Characteristics And Influencing Factors Of Atmospheric Mercury Sources And Sinks In Nanjing

Mercury is a heavy metal with persistency and neurotoxicity,which can be transported for a long distance in the atmosphere and spread globally before being deposited to earth's surfaces through dry and wet deposition,causing adverse effects on humans and ecosystems.The assessment of Hg impacts on humans,animals and ecosystems and the establishment of Hg emission control policies all require the accurate understanding of the source and sink characteristics of atmospheric Hg.This study developed a comprehensive method for the measurement of atmospheric mercury concentration and deposition at the Xianlin campus of Nanjing University?NJU?.Based on the measurement of speciated Hg concentrations,the source characteristics and the impacts of meteorological factors and regional sources on the atmospheric Hg were studied using the generalized additive model?GAM?and HYSPLIT model and the results showed that the reduction of anthropogenic Hg emissions was the main driver of the decrease of gaseous elemental mercury?GEM?at the NJU site.In addition,we conducted the observation of gaseous oxidized mercury?GOM?and total atmospheric mercury?TAM?dry deposition for the first time in China,and the sink characteristics were identified.The key influencing factors of atmospheric Hg dry deposition were studied by comparing with the simulations of speciated Hg,then the parameterized scheme of speciated Hg dry deposition model were modified.In this study,two campaigns with continuous measurements of GEM,GOM and particulate-bound mercury?PBM?were conducted at the suburban NJU site during the periods of August 2014 to July 2015 and May 2018 to April 2019.The overall mean GEM,GOM and PBM concentrations decreased significantly from 3.77±1.32 ng m^-3,35.97±58.45 pg m^-3 and 114.07±170.29 pg m^-3 to 3.24±1.26 ng m^-3,15.97±23.38pg m^-3 and 66.22±110.07 pg m^-3,respectively.The average decreasing rate of GEM was-0.13 ng m^-3 yr^-1 or-3.7%yr^-1,more aggressive than the declining trends in North America and Western Europe,indicating the effectiveness of air pollution control measures taken by China in recent years.In former campaign,GEM concentrations at NJU site did not show an obvious seasonal variation due to the great influence of local signals.In latter campaign,with the decrease of the impact of local signals,the impact of long-range transport increased.High GEM concentration air masses from Anhui province and North China Plain transported to the NJU site in spring and winter,respectively,making GEM higher in these two seasons.GAMs were utilized in this study to first subtract the local emission contribution and then quantify the impacts of meteorological factors and regional sources on the GEM concentration.The overall model performance is satisfactory,with the R² values for two campaigns were 0.43 and0.57,respectively.The residuals of GAM were likely to reflect the impact of local signals,and the better performance of GAM for latter campaign implied the declining influence of local anthropogenic emissions at NJU.The impact of relative humidity on GEM was crucial via the reduction of Hg^?II?on particles in the atmosphere,the mechanism of which requires further investigation.Air temperature and wind speed affect GEM by soil Hg^?0?evasion and GEM stagnation,respectively.The main sources of GEM at NJU were from the east and the northeast?0–150°?,but air masses from these regions contained relatively low GEM on average,while air parcels from the southwest??210°?had slightly higher GEM.Additionally,the major sources of GEM at NJU were concentrated within 600 km radius.Atmospheric Hg wet deposition was continuously measured during the period from November 2018 to November 2019.During the observation period,the wet deposition flux at the NJU site was 4.84?g m^-2,equivalent to some background site in Northern Hemisphere since the relatively less precipitation during the observation period.However,the volume-weighted concentration of total mercury was 7.13 ng L^-1,still at a high level.Atmospheric mercury wet deposition flux at NJU was mainly affected by precipitation depth?R²=0.6?.The correlation analysis between total mercury concentrations and other main anion and cation in precipitation at NJU indicated that the source of mercury in precipitation at NJU was relatively complex.GOM dry deposition was also measured during the period of July 2018 to November2019,and the overall averaged GOM dry deposition flux was 0.75±0.51 ng m^-2 h^-1,equivalent to some urban sites in Northern America.GOM dry deposition flux presented an evident seasonal variation,with the highest flux in summer(0.96 ng m^-2h^-1)and the lowest flux in winter(0.63 ng m^-2 h^-1),which possibly due to the relatively high GOM concentration and strong air turbulence in summer.Moreover,TAM dry deposition was also measured by surrogate surface during April 2019 and November2019.The overall averaged TAM dry deposition flux was 2.40±1.16 ng m^-2 h^-1,higher than that of most site in North America.If take the observed averaged TAM dry deposition as annual averaged,the annual TAM dry deposition at NJU was 21.02?g m^-2 y^-1,much high than wet deposition flux,indicating that dry deposition was the main contributor for atmospheric Hg deposition at NJU.This study also conducted simulations of speciated Hg dry deposition using dry deposition models based on the observations of speciated Hg concentration,and the simulated depositions was then compared with depositions measured using the surrogate surfaces.Due to the underestimation of GOM concentrations and the difference of the dry deposition velocity of different GOM components,a scaling factor of 3 was used in this study to adjust the GOM concentrations and?and?values were set at 2 to get a more accurate simulated dry deposition flux(0.76±0.39 ng m^-2 h^-1).In addition,the averaged simulated dry deposition flux of GEM and PBM by the modified dry deposition model were 1.90±2.22 ng m^-2 h^-1 and 0.43±0.38 ng m^-2 h^-1,respectively.GEM was the main species of atmospheric mercury dry deposition at NJU,with its contribution share of 61.5%.However,the contribution share of reactive mercury?RM?dry deposition at NJU was higher?38.5%?than most of other sites,mainly because of the high RM concentration at the NJU site.