Fog Water Precipitation In Heavy Fog And Its Impact On Air Pollutant

The stable atmospheric boundary layer is conducive to the accumulation of atmospheric pollutants and the occurrence of fog.At the same time,fog can have important effects on the production and elimination of pollutants and reaction paths,which can change the composition characteristics of pollutants in the atmosphere.Comprehensive observation experiments of fog were carried out in Nanjing in the winter of 2013 and 2017.Based on the obtained physical and chemical comprehensive observation data,the microphysical quantities and deposition characteristics in dense fog and the relationship between the two were analyzed,and the changing characteristics of pollutants and the effects of dense fog on pollutants were discussed.The following conclusions were mainly obtained:（1）During the observation periods of 2013 and 2017,the duration of dense fog（visibility<200 m）was 18 h 15 min and 5 h 35 min,respectively,and the liquid water content（LWC）and number concentration（N）reached a maximum of 0.41 g m^-3 and 1145 cm^-3.Fitting the microphysical quantities of fog revealed that the weighted average fall velocity of fog droplets obeyed a power law with LWC/N distribution,the gravitational settling flux was linearly correlated with LWC and followed a power-law distribution with LWC^5/3N^-2/3,the correlations were all significant（R²=0.82,0.97 and 0.98,respectively）.（2）Fog water deposition was valued using the parameterization scheme,and the results showed that the fog water deposition was mainly concentrated during the dense fog,and the total fog water deposition was 314.19 g m^-2 and 81.78 g m^-2 in 2013 and 2017,respectively.This difference was mainly caused by the difference in the duration of dense fog.Small droplets（<16μm diameter）contributed the most to the total fog-water deposition,accounting for 41-48%of the total fog-water deposition.In comparison,it was found that the parametric scheme of Katata et al.could better estimate the fog-water deposition because it takes into account the influence of the subsurface and turbulence characteristics.The results of fitting visibility,LWC,N,and deposition fluxes show that there is a nonlinear correlation between visibility and LWC and N.The fog-water deposition flux and visibility are in accordance with the power-law distribution,and the fitted correlation is good（R²≥0.89）.（3）Dense fog has a scavenging effect on both gas-phase and particle-phase pollutants in the atmosphere.The total values of fog water species nitrogen and sulfur deposition were51.46 mg m^-2 and 9.05 mg m^-2 in 2013 and 5.06 mg m^-2 and 3.30 mg m^-2 in 2017,respectively,with the highest contribution of 55%-67%from small fog droplets,which is due to the ion the enrichment of small droplets.In comparison with the reported values for the same period in previous years,the total nitrogen deposition from persistent dense fog events can reach 21.6%-26.5%of the total wet deposition for the month.（4）The VOCs concentration in the dense fog was reduced,the total VOCs removal rate was close to 30%,which may be attributed to an indirect/direct removal effect,in which the enhanced collision and deposition of fog droplets promote the redistribution of VOCs gas-aqueous/particle partitioning,and remove them from the atmosphere by fog water deposition.LWC is first positively correlated with PM_2.5 and VOCs,and then turns to be negatively correlated.