin the whole near-surface UBL during heavily polluted episodes,resulting in smaller proportion of H and larger proportion of G.Weak thermal forcing effects caused by insufficient availability of net radiation energy at the surface and weak dynamic motion associated with weak winds were both responsible for the large proportion of heat storage in urban buildings.Combining the Doppler lidar detection data and the 325 m meteorological tower vertical gradient observations,this paper uses the threshold method and the Richardson number method to retrieve Beijing urban BLH in December 2016,which better describes the daily evolution process of atmospheric boundary layer,especially under the heavily polluted conditions.The highest daytime urban BLH observed was up to 1000 m;however,under heavily polluted conditions with weak wind,the maximum daytime urban BLH was only approximately 300 m.The variations of DSR,WS,H,friction velocity(_*),and turbulence kinetic energy(TKE/m)correlated positively with the BLH,while there was a negative correlation between BLH and PM_2.5.For the diurnal cycle,under weak wind conditions,the pattern of urban BLH was largely modulated by thermal forcing of solar radiation.While under strong wind conditions,the pattern of urban BLH was largely modulated both by thermal forcing and dynamical forcing.The durations of morning transition in the atmospheric boundary layer increases under heavily polluted conditions.Our work highlights that weak wind speed(weak dynamic motion)and heavy aerosol pollution(weak thermal forcing due to the effect of cooling)under heavily polluted conditions can dramatically inhibit the development of the urban boundary layer.