The influence of ozone on atmospheric emissions of gaseous elemental mercury and reactive gaseous mercury from substrates

Experiments were performed to investigate the effect of ozone (O 3 ) on mercury (Hg) emission from a variety of Hg-bearing substrates. Substrates with Hg(II) as the dominant Hg phase exhibited a 1.7 to 51-fold increase in elemental Hg (Hg o ) flux and a 1.3 to 8.6-fold increase in reactive gaseous mercury (RGM) flux in the presence of O 3 -enriched clean (50 ppb O 3 ; 8 substrates) and ambient air (up to ∼70 ppb O 3 ; 6 substrates), relative to clean air (oxidant and Hg free air). In contrast, Hg o fluxes from two artificially Hg o -amended substrates decreased by more than 75% during exposure to O 3 -enriched clean air relative to clean air. Reactive gaseous mercury emissions from Hg o -amended substrates increased immediately after exposure to O 3 but then decreased rapidly. These experimental results demonstrate that O 3 is very important in controlling Hg emissions from substrates. The chemical mechanisms that produced these trends are not known but potentially involve heterogenous reactions between O 3 , the substrate, and Hg. Our experiments suggest they are not homogenous gas-phase reactions. Comparison of the influence of O 3 versus light on increasing Hg o emissions from dry Hg(II)-bearing substrates demonstrated that they have a similar amount of influence although O 3 appeared to be slightly more dominant. Experiments using water-saturated substrates showed that the presence of high-substrate moisture content minimizes reactions between atmospheric O 3 and substrate-bound Hg. Using conservative calculations developed in this paper, we conclude that because O 3 concentrations have roughly doubled in the last 100 years, this could have increased Hg o emissions from terrestrial substrates by 65–72%.