The Distinction Scheme Of Haze-fog Based On Visibility,relative Humidity And PM_2.5 Concentration And Its Spatio-temporal Distribution And Physical And Chemical Properties

In recent years,the frequent occurrence of fog-haze events in eastern and central China has resulted in severe reduction of visibility,which had a huge impact on human health.However,in the current researches,there are various fog-haze distinction schemes,most of which are based on relative humidity and visibility,and the thresholds are not unified,and it is difficult to distinguish mist and haze.Based on a large amount of first-hand meteorological and environmental observation data,we studied the correlation between relative humidity,visibility and PM_2.5 concentration,proposed a new method to determine the relative humidity threshold,and determined 100μg m^-3as the threshold for visibility to decrease to less than 10 km.A fog-haze distinction scheme based on visibility,relative humidity and PM_2.5 mass concentration was proposed,and the temporal and spatial changes of fog,haze and mist.It shows that the occurrence frequency of the three weather phenomena increases first during 2013-2016,and then decreases after 2017.The haze is mainly distributed in the North China Plain,the Central China Plain,the Yangtze River Delta and the Sichuan Basin.The frequency of regional fog in China is relatively low.In terms of seasonal distribution,the number of haze days is concentrated in winter,while mist mostly occurs in spring and autumn.Clean fog mainly occurs in high mountain stations and clean areas with high relative humidity,while polluted fog mostly occurs in densely populated cities.Current researches show that black carbon aerosol can weaken the East Asian monsoon and influence haze phenomena in north China area,also considering the secondary aerosol play an important role in the process of haze,this paper dealt with CAWNET black carbon monitoring data and the aerosol composition observation data,obtained black carbon aerosol concentration distribution and long-term trends;The variation characteristics of the main components of aerosols in fog-haze and clean weather in three typical cities in autumn and winter from 2016 to 2018 were analyzed.The results show that the proportion of clean weather in Beijing is increasing year by year.Most fog-haze events are accompanied by intense air pollution.The occurrence and migration of haze is usually related to the growth of aerosol hygroscopicity.Meteorological conditions such as relative humidity play an important role in the process of heavy pollution episodes.During 2017-2018,nitrate aerosol concentration increased more significantly in haze days,but sulfate concentration did not.Nitrate aerosol has gradually become the main secondary aerosol in heavy pollution episodes of Beijing area.The change of black carbon aerosol is relatively stable.Compared with clean days,in haze days,BC aerosol decreased about 1%,sulfate increased about 2-3%,OC accounted for a slight decrease,and nitrate increased about 10%.Shijiazhuang station has a higher proportion of black carbon aerosol,and the concentration of aerosol directly discharged into the atmosphere was higher than that in Beijing.The Taiyuan region has a higher proportion of black carbon,but has fewer haze days,possibly due to low relative humidity levels.