Study On Aerosol Chemistry And Cloud Microphysical Properties At Mt.Tai

Cloud is a heterogeneous system that includes gas,liquid and solid phases.To study the cloud process and the concentrations of pollutants in the cloud samples is important for revealing the formation,migration and transformation of contaminated components in the atmosphere.Mt.Tai is the highest point of the North China Plain.The pollution level at the summit of Mt.Tai can represent the background value of North China Plain.This study was from April to October 2014,and June to August 2015.The main research object was a total of 46 cloud events and 78 cloud water samples collected and monitored during this period.The aim is studying the chemical compositions of cloud water,the relationships between particles and the chemical components of clouds,and the microphysical characteristics of cloud events.Cloud samples were collected using the CASCC2(Caltech Active Strand Cloud water Collector Version 2)Cloud Sampler developed by Colorado State University.The pH and the conductivity were measured in situ.The water-soluble ions,polycyclic aromatic hydrocarbons and heavy metals were analyzed in laboratory.Cloud events were monitored using a Fog Monitor(Model FM-100,Droplet Measurement Technologies).The obtained microphysical characteristics(including cloud droplet concentration,liquid water content,equivalent diameter and median volume diameter)combined with the chemical compositions of the cloud samples,the meteorological data and PM2.5 concentrations were analyzed to investigate the formation and development characteristics of the cloud droplets and the transformation of chemical compositions between aerosol phase and cloud phase in the atmosphere.It was found that the volume weighted mean pH of cloud water in 2014 and in 2015 were 5.87 and 5.62,respectively,which were much higher than that observed in 2007-2008 with the observed pH value of 3.86[1].The electrical conductivity of clouds at Mt.Tai was higher than those in other orographic regions all over the world,indicating that the concentrations of cloud pollutants at Mt.Tai were high.The main water-soluble ions in the cloud water were NH4+,SO42+,Ca2+ and NO3-.Among them,NH4+and Ca2+ were the most important cations in neutralizing the acid of cloud water.The increase of their concentrations lead to the rise of pH values of cloud water in 2014 and in 2015.(NH4)2SO4 and NH4NO3 were the main hygroscopic components in the atmosphere.Occurrence of cloud events always accompanied with the increase in the concentrations of NH4+,SO42-and NO3-in the atmosphere.PM2.5 was likely to be the main source of soluble ions in the cloud water.The soluble ions in PM2.5 enter the cloud phase through dissolution,diffusion,dilution and liquid phase reactions.The higher the concentration of PM2.5,the higher the total ion concentration and the smaller the pH value in cloud water.The analysis of the backward trajectory showed that the air mass sources had a certain influence on the concentration of pollutants in the cloud,induceing the difference of chemical components during these two years.By studying the microphysical parameters,it represented that the liquid water content and the number concentration of the cloud droplets were various in different cloud events.But the observed droplet diameters were all smaller than 26.0 μm,most of them were between 6.0-9.0 μm and the peak number concentrations of cloud droplets appeared at 7.0 μm.It was found that the increase of relative humidity in the atmosphere and the decrease of the concentration of PM2.5 would promoted the decrease of the number of cloud droplets and the formation of large cloud droplets.At the same time,the liquid water content increased.Therefore,when estimating the concentrations of pollutants in the cloud,the dilution effect of cloud water should be taken into account.Finally,we summarized the formation mechanism of cloud droplets.The higher the concentration of PM2.5,the higher the concentration of cloud condensed nuclei in the atmosphere.It would increase the competitions of atmospheric water between cloud droplets,and resulting in the formation of cloud droplets with small sizes and the increase of soluble ion concentrations in cloud water.