| With the development of social economy and the acceleration of urbanization,the problem of air pollution is becoming more and more serious,which has become one of the environmental and people’s livelihood issues widely concerned by the public.Atmospheric aerosol pollutants can lead to the generation of haze,which will not only significantly reduce visibility,then affect traffic safety,but also may enter the human body through the respiratory tract,causing respiratory and cardiovascular diseases.In addition,some atmospheric components,which are inherently toxic,are likely to pose greater health risks to organisms once they enter the bodies.Sulfate aerosol and atmospheric mercury are two typical components of atmospheric pollutants.As a secondary aerosol,sulfate can account for a large mass ratio(10%～30%)in fine atmospheric particles(such as PM2.5)during urban haze period,and to be an important driving factor leading to urban haze.However,at present,there are still great uncertainties about the physical or chemical mechanism of the rapid growth of sulfate during urban haze,as well as the corresponding influencing factors.Mercury(Hg)is a highly toxic metal.Due to its high volatility and high atmospheric residence time,its gaseous elemental form of mercury(accounting for 90%of the total atmospheric mercury form)can be transported over long distances with atmospheric circulation.Once depositing and entering the water and soil,it may be methylated under the action of microorganisms and be converted into the form of methylmercury which is more toxic and can be enriched through the food chain.The long-distance migration of mercury offers it more harmful potential to the environment on a larger spatial scale,which requires more explorations of its the migration and transformation mechanisms from a larger spatial perspective.From the inland to the ocean,and then to the polar regions,although a large number of previous studies have been conducted on the cycle mechanisms of atmospheric mercury,there are still many deficiencies in the researches on some key scientific issues due to the limited relevant observations in some regions.This paper conducted several field observations,including the PM2.5 samples collected in Beijing from October 2014 to January 2015 and corresponding observations of atmospheric trace components,observation of gaseous elemental mercury in Hefei from January to October 2016,and observations of atmospheric and surface seawater mercury during the 34th Chinese National Antarctic Research Expedition(34th Chinare),combined with laboratory chemical analyses,mathematical statistics and model simulations,to discuss the key processes of transport and transformation of atmospheric sulfate aerosol and mercury in this paper.The main conclusions are presented as follows:(1)Analysis of the chemical compositions of PM2.5 collected during haze periods in Beijing shows that during the aggravating process of haze,the mass fraction of sulfate in PM2.5 increased while that of nitrate declined slightly.Besides,the relative growth rate of sulfate is higher than that of nitrate.These characteristics suggest that with the worsen of urban haze,the relative contribution of stationary emission sources to urban haze in Beijing will increase compared with that of mobile emission sources.Further research indicates that it may be attributed to regional transport of sulfate from heavy industrial areas accompanied by increased sulfate secondary transformation during polluted periods.(2)The variation features of the production rate of sulfate heterogeneous S(Ⅳ)+NO2 pathway(PS(Ⅳ)+NO2)and its relative importance among the sulfate total heterogeneous production pathway(PS(Ⅳ)+NO2/Phet)during the haze evolution in Beijing was investigated in this study.The calculated PS(Ⅳ)+NO2 ranged from 1.97×10-4 to 5.91 μg·m-3·h-1.The PS(Ⅳ)+NO2/Phet was generally correlated positively with PM2.5 concentrations,indicating the increased relative importance of S(Ⅳ)+ NO2 pathway on haze days.However,due to the mutual restriction between aerosol pH and aerosol liquid water content(ALWC)during haze evolution,the relative contribution of the S(Ⅳ)+NO2 pathway to sulfate heterogeneous formation was generally limited to 40%.(3)The characteristics and corresponding influencing factors of sulfate heterogeneous transmission-metal-ions(TMI)catalyzed oxidation pathway during haze evolution in Beijing was investigated in this study.It was found that although previous studies have shown a high sensitivity of the TMI-catalyzed pathway to aerosol pH,sulfate production rates of this pathway did not always increase in haze aggravation stages with higher aerosol acidity,suggesting that aerosol pH would not necessarily be the main control factors of this pathway.Further exploration suggests that the highly variable dissolved Fe(Ⅲ)and Mn(Ⅱ)concentrations and ionic strength during haze evolution processes would considerably influence the production rate of the TMI-catalyzed pathway.This further implies that the north or northwest air masses,which often carry crustal-source particulates,may have a non-negligible effect on sulfate formation during the generation and/or dissipation stages of haze in Beijing by influencing aerosol pH and transition-metal contents.(4)The seasonal characteristics of atmospheric gaseous elemental mercury(GEM)in Hefei in the winter,spring and autumn in 2016 was explored in this study.Higher GEM concentrations in spring were associated with more abundant anthropogenic emissions and biomass burning before spring ploughing in this season.GEM concentrations in three seasons obviously decreased from 2013～2014 to 2016(22.4%～49.2%),reflecting the effectiveness of air quality control measures for the coordinated control of atmospheric Hg.The relative contributions of three categories of anthropogenic GEM sources among their regional transmissions in each season was further quantified in this study.It showed that their relative contributions in three seasons were similar,with industrial sources accounting for the highest contribution(56%～59.6%);Intentional use and product waste associated sectors followed(24.5%～27.1%);and the stationary combustion sources accounted for the lowest(15.6%～16.9%),implying control of industrial Hg emissions should be given priority in future measures.This study compared the simulated results of the GEOS-Chem model with the field observation results.It is found that the GEOS-Chem model significantly overestimated the actual observed value of GEM by 75%,suggesting that timely updating mercury emission inventory data is needed to elevate the accuracy of model simulation of atmospheric mercury under the background of implementing "Minamata Convention" and a series of air pollution control policies such as the "Beautiful China"strategy in the future.(5)The observations of atmospheric mercury in western Pacific displayed that,as the border between the northern and southern hemispheres,the role of tropical western Pacific in the global transport and the hemispheric distribution of atmospheric Hg might be special.The calm weather in doldrums in the tropical Western Pacific would not conducive to the horizontal transport and diffusion of GEM,which would impede its inter-hemispheric transmission.Additionally,the low levels of O3 and sea-salt aerosols in doldrums may also reduce the oxidization and the subsequent dry deposition of GEM in the boundary layer,which would further facilitate GEM accumulation in this area and delay its migration.In addition,the Intertropical Convergence Zone(ITCZ)in the tropical western Pacific,characterized by intense convective precipitation and strong solar radiation,can lead to elevated DGM levels in the surface seawater due to the abundant wet deposition and subsequent photoreduction of Hg.It can increase the marine release of HgO and,in turn,impact the latitudinal distribution of GEM and extend the lifetime of Hg in actively cycling reservoirs.(6)The large-scale observations of atmospheric mercury species in Southern Ocean displayed that the large discrepancy in the hydrological environments appear to impact the atmospheric mercury cycle in the Antarctic marine boundary layer,causing their large spatial differences.The Antarctic continent can serve as an important source of gaseous oxidized mercury(GOM)during sunlit summer period,due to the significant in-situ oxidation and the transmission of GOM-rich air masses thereto from the Antarctic Plateau via katabatic wind.Significant in-situ oxidation of GEM could also occur in sea-ice regions,while the uptake of high content of sea-salt aerosols might efficiently eliminate atmospheric GOM in the summertime.It would be an important factor that influence the deposition flux of Hg,and can explain the phenomenon observed in previous studies that atmospheric oxidized Hg in the sea-ice area mainly takes the form of particle-bound mercury(PBM).In the oceanic region,the lowest level of GOM is attributed to both the uptake of sea-salt aerosols and the seasonal melting of first-year sea-ice.These features indicated that the impact of various hydrological environments should be seriously considered in the estimation of Hg deposition in the Antarctic area.We first compared the simulated GEM and GOM concentrations from GEOS-Chem model with their observed results among Antarctic marine boundary layer,and found that the GEOS-Chem model significantly underestimated GEM while overestimated GOM in standard mode,compared with our observations,suggesting the corresponding migration and transformation mechanisms of mercury(e.g.transport from Antarctic continent,the impact of sea-ice and hydrological environment)in model need further optimizations in Antarctic MBL... |
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