Observational Analysis Of Boundary Layer And Entrainment Structures Of A Heavy Haze Pollution Event In The Middle And Lower Reaches Of The Yangtze River

With rapid development of urbanization and industrialization,anthropogenic emissions have increased substantially.As a result,air pollution events have become more serious with higher frequency over the past two decades in China.Aerosol radiative effect exerts an important impact on thermal and dynamical structures as well as entrainment processes in the atmospheric boundary layer?ABL?with heavily polluted conditions.Characteristics of the ABL and entrainment structures of a heavy haze pollution event are analyzed by using the vertical profiles of PM_2.5?particulate matter smaller than 2.5micrometers?,temperature,relative humidity and winds measured by unmanned aerial vehicle?UAV?,Lidar extinction coefficient measurements,and surface observations during the period from December 23,2017 to January 18,2018 in Nanjing,China.The feasibility and advantages of using the PM_2.5profile detected by the UAV to determine the height of the boundary layer and the thickness of the entrainment zone was discussed.The impacts of aerosols on the boundary layer and entrainment structures were assessed by comparing a haze-polluted day with a day with less pollution.The conclusions are summarized as follows.?1?A clear three-layer structure was observed in vertical profile of PM_2.5concentrations during daytime when turbulent activity was active.PM_2.5profiles showed very similar pattern to that of specific humidity,but different from potential temperature.In addition,the fastest increase in PM_2.5was observed near the top of the ABL.?2?The PM_2.5concentration profiles measured by the UAV were successfully applied to determine the ABL height,leading to a substantial reduction in uncertainty associated with potential-temperature gradient method.As compared to other methods such as potential temperature gradient method and 1.5K potential temperature method,the PM_2.5gradient method showed more reasonable results in determining the ABL heights and location of entrainment zone.?3?High concentrations of aerosols attenuated surface-reaching solar radiation,reduced surface heat flux,diminished intensity of turbulence,increased the stability of the ABL,delayed development of the convective boundary layer,and inhibited development of the ABL.The result also indicated that entrainment-zone depth increased with increasing surface PM_2.5concentrations,which was consistent with other modeling studies.?4?Entrainment rate normalized with convective velocity did not follow-1 power function with the convective Richardson number under heavy aerosol pollution conditions due to aerosol radiative effect.The higher the haze concentration,the farther the normalized entrainment velocity as a function of the convection Richardson number deviated from the-1power function.This finding further validated the related modeling studies such as Large Eddy Simulation?LES?.