Photochemistry of oxidized Hg(I) and Hg(II) species suggests missing mercury oxidation in the troposphere

Abstract

Mercury (Hg), a global contaminant, is emitted mainly in its elemental form Hg0 to the atmosphere where it is oxidized to reactive HgII compounds, which efficiently deposit to surface ecosystems. Therefore, the chemical cycling between the elemental and oxidized Hg forms in the atmosphere determines the scale and geographical pattern of global Hg deposition. Recent advances in the photochemistry of gas-phase oxidized HgI and HgII species postulate their photodissociation back to Hg0 as a crucial step in the atmospheric Hg redox cycle. However, the significance of these photodissociation mechanisms on atmospheric Hg chemistry, lifetime, and surface deposition remains uncertain. Here we implement a comprehensive and quantitative mechanism of the photochemical and thermal atmospheric reactions between Hg0, HgI, and HgII species in a global model and evaluate the results against atmospheric Hg observations. We find that the photochemistry of HgI and HgII leads to insufficient Hg oxidation globally. The combined efficient photoreduction of HgI and HgII to Hg0 competes with thermal oxidation of Hg0, resulting in a large model overestimation of 99% of measured Hg0 and underestimation of 51% of oxidized Hg and ∼66% of HgII wet deposition. This in turn leads to a significant increase in the calculated global atmospheric Hg lifetime of 20 mo, which is unrealistically longer than the 3–6-mo range based on observed atmospheric Hg variability. These results show that the HgI and HgII photoreduction processes largely offset the efficiency of bromine-initiated Hg0 oxidation and reveal missing Hg oxidation processes in the troposphere.

Development of an Understanding of Reactive Mercury in Ambient Air: A Review

“…, ClO, BrO, ClBr) [1,2]. A more recent paper by Saiz-Lopez et al [3] provides an update on current thinking regarding our understanding with respect to reactions and points out, using a global model based on bromine-induced GEM oxidation that other oxidation mechanisms are needed in the troposphere to explain observations. In 1979, Fogg and Fitzgerald [4] postulated that since GEM is not highly water soluble, concentrations measured in precipitation could not be explained by GEM alone.…”

Dynamics of gaseous oxidized mercury at Villum Research Station during the High Arctic summer

“…7). While ozone mixing ratios were high during GOM enhancement events, they are an order of magnitude below levels reported in the upper troposphere-lower stratosphere (Talbot et al, 2007), and given the slow rate reaction coefficient (Pal and Ariya, 2004), ozone is an improbable first oxidant of mercury during these campaigns (Calvert and Lindberg, 2005), although ozone has recently been identified as a second oxidant of Hg I (Saiz-Lopez et al, 2020). While ozone might be acting as a second oxidant of Hg I , any depletions of ozone during GEM oxidation, either through reaction with the Hg I intermediate or with halogen species, is likely masked by the elevated levels of ozone in the free troposphere.…”

“…2 and 7). Ozone could originate from both anthropogenic and natural sources, and ozone is only slightly water soluble, leading to less efficient wet removal (Sander, 2015). GOM observed during Event 5a appears to be the result of emissions either from active fires or anthropogenic sources in northern Scandinavia.…”

“…Of which Br, I, and OH have been postulated to be the main species for Y , both globally and in the Arctic (Goodsite et al, 2004(Goodsite et al, , 2012, while NO 2 , HO 2 , ClO, or BrO have been demonstrated to be candidates for Y by Dibble et al (2012). Recently, ozone was proposed to be a missing oxidation pathway of HgBr (Saiz-Lopez et al, 2020). Sources of these reactive halogen species include emissions from sea ice, snowpack, frost flowers, refreezing leads, sea salt aerosol, and labile halogen reservoir species (i.e., halocarbons and inorganic bromine) (Brooks et al, 2006;Kaleschke et al, 2004;Peterson et al, 2018Peterson et al, , 2019Simpson et al, 2015).…”

Multiconfigurational Quantum Chemistry Determinations of Absorption Cross Sections (σ) in the Gas Phase and Molar Extinction Coefficients (ε) in Aqueous Solution and Air–Water Interface

“… 1 , 3 , 4 However, in many situations, a more accurate description is required, and it is needed to go beyond this single-geometry approximation, for example, when vibrations strongly affect the absorption properties 5 or when photochemical rates ( J ) are required to build kinetic models of the physical and chemical processes induced by light. 6 − 10 For the latter, absorption intensities (cross sections (σ)) over the wavelength (σ(λ)) must be computed, J = ∫ ϕ(λ) σ(λ) I (λ) dλ being the formula for the photochemical rate J , where ϕ(λ) and I (λ) are the quantum yield of the process and the irradiance of the light source, respectively.…”