Characterization,Transformation And Transport Ofatmospheric Mercury At Mountain Sites

Mercury?Hg?is a highly neurotoxic global heavy metal pollutant that is readily volatile,persistent and bioaccumulative.Once released into environment,mercuy can perform global biogeochemical cycles in different forms among air,surface water,soil and biosphere.As an important medium for mercury transport and transformation,the atmosphere plays an important role in the global mercury cycle.To understand the atmospheric mercury background concentrations in North China and Hong Kong where the air is seriously polluted due to intensive human activity,and the facors influencing atmospheric mercury distribution features,the chemical transformation machenisms,potential source regions and long-range transport,field campaigns were carried out at Mt.Tai in North China during June-August,2015 and at Mt.Tai Mo Shan?Mt.TMS?in Hong Kong during November,2016-April,2017,to study the total gaseous mercury?TGM?,particulate bound mercury?PBM?and speciated mercury in cloudwater.We analyzed mercury concentrations and chemical speciation,TGM diurnal patterns,transport and transformation mechanisms in cloud processes.A variety of source analytical methods were employed to evaluate the contribution of human activities to mercury in the upper atmosphere.The results showed that TGM concentrations at Mt.Tai vaired with small amplitude,while those at Mt.TMS sharply increased by 1-3 ng m^-3 in a short time.The averaged concentrations of TGM at Mt.Tai and Mt.TMS were 2.18±0.56 and 2.26±0.64 ng m^-3,respectively,representing the TGM background concentrations in North China and Hong Kong,which were moderate compared with TGM concentrations in other background area in China.TGM concentrations were apparently related to wind direction,with abundant TGM from southwest wind at Mt.Tai and obviously higher TGM from northeast wind at Mt.TMS.TGM at Mt.Tai and Mt.TMS both showed significantly positive correlations with CO,indicating the contributions of long-range transport,and the good correlations with PM2.5 and O3 suggested contributions from regional pollutants and photochemical reactions,while the insignificant correlations with NO implied relatively less anthropogenic effects.With the criteria of NO/NOx ratio less than 0.3,long-range transport was estimated to contribute 69.5%and 54.0%of TGM at Mt.Tai and Mt.TMS,respectively.The diurnal pattern of TGM at Mt.Tai was similar to that of CO,showing highest concentrations at midday and lowest at sunrise with relatively higher concentrations before wee hours,which was mainly controlled by the varying boundary layer.TGM at Mt.TMS showed higher concentrations at nighttime and lower at daytime with a dramatic decline at 8:00-14:00 when solar radiation strengthened.We observed four distinct TGM depletion events during TMS campaign,which commonly occurred between 9:00-12:00 and rapidly decreased to less than 0.5 ng m^-3 with decreased O3 and increased NO2.To our knowledge,this is the first time that atmospheric mercury depletion event was observed in low latitude marine boundary layer,which was likely attributed to two-step photooxidation of TGM induced by concurrence of increased solar radiation,elevated halogen radicals and high concentraions of NO2.The average PBM concentraions at Mt.Tai and Mt.TMS were 84.1 and 116.2 pg m^-3,respectively,much higher that those at most background sites in China,which indicated severe PBM pollution in North China and Hong Kong.Mercry content in PM2.5 at Mt.Tai and Mt.TMS were 4.8 and 19.8 ?g/g,respectively.The distinct differences were caused by PM2.5 mass concentrations,temperature and relative humidity,which controlled the adsorption and absorption of reactive mercury to particulate matters via changing particle size,influencing gas-particle equilibrium of reactive mercury and liquid water content in sea salt aerosols.In addition,aerosols at Mt.TMS had higher chloride content which favored the complexation with reactive mercury and thus promote PBM formation.Clouwater pH values at Mt.Tai and Mt.TMS were 5.79?ranging 4.82-6.95?and 3.63?ranging 2.96-5.94?,respectively,and the volume weighted mean concentrations of total mercury?THg?,dissolved mercury?DHg?and methylmercury?MeHg?were 47.6 and 25.8 ng L^-1,16.8 and 16.0 ng L^-1 and 0.14 and 0.06 ng L^-1 respectively.Particulate mercury?PHg?in cloudwate at Mt.Tai showd higher proportion?65.7%?than that at Mt.TMS?36.3%?.The significant positive relationships of PHg with the droplet number concentratin?Nd?and conductivity at Mt.Tai indicated the direct contributions by aerosol particles.The lower pH range at Mt.TMS had obvious dissolution effects on DHg,and the liquid water content?LWC?at both sites had apparent dilution effects of DHg.Both pH and LWC had little effects on MeHg.Thermodynamic equilibrium model results showed that the Hg-dissolved organic matter?DOM?complexes was the major chemical speciation of DHg,whereas higher pH value and chloride concentraions caused decrease in Hg-DOM formation due to increased OH-and Cl-competition with DOM for Hg.Studies on continuous cloud events at Mt.Tai revealed that cloud processing not only affected concentrations of speciated Hg in cloudwater,but also changed the Hg chemical speciation.Based on Henry's law and cloud scavenging efficiency of PBM,it was estimated that cloudwater Hg at Mt.Tai,particularly the PHg,were mainly from cloud scavenging of PBM.The adsorption coefficient?Kad?of cloud residues on Hg was inversely related with cloud residues content in power law.The measured mean Kad of 69.8 L g-1 was much higher than that used in modeling.SEM-EDS and HYSPLIT analyses on typical cloud residue particles showed that the fly ash particles released by industry were featured with higher specific surface area and more surface carbon binding sites,leading to stronger physical and chemical adsorption capacity of Hg compared to mineral particles.Therefore,abundant fly ash particles in cloud residues should have enhanced Kad.After excluding many sources,the cloudwater MeHg was speculated to be mianly formed via abiotic methlation of Hg.Comparing TGM diurnal pattern between in-cloud day and clear day,we found the TGM concentrations in in-cloud day were notably higher than those in clear day,which contradicted the cloud scavenging effect.The concentration differences were extremely high(0.4-1.2 ng m^-3)at 7:00-14:00 when the cloudwater DHg and PHg concentrations increased significantly and PM2.5 mass also showed increasing tendency.The result indicated that PBM contributions and aqueous photoreduction of cloudwater Hg caused the increased TGM in in-cloud day,and photolysis of Hg-DOM complexes was calculated to be likely the major reduction pathway.Principle factor analysis?PCA?showed that TGM at Mt.Tai and Mt.TMS were both mainly long-range transported,and TGM at Mt.Tai was also impacted by Hg redox reactions.Positive matrix factorization?PMF?results indicated different sources for various speciated mercury in cloudwater.Cloudwater DHg at Mt.Tai mainly came from industry emission and marine source,PHg was contributed by dust/aerosol particles and MeHg from abiotic methylation.In contrast,cloudwater MeHg at Mt.TMS was largely from marine and partly from soil,whereas DHg and PHg were probably related with gaseous mercury conversion.Potential soure region distribution?PSCF?analysis showed that southwest China,Henan Province,and parts of Shandong Province and Yangtze River Delta were the most likely source regions of TGM at Mt.Tai,meanwhile Hunan,Guangxi and Guangdong Provinces were the major source regions of TGM at Mt.TMS,both of which agreed with anthropogenic mercury emission distribution in China,implying more significant contributions of long-range transport compared to local/regional emission sources.Southwest China was the source regions of cloudwater Hg at Mt.Tai,however,the Yellow sea was also an important source region of DHg but contributed litte to PHg and MeHg.All cloudwater Hg species at Mt.TMS originated from south China sea and west Pacific Ocean,displaying similar soure region distributions.The ?TGM/?CO ratios for atmospheric mercury pollution long-range transport events observed at Mt.Tai and Mt.TMS were 0.0081 and 0.0059±0.0019 ng m^-3 ppb-1,agreeing well with the character of East Asia pollution plume(0.0056 ng m^-3 ppb-1),especially for that observed at Mt.TMS.Accordingly,the anthropogenic mercury emission was estimated to be 817±263 t/a.This research used the mountain site as platfoms to elucidate atmospheric mercury concentrations,speciation distributions and transformation among gas-,particulate and aqueous-phase in the planet boundary layer and marine boundary layer.We found the atmosphere above North China and Hong Kong were already polluted by atmospheric mercury,especially the cloudwater.Boundary layer variation and photoredox controlled the spatiotemporal distribution of atmospheric mercury in terrestrial and marine environment.The contribution of aerosols to clouwater Hg,adsorption by cloud residues and aqueous-phase photo reduction significantly affected the chemical transformation of atmospheric mercury.Additionally,the atmospheric mercury pollution could be long-range transported to mountain sites in China,and even the west Pacific Ocean.The results are of significant importance for evaluating human activities contributions to atmospheric mercury and further understanding global mercury geochemical cycles.