Effects of relative humidity and CO(g) on the O3-initiated oxidation reaction of Hg0(g): kinetic & product studies

Abstract

Ozone is assumed to be the predominant tropospheric oxidant of gaseous elemental mercury (Hg⁰(g)), defining mercury global atmospheric lifetime. In this study we have examined the effects of two atmospherically relevant polar compounds, H₂O(g) and CO(g), on the absolute rate coefficient of the O₃-initiated oxidation of Hg⁰(g), at 296 ± 2 K using gas chromatography coupled to mass spectrometry (GC-MS). In CO-added experiments, we observed a significant increase in the reaction rate that could be explained by pure gas-phase chemistry. In contrast, we found the apparent rate constant, k_net, varied with the surface-to-volume ratio (0.6 to 5.5 L flasks) in water-added experiments. We have observed small increases in k_net for nonzero relative humidity, RH < 100%, but substantial increase at RH ≥ 100%. Product studies were performed using mass spectrometry and high resolution transmission electron microscopy coupled to an electron dispersive spectrometer (HRTEM-EDS). Our results give evidence for enhanced chain growth of HgO(s) on a carbon grid at RH = 50%. A water/surface/ozone independent ozone oxidation rate is estimated to be (6.2 ± (1.1; tσ/√n) × 10⁻¹⁹ cm³ molecule⁻¹ s⁻¹. The total uncertainty associated with the ensemble of experiments amount to approximately ≤20%. The atmospheric implications of our results and the effect of an added reaction partner in homogeneous and heterogeneous atmospheric chemistry will be discussed.