Chemical Characteristics Of Fog Water And Aerosol In Three Grades In Nanjing And Lusha

The chemical characteristics of fog water are closely related to the size of fog droplets.In order to explore the chemical composition of fog droplets with different sizes in Nanjing and Mt.Lushan,and the influence of Fog Process in Nanjing on the chemical composition and size distribution of aerosol particles.In this study,fog observation campaigns were carried out in Nanjing and Mt.Lushan in the winter of 2013-2017.170 three-stage fog water samples from 8fog processes were collected by using the three-stage CASCC,and their chemical components were determined.The sources of pollutants in fog droplets were discussed from the regional and emission source types,and the droplet spectrum distribution in the contract period was concluded aerosol particle spectrum distribution and chemical composition of particle size aerosol,in-depth analysis of fog microphysical quantity and aerosol spectrum distribution during Nanjing fog in the winter of 2017,and comparative analysis of the chemical composition of three-stage fog water and particle size aerosol before,in and after fog.The main conclusions are as follows:（1）The pH value,electrical conductivity,total ion concentration,water-soluble ion components and other chemical characteristics of Nanjing three-stage fog water are size dependent.The small fog droplets have stronger acidity,higher electrical conductivity and higher total ion concentration.The total ion concentration of Nanjing fog water is about 10times that of Mt.Lushan fog water,which shows that the pollution degree of urban fog water is higher than that of Mt.Lushan.（2）The concentration of different water-soluble ions in droplets showed size dependence:Ca²⁺was enriched at≥22μm large droplets,SO₄^2-,NO₃^-and NH₄⁺are enriched in 4-16μm small droplets.The changes of the total ion concentration of three-stage fog water and the mass concentration of PM_2.5 and PM_2.5-10 in the air with the fog process show that:due to the faster settling speed of larger droplets,2.5-10μm coarse particles respond faster to fog removal,and the competition between sedimentation and gravity sedimentation leads to the maximum reduction of the mass concentration of fine particles<2.5μm.The ion concentration of Mt.Lushan fog water shows a"U"trend of first decreasing and then increasing with the generation and dissipation of fog.（3）PMF source analysis shows that there are significant differences in the contribution rates of chemical components in different size droplets from different pollution sources.Soil sources mainly affect the Ca²⁺content of large droplets,and the contribution of industrial sources and motor vehicles more affects the NO₃^-and SO₄^2-concentrations of medium and small droplets.Soil source,industrial source and motor vehicle are the main contribution sources of Nanjing fog water,while industrial source,motor vehicle,soil source,biomass combustion and secondary aerosol are the main contribution sources of Mt.Lushan fog water.（4）In the winter of 2017,the distribution of fog droplet liquid water content with particle size in the first Fog Process in Nanjing was"V",with the lowest value at 7μm.The distribution of fog droplet liquid water content with particle size in the second fog process was three peaks,with the peaks at 5μm,15μm and 21.5μm respectively.（5）In the stage of fog formation and development,the mass concentration of aerosol with small particle size decreases and that of aerosol with large particle size increases.In the stage of fog maturity,the mass concentration of aerosol particles reaches the lowest in the whole particle size section,and the mass concentration of aerosol with large particle size decreases significantly.Compared with that before fog,the peak value of aerosol mass concentration after fog moves to the direction of large particle size.Before fog,the water-soluble ion components of aerosol are enriched in small particles with particle size<0.43μm.With the fog process,nucleation and moisture absorption increase,making the water-soluble ions enriched in the larger particle size segment.The newly generated aerosol in the fog is released with the evaporation of fog droplets,resulting in the increase of NO₃^-,SO₄^2-and NH₄⁺concentrations after fog.（6）The neutralization rate of aerosol with smaller particle size is higher,and the acidity of new droplets in the early stage of fog formation is strong.With the gradual neutralization of fog process,the pH value of fog water increases gradually.These results are the basis for further study on the environmental and ecological impact of fog on the Yangtze River Delta,and can provide a scientific basis for the prevention and control of urban air pollution.