Impacts Of Light Radiation And Ozone Tothe Chemical Transformation Of Reactive Gaseous Mercury

Mercury is the only heavy metal pollutant which exists in gaseous state in the atmosphere.There are three main forms of atmospheric mercury:gaseous elemental mercury?GEM?,reactive gaseous mercury?RGM?and particle-bound mercury?PBM?.Although the concentration is low,RGM has potential threats to the aquatic ecosystem and human health when deposite to the terrestrial landscape.Photo-chemical transformation of RGM is an important part of mercury chemical cycle in the atmosphere.In this study,the capture efficiency of the four RGM sampling method?KCl coated quartz sand tube?KCl-S?,KCl coated denuder?KCl-D?,KCl coated glass fiber filter?KCl-F?and cation exchange membrane?CEM??on RGM were compared under different humidity conditions?low humidity:²0%,medium humidity:50-70%and high humidity:¹00%?.The effects of different light conditions?ultraviolet light?200-400 nm?,visible light?400-780 nm?and simulated sunlight?200-1000 nm??and radiation intensity on the chemical transformation of RGM?HgCl₂?and the impacts of O₃ on the photo-chemical transformation of RGM were studied as well.After laboratory experiment and outdoor monitoring,the results showed that KCl-S has higher sampling efficiency on RGM than KCl-D and KCl-F.Besides,KCl-S is more stable than three other methods.KCl-S has no significant effect by the relative humidity,although the RGM sampling efficiency of KCl-S is slightly lower than CEM,which is about 83%of that of CEM.KCl-S has the advantages in low blank,convenient measured method and low cost.Light promotes the reduction of RGM,and the reduction rate of RGM increases with the increase of radiation intensity.Ultraviolet light has the most significant effect on the reduction of RGM,under which the ratio of GEM increased 3.3-4.1 times than that under dark conditions.However,visible light has a relatively small effect,where the ratio of GEM increased only 59.9-129.2%compared with that under dark conditions.O₃ promotes the reduction of RGM under dark and visible light.The possible reaction is HgCl₂+2O₃?Hg⁰+2O₂+2ClO·.Under ultraviolet and simulated sunlight conditions,O₃ inhibited the reduction of RGM to some extent,and the inhibition intensified with the increase of O₃ concentration.The increased GEM ratio compared to the dark condition decreased from 405.1%to 330.6%under ultraviolet light when concentration of O₃ increased from 0 ppb to 100 ppb.It may be caused by halogen radicals such as ClO·which were produced under ultraviolet light.The results of atmospheric mercury monitoring in the Mount Qomolangma region in Tibet are basically consistent with the results of laboratory simulation.Field monitoring showed that RGM concentration showed a significant diurnal variation trend,which was positively correlated with solar radiation intensity and ozone concentration,indicating that O₃ inhibited the reduction of RGM under sunlight,especially ultraviolet light.The results showed that the synergistic mechanism of O₃ and light?wavelength and radiation intensity?and the induced radicals by O₃ and light should be the important factor of the RGM transformation process in the atmosphere.