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 > or = 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; tsigma/ radicaln) x 10(-19) cm3 molecule(-1) s(-1). The total uncertainty associated with the ensemble of experiments amount to approximately < or = 20%. The atmospheric implications of our results and the effect of an added reaction partner in homogeneous and heterogeneous atmospheric chemistry will be discussed.

Recent Advances in Atmospheric Chemistry of Mercury

“…Despite the potential significant role of heterogeneous Hg reactions on atmospheric Hg chemistry and model simulations [37,92], scare data is available on mercury reactions and equilibrium processes on atmospheric surfaces such as aerosols [48]. A systematic understanding of the surface chemistry of Hg is extremely difficult [48,93] due to the varying concentration, size distribution and composition of aerosols at different locations, times and meteorological conditions [94,95].…”

A new mechanism for atmospheric mercury redox chemistry: implications for the global mercury budget

“…It has been postulated that intermediate products like HgO 3 could lead to the formation of stable (HgO) n oligomers or HgO (s) via decomposition to OHgOO (g) (Subir et al, 2011), but these must be stabilized through heterogeneous reactions on atmospheric aerosols (Calvert and Lindberg, 2005), and the associated mechanism is unlikely to be significant in the atmosphere (Hynes et al, 2009). A gas-phase reaction has been reported in chamber studies (Hall, 1995;Pal and Ariya, 2004;Sumner et al, 2005;Snider et al, 2008;Rutter et al, 2012), but this is likely to have been influenced by the walls of the chamber (as seen in Pal and and presence of secondary organic aerosols (in Rutter et al, 2012). j Atmospheric concentrations of I and IO (Dix et al, 2013;Prados-Roman et al, 2015;Volkamer et al, 2015) are too low for these reactions to be significant.…”

Mechanistic Study on the Ozone‐Mercury Reaction and the Effect of Water and Water Dimer Molecules

“…Our calculated rate constants in wet reaction with two and one water molecule are 4.76 × 10 -20 and 1.11 × 10 -25 cm 3 molec -1 s -1 , respectively, that are comparable with the experimental vales. Experimental values predict that rate constant increases with relative humidity [12][13] Fig. 6, it can be seen that the rate constant k increases rapidly with the number of water molecule in reaction.…”

“…The effect of water molecules on gas phase reaction has not been extensively studied. Ariya 12 and Snider 13 found that the rate constant varied between 3 × 10 -19 and 30 × 10 -19 cm 3 molelules -1 s -1 as the relative humidity was varied.…”

“…[11][12][13] In the present work, we report thermodynamic and kinetic studies of the gas phase reaction of elemental mercury with ozone (dry) and have also probed the effect of water and water dimers on the reaction.…”