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

Abstract

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%.