Taking Qingdao as an area of concern, this paper analyzes the dynamic change characteristics and influencing factors of visibility during the fog-haze process using meteorological data including visibility at Qingdao, Pingdu, Huangdao and Jiaozhou stations and observation data such as PM2.5 concentration and water-soluble ion concentration at nearby environmental monitoring stations from November 16 to 20, 2021.The results show that the whole haze process in Qingdao from November 16 to 20, 2021 experienced the transitions of light fog, dry haze, wet haze, dry haze, wet haze, dry haze, wet haze, and light fog.Geographical discrepancies make the inland stations experience no light fog stage and the haze stage starting 8 hours earlier than the coastal stations.In the fog and haze process, the atmosphere appears stable junction and a certain humidity condition with the ground wind speed being less than 2 m·s-1 and the vertical movement speed being 0.0~0.5 Pa·s-1.The large amount of water vapor carried by the easterly wind and the variation of humidity layer above 80% are significant factors for the transition between dry and wet haze.Visibility gradually decreases with the enhancement of PM2.5 concentration and relative humidity.When the relative humidity increases, the correlation between visibility and PM2.5 concentration decreases.Compared with the dry haze stage, the PM2.5 concentration in wet haze increases by 14.1%, visibility decreases by 47.4%, and visibility decreases by 7.6% after the transition from haze to fog.In the dry haze stage, visibility is significantly affected by the PM2.5 level.In the wet haze stage visibility is affected by both PM2.5 level and relative humidity, whereas in the fog phase visibility is significantly affected by relative humidity.In the haze stage, the SNA concentration (SO42-, NO3-, NH4+), SOR (sulfur oxidation rate), and liquid water content in particulate matter gradually increase with the enhancement of pollution level leading to visibility gradually decreasing.The backward trajectories indicate that during the haze period, the air masses originate from inland, passing through Jiangsu provrce, He'nan provice, and other places to Qingdao with a deep pollution degree, while during the fog period, the air masses reach Qingdao through the coastal areas with high humidity and low pollution degree.
