Influence Of Meteorological Conditions To The Variation In Winter Haze And Fog Events From 2013 To 2019 In Middle-Eastern China

Severe air pollution events accompanied by high PM_2.5concentrations have been repeatedly observed in Middle-Eastern China since 2013.Frequent air pollution events and their causes in this region attached widespread attention.The month of January was selected to represent the winter season.Based on PM_2.5concentration data provided by China Environmental Monitoring Center,ground observation data from China Meteorological Administration and reanalysis data from ECMWF,selected PM_2.5concentration and visibility as the representatives of ambient air quality respectively,the causes of winter air pollution from 2013 to 2019 in Middle-Eastern China was investigated in depth.The main study contents and conclusions are summarized as follows:1)Contribution of meteorological conditions to changing in regional PM_2.5concentrationBased on values of upper-level MCI,the years 2014,2015,2017,and 2019 were defined as meteorology-haze years and the years 2016 and 2018 were defined as meteorology-clean years.A change in meteorological conditions may lead to a 26%change in PM_2.5concentration between 2014 and 2013(two meteorology-haze years)and 16–20%changes in PM_2.5concentration between meteorology-haze years and meteorology-clean years.Changes in pollutant emissions may cause 21–47%changes in PM_2.5concentration between each two meteorology-haze years.A comparison of two meteorology-clean years and pollutant emissions in 2018 may be reduced by 40%compared with 2016.Overall,changes in emissions had a greater influence on changes in PM_2.5compared with meteorological conditions.2)Relative contribution of PM_2.5concentration and relative humidity to visibilitywhen RH<70%and PM_2.5concentration<75?g/m³,visibility in Middle-Eastern China was usually higher than 10km.The increase in PM_2.5concentration was the dominant factor for the rapid decrease in visibility.The combination of the increase in RH(70-85%)and the increase in PM_2.5concentration(75-200?g/m³)resulted in the further decrease of visibility(10-5km).The decrease in visibility(5-2km)mostly depended on the further increase of RH(85-95%).The correlation between PM_2.5concentration and visibility was weakened.The decrease in visibility to 2km or even lower was mainly due to the extinction of droplets under the near saturation of water vapor(RH>95%),and had little relation with changing of PM_2.5concentration.3)The multiple regression equations of visibility,PM_2.5concentration and ambient humidityCompared with establishing the visibility fitting equation directly without grouping,establishing the visibility fitting equation according to the RH above or below 85%respectively can greatly optimize the multivariate regression models.RMSE for visibility fittings with RH>85%decreased from 3.7-9.2 to 0.5-0.7.Visibility in January of 2015,2016,2018 and 2019 were well reproduced by these fitting models.Correlation coefficients between the observed visibility and the calculated visibility were all higher than 0.91.It provides new visibility parameterization for the Haze-Fog numerical prediction system.