Impact Of Mountain-Valley Wind Circulation And Rainfall On PM_2.5 In Beijing

Fine particulate matter PM_2.5 has already been the primary pollutants in Beijing.Thus,the basis of controlling the air pollution is to have a comprehensive understanding of its variation characteristics and impact mechanism.Meteorological conditions is the key factor that can influence the concentrations of pm2.5’sevolution.The impact of mountain-valley wind circulation on the typical examples of pollution is analyzed through the selected pollution process,combining with the hourly PM_2.5 concentrations and meteorological data which is in the Haidian、Shangdianzi and Lishuiqiao of Autumn and Winter from 2013 to 2015,and also the data of Tower of atmospheric、wind profile of Haidian and automatic meteorological stations in the same time.At the same time,the impact of collision removal of rainfall on aerosol particles of different sizes is analyzed through the calculation of Stokes number,combining with the hourly PM 2.5 concentrations and meteorological data in Haidian from October 2012 to October 2014,and also the size distribution data in a selected rainfall process.The analysis show that,the average wind speed of valley wind is greater than the mountain wind,and they all will be "broken" during the conversion time in the mountain-valley wind days.In contrast with the mountain wind,the average duration of valley wind in autumn is longer than which is in winter,and the start time of valley wind in autumn is earlier than the same wind in winter;influenced by the topography of Beijing area,the direction boundary of the transformation between mountain-valley wind is northeast – southwest.The frontier of mountain wind in autumn can fall down to the South Second Ring Road,and it can be pressed to the South Third Ring Road in winter;the average thickness of valley wind is greater than the mountain wind.Whether the moment is in autumn or winter,in the south,the average time when the PM_2.5 concentration began to rise,is earlier than in the north in a day;the time which concentration of pollutants began to rise in the fall is earlier than in the winter,but the time which began to decline is opposite.The transition zone of different PM_2.5 concentration in Beijing in autumn or winter locates in South Second Ring Road（South Third Ring Road）,and it will move to south over time.Duration autumn（winter）season this phenomenon is about 4（2）hours.During the transformation period,wind speed is very small and the concentrations of PM_2.5 can rise,also suburbs is earlier than the city of the ending time of valley wind.Through the simulation example,we found that the city center had a northeast-southwest higher boundary layer about 900 m^-1200 m at 14 pm which is associated with the distribution of wind field in the region.The boundary layer height is about 40-80 m in the night,and potential temperature is low at surface layer,which is as the height increases.The potential temperature level is lower than the west,and in vertical direction,the mixing layer height is low which is about 500 meters.The calculation results of Stokes number show that the raindrops had little effect of direct collision removal on aerosol particles smaller than 1μm,and had more effect on aerosol particles larger than 1μm.Based on the statistical analysis of the observation data,the precipitation processes or the precipitation hours with significantly decreased PM_2.5 were quite limited.However,PM_2.5 concentrations were increased in 43.2% of the precipitation hours.By analyzing the size distribution data of aerosol particles during a typical precipitation process,we found that the precipitation had significant scavenging effect on Aitken mode particles（<0.1μm）and coarse mode particles（>1.0μm）,except for the accumulation mode particles.Since the accumulation mode aerosols contribute most of the mass of PM_2.5,the rainfall processes only had minor influence of collision scavenging on PM_2.5.This research shows that: PM_2.5 concentrations was mainly affected by the meteorological conditions.Among them,the mountain-valley wind circulation has significant effects of PM_2.5 concentrations,and the rainfall processes only had minor influence of collision scavenging on PM_2.5.The study has certain guiding significance to the construction of urban planning.