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

Abstract 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 HgIX (X = Br, Cl), which has a quicker photolysis rate than the HgIIX2. Our results suggest that the impacts of solar radiation and O3 on reductions of HgII 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.