Effects Of Meteorological And Atmospheric Circulation Factors To Aerosol Pollution During Winter In North China

Using the AOD provided by MERRA-2 as the indicator of aerosols pollution strength.The effect of meteorological and atmospheric circulation factors on aerosols during aerosol pollution in North China was investigated.K-mean clustering analysis was used to locate a kind of aerosol pollution event in winter over North China.By using regression analysis,the relation between meteorological factors and aerosol pollutants during pollution events was investigated.During the aerosol pollution events,the effect of atmospheric circulations on meteorological factors in North China was revealed by stepwise regression analysis,and the source of anomaly of atmospheric circulations was studied.Yearly AOD in North China had an increasing trend over the period1980-2018.In winter daily AOD in North China had an increasing trend of0.0014/day and a time period of 10 days.The area average AOD in North China was the highest in summer（0.42）,and the lowest in winter（0.32）.The high AOD center was located in the south part of North China in winter,and moved northward till it reached Hebei in summer.The K-mean method was used to clustering the winter weather pattern in North China,and a kind of weather pattern（type-5）which has significant effect on aerosol pollution strength is found.From 1982 to 2016,there were324 days of this weather pattern,accounting for 10.28%of the winter days in the past 35 years in North China,with an average of 9.25 days per year.The positive anomaly of AOD appeared in North China from days 0 to 2after the occurrence of type-5,which means the accumulation of aerosols.The positive anomaly of AOD decreased gradually from days 3 to 5 after the occurrence type-5,and the center moved southward and disappeared,which means the dissipation of aerosols in North China.Through regression,the effects of different meteorological factors on AOD in North China under type-5 process was analyzed.It was found that zonal wind（u wind）and relative humidity have the most significant effects on AOD in North China.After the analysis of aerosol flux and relative humidity,it was found that under type-5 process,the accumulation of aerosol was enhanced（the average value of aerosol flux divergence was-0.054μg/s~2m~2）over heavy emission area（113°-118°E,33°-40°N）,the accumulation of aerosol leaded to the increasing of AOD in North China,and the increasing of relative humidity in North China was conducive to the aerosol hygroscopic growth.During type-5 process,the boundary layer height in North China decreased,which would enhance the accumulation of aerosols in the lower layer,and further conducive to the occurrence of haze.The East Asian Trough,the East Asian Winter Monsoon and Siberian High were the main influencing circulation systems of meteorological factors in North China under type-5 process.The East Asian Winter Monsoon weakened over day-1 to day 1,and was weakest on day 1（low level wind index LLW is 1.20）,and then returned to normal after day 3.The anomaly change of East Asian monsoon would cause east wind anomaly,which would lead to accumulation of aerosol and the increase of AOD in North China.The East Asia Trough weakened over day-5 to day 4（minimum East Asia trough index was 0.79 in day 1）.This change would lead to the increase of relative humidity near the surface,which would enhance the growth of aerosol hygroscopic growth in North China.The intensity of Siberian High weakened over day-5 to day 0,and began to recover on day1.After the recovery of Siberian High,the index was slightly higher than the average in winter（the Siberian high index was 0.16 at the maximum）.The Siberian High reduced the boundary layer height and increased the relative humidity in the northwest,Henan and Hebei,which was conducive to the increase of AOD after day 0.At 200h Pa and 500h Pa,the geopotential height anomaly appeared in the northern hemisphere with the form of wave train over the whole type-5process.Through the analysis of the horizontal distribution,vertical structure and wave flux distribution of the wave train,it could be found that the source of the wave train might come from 200h Pa and continued to propagate to the southeast.The vertical structure shows that wave train was barotropic,which indicated that the anomaly of geopotential height came from an external force.When the wave train propagated over Lake Baikal（90°E,55°N）at day-2,the Siberian high was weakened.At day 0,the wave train reached the Korean Peninsula（120°E,45°N）,caused the weakening of the East Asian Trough and the East Asian winter monsoon.These Changes in atmosphere circulations would lead to the anomaly of aerosol transport,relative humidity,thereby increased the AOD in North China.The decrease of the boundary layer height leaded to the aerosol pollution in North China.