A potential route for photolytic reduction of HgCl₂ and HgBr₂ in dry air and analysis about the impacts from Ozone

Among various gaseous oxidized mercury species, HgCl₂ and HgBr₂ are known to be more recalcitrant to direct photolysis under radiations. In this study, we examined a possible route to reduce HgCl₂ and HgBr₂ in the dry air under UV, visible and solar radiations with different levels of irradiances. Potential impact of O₃ on the reductions of HgCl₂ and HgBr₂ were discussed. The laboratory results showed that if without any radiation, O₃ would not influence the chemical conversions between HgCl₂/HgBr₂ and Hg⁰. However, if with involvement of radiation, a rapid reduction of HgCl₂ and HgBr₂ was observed, resulting in a significant production of Hg⁰ in the air. The magnitudes of reduction reactions heavily depended on the frequency of radiations as well as the irradiances. UV light had the strongest capacity to promote photoreduction of HgCl₂ and HgBr₂ even at relatively low irradiances. A relationship analysis indicated that the reduction of HgCl₂ and HgBr₂ was possibly related to the O₃ concentrations in the air. We proposed that O₃ might be involved in productions of the unstable intermediate Hg^IX (X = Br, Cl), which has a quicker photolysis rate than the Hg^IIX₂. Our results suggest that the impacts of solar radiation and O₃ on reductions of Hg^II species in the atmosphere are worth of being investigated theoretically and in both laboratory and field studies in the future, since it might be an important but lesser-known part in the atmospheric Hg chemistry.