Observation Analysis And Simulation Study Of A Fog Process In Yangtze River Delta

The Yangtze River Delta?YRD?region,located on the west coast of the Pacific Ocean,is the fastest growing economic development region in China.Fog,haze and other extreme weather events over the YRD have increased during the past decades.During a dense radiation fog event from the evening of 2 Jan to noon on 3 Jan 2017 at Shouxian,a rural site in Anhui Province,China,surface meteorological data and vertical profiles of particulate matter?PM?and meteorological parameters were collected using a tethered balloon within the lower troposphere?1000 m?.This study assessed the evolution of the PM_2.5 profile with the planetary boundary layer?PBL?structure and how PM_2.5 concentration was affected by fog.In addition,a comparative analysis of a weak fog process in Nanjing was conducted.Due to the wide range and the large impact of the fog process,this study further simulated the dense fog process in ShouXian with the WRF-Chem model,and reached the following conclusions:?1?On the surface,the average diurnal variation of aerosol particles had bimodal distributions before fog formation,which was mainly influenced by diurnal variation of the mixing level depth?MLD?.The reduction of ground visibility was mainly affected by the extinction of accumulation mode particles in the meantime.Aerosol number concentrations decreased remarkably and the PM_2.5 concentrations were strongly scavenged from 150?g/m³to 45?g/m³ during the fog process.?2?The vertical distribution of PM_2.5 concentrations was affected by the PBL height and the vertical structure of fog.At 05:00 LT?local time?,in the early morning before the fog event,the PM_2.5 in the stable layer?260?g/m³?was slightly higher than that above the residual layer?200?g/m³?.At 14:00 LT,the PM_2.5 was well mixed beneath500m with a concentration of 310?g/m³.After 20:00 LT,when the fog formed,the PM_2.5 was scavenged from the bottom to up,and the scavenging height was dominated by the fog top height.?3?The vertical development of fog was promoted by turbulence mixing and radiation cooling at the fog top.Turbulence mixing enhanced the efficiency of fog droplets scavenging fine particles by the collision-coalescence process.Meanwhile,the PM_2.5 scavenging height was in accordance with the turbulence development height.?4?There were significant differences in the fog process between Nanjing and Shouxian.The fog process in Nanjing was short and the intensity was small.The PM_2.5 concentration tended to increase during the fog process.However,the fog of Shouxian was dense and a large number of particles were removed in the fog.It could be seen that the intensity of the fog had different effects on the aerosol particles.By replacing the rural underlying surface of Shouxian with the urban underlying surface,the influence of urban heat island effect on the fog process was discussed.During the day,the difference between urban and rural net radiation is positive,indicating that the urban underlying surface increases the net radiant flux of the surface.The difference of latent heat between rural areas urban is about 100 Wm^-2.The water vapor conditions in rural areas are better than those in urban areas,which is more conducive to the formation of fog.During the day,the heat storage capacity of urban is higher than that of the countryside.the maximum value of urban heat storage can reach 180 Wm^-2,while only 100 Wm^-2 in rural areas.In the evening,the heat stored during the day is used for night release.The heat released by the city is greater than that released by the rural,which makes the city cool down more slowly than the countryside.This is the reason why the urban fog process weaker than the rural.