Impacts Of Synoptic Patterns And Boundary Layer Characteristics On Air Pollution Over Middle-Lower Yangtze Area

The subjective classification method,combining the radiosonde,conventional surface meteorological observation data,ERA-interim reanalysis dataset every six hours,the Lagrangian trajectory diffusion model HYSPLIT and PM_2.5 mass concentration data from the Ministry of Environmental Protection,was used to identify the synoptic types in middle and lower reaches of the Yangtze River when large-scale air pollution process occurred from 2014to 2016.Then a transmission-type pollution weather process?March 7-11,2015?and a static-type pollution weather process?January 14-20,2014?were selected to analyze the characteristics of the inversion layer in the atmospheric boundary layer,the causes of formation,and the impact on PM_2.5 concentration at each site.The result showed that from 2014 to 2016,large-scale PM_2.5pollution weather in the middle and lower reaches of the Yangtze River mainly occurred in the autumn and winter.Among them,the sum of three types of ahead of the cold front,uniform pressure fields and the bottom of high pressure accounts for 3/4.However,the surface weather situation is not invariable when large-scale contaminated weather occurred,but converting between multiple synoptic patterns.The cold front can cause air pollutants in the upstream area to be transported with the cold air to the Middle-Lower Yangtze Area.As a result,large-scale transmission-type pollution weather appeared in the Middle-Lower Yangtze Area.First,the pollution weather appeared in North China in March 7-8,2015.Subsequently,the heavy pollution area moved to Middle-Lower Yangtze Area.As the weather system moved,the boundary layer structure in the same part of the weather system has common characteristics,multi-level inversion occurred during the pollution period.The formation of this multi-level inversion is due to the radiation cooling near surface at night,frontal inversion of the cold front moving,sinking motion above the boundary layer.Except for the Beijing Station,pollutants will accumulate before the arrival of the cold front,the wind speed is low and the relative humidity is high.When the cold front crossing,due to the influence of pollutants in the upstream area with the transport of cold air,the PM_2.5 concentration will increase significantly.The shape of uniform pressure fields is second only to the cold front,the ground is mainly controlled by a uniform pressure field,and it's beneficial to the accumulation of pollutants and then became a stable pollution weather process.Because of the radiation cooling near surface at night,sinking motion above the boundary layer,the two layers of inversions appeared at each station at different times.And the ground wind speed of each station is small and there is no dominant wind direction,the relative humidity is high,visibility remains around 3km.When cold air crosses,the ground wind speed at each station increased rapidly and the wind direction becomes northerly or northwest,the atmospheric diffusion ability of horizontal and vertical were increase.The pollutants gradually dissipated.