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 O-3 on the reductions of HgCl2 and HgBr2 were discussed. The laboratory results showed that if without any radiation, O-3 would not influence the chemical conversions between HgCl2/HgBr2 and Hg-0. However, if with involvement of radiation, a rapid reduction of HgCl2 and HgBr2 was observed, resulting in a significant production of Hg-0 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 O-3 concentrations in the air. We proposed that O-3 might be involved in productions of the unstable intermediate (HgX)-X-I (X = Br, Cl), which has a quicker photolysis rate than the (HgX2)-X-II. Our results suggest that the impacts of solar radiation and O-3 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.

Automated summary

The study examined methods to reduce HgCl2 and HgBr2 in dry air under UV, visible and solar radiations with different irradiances, discussing the impact of O-3 on the reductions. Results showed rapid reduction of HgCl2 and HgBr2 under radiation, with the magnitude depending on frequency and intensity, suggesting a possible relationship with O-3 concentrations. Potential involvement of O-3 in generating unstable intermediates was proposed for further investigation in atmospheric Hg chemistry.