Characteristics Of Atmospheric Mercury Deposition And Isotopes In Xiamen

Mercury（Hg）has been regarded as a global heavy metal contaminant of high neurotoxicity.Once emitted from its sources,Hg species perform global biogeochemical cycles among atmosphere,water,sediment and biosphere due to its high volatility and good migration properties.Atmospheric deposition is a complex physical and chemical process and a principal pathway of atmospheric mercury entering terrestrial and aquatic environments.In order to understand the characteristics and isotopic composition of mercury in dry and wet depositions in coastal cities,Hg dry and wet deposition samples were collected in Xiamen,southeast coastal city of China in this study.The characteristics,the influencing factors,and each contribution of atmospheric Hg dry and wet deposition at different environmental functionareaswerestudied.Additionally,mercuryisotopic compositions of the dry and wet Hg deposition were analyzed.Isotope tracer technology and backward trajectories analysis were applied to investigate the sources and the transformation of atmospheric Hg.The main work and conclusions are as follows:（1）Weekly precipitation samples were collected from June 2016 to May 2017 and August 2017 to July 2018 at urban,suburban and rural sites in Xiamen,and concentrations of total mercury（THg）were determined according to EPA 1631 method.The volume-weighted mean concentrations of THg were 7.70,7.55 and 7.15 ng·L^-11 and the wet deposition fluxes of THg were 7.19,6.66 and 8.19μg·m^-2·yr^-11 at the urban,suburban,and rural sites,respectively.One-way Anova test revealed that no significant difference of THg concentration among the three sites,with the slightly higher values at the urban and suburban sites.However,the seasonal variation of THg concentration strongly depended on the types of locations.The highest THg concentration was observed in summer at the urban and suburban sites but in winter at the rural site.THg deposition flux was more subjected to precipitation amount at the rural site.Whereas,THg deposition fluxes at the urban and suburban areas were closely correlated to THg concentration,reflecting the existence of continuous Hg emission sources.Principal component analysis results suggest that oxidation processes in the atmosphere were probably the important contributions of Hg in precipitation,especially at the rural area.（2）Atmospheric dry deposition samples from August 2017 to July2018 at the three sites in Xiamen were analyzed for the concentrations of particulate bounded mercury（PBM）and reactive gaseous mercury（RGM）using bath heating-antiaquar hydrolysis combined with cold vapour atomic fluorescence spectrometer detection.The average concentrations at the urban,suburban,and rural sites were 46.5,47.9,and 30.6 pg·m^-33 for PBM,and 2.8,2.6,and 1.7 pg·m^-33 for RGM.Dry deposition fluxes of PBM+RGM were 9.2 and 5.1μg·m^-2·yr^-11 at the urban and rural sites（lacking of the autumn values at the suburban site）,respectively.PBM and RGM concentrations were significantly different among the three sites,with the higher concentration at the urban and suburban sites.The seasonality of PBM concentration was consistent among the three sites,with the highest PBM concentration in winter.However,the seasonal pattern of RGM strongly depended on the types of locations,with an elevated summer RGM concentration at the urban and suburban sites and a higher winter RGM at the rural site.The dry deposition flux of Hg（PBM and RGM）was 1.8,1.7,and 1.0 times of the wet deposition flux at the urban,suburban,and rural sites,respectively,suggesting that dry deposition appears to be more predominant removal pathway of atmospheric Hg in the areas that is strongly impacted by local Hg emission sources.（3）Isotopic composition of THg in precipitation and PBM in dry deposition at the urban,suburban and rural sites in Xiamen was analyzed using multi-receiver inductively coupled plasma spectrometer.Both THg in precipitation and PBM in dry deposition were characterized by negative mass-dependent fractionation of even isotopes(MDF,δ²⁰²Hg=-0.7 and-0.72‰)and slightly positive mass-independent fractionation of even isotopes(even-MIF,Δ²⁰⁰Hg=0.13‰and 0.08‰).MIF of odd isotopes was positive in precipitation THg(Δ¹⁹⁹Hg,0.24‰),whereas it was negative in dry deposited PBM（-0.13‰）.Theδ²⁰²Hg values in precipitation at the urban site was closer to the values of coal-burning flue gas in Xiamen than those at the suburban and rural sites,suggesting a greater effect of coal combustion.TheΔ¹⁹⁹Hg values of precipitation and PBM samples were higher at rural site than urban site,indicating thatΔ¹⁹⁹Hg values were more influenced by the extensive photo-reduction under the long-distance air mass transport at the rural site.TheΔ¹⁹⁹Hg values of precipitation and PBM samples reveal that the sources of atmospheric mercury were relatively complex at the suburban site.Backward trajectories analysis indicates a higher values ofΔ¹⁹⁹Hg relating to long-distance terrestrial air mass transport and lowerδ²⁰²Hg under the short-distance terrestrial air mass.The results suggest the influence of extensive photo-reduction and accumulation of local emissions,respectively.On the other hand,theΔ¹⁹⁹Hg andΔ²⁰¹Hg values of atmospheric PBM were elevated and theδ²⁰²Hg was positive relating to the marine air mass,reflecting the highδ²⁰²Hg in the ocean splash,and the accumulation of odd-MIF during the transport.