Photodissociation Mechanisms of Major Mercury(II) Species in the Atmospheric Chemical Cycle of Mercury

Abstract

Mercury is a contaminant of global concern that is transported throughout the atmosphere as elemental mercury Hg0 and its oxidized forms HgI and HgII. The efficient gas‐phase photolysis of HgII and HgI has recently been reported. However, whether the photolysis of HgII leads to other stable HgII species, to HgI, or to Hg0 and its competition with thermal reactivity remain unknown. Herein, we show that all oxidized forms of mercury rapidly revert directly and indirectly to Hg0 by photolysis. Results are based on non‐adiabatic dynamics simulations, in which the photoproduct ratios were determined with maximum errors of 3%. We construct for the first time a complete quantitative mechanism of the photochemical and thermal conversion between atmospheric HgII, HgI, and Hg0 compounds. These results reveal new fundamental chemistry that has broad implications for the global atmospheric Hg cycle. Thus, photoreduction clearly competes with thermal oxidation, with Hg0 being the main photoproduct of HgII photolysis in the atmosphere, which significantly increases the lifetime of this metal in the environment.

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

“…Fig. 1 shows the main reactions for thermal and photochemical conversion between Hg 0 , Hg I , and Hg II species ( 25 ). syn -HgBrONO and HgBrOOH are considered the major gaseous oxidized Hg II species from Br-initiated two-step Hg 0 oxidation ( 6 , 26 , 27 ).…”

“…syn -HgBrONO and HgBrOOH are considered the major gaseous oxidized Hg II species from Br-initiated two-step Hg 0 oxidation ( 6 , 26 , 27 ). It has been calculated that following gas-phase photolysis, syn -HgBrONO breaks down to HgBrO (90%) and HgBr (10%) ( 24 , 25 ), and HgBrOOH yields Hg 0 (66%), HgBrO (31%), and HgBr (3%) ( 25 ). The computed tropospheric photolysis rate of the HgBrO radical (2.95 × 10 −2 ⋅s −1 ) ( 25 ) is too slow to compete with the proposed reaction of the radical with methane [pseudo–first-order rate, k ′ = 11 s −1 at 298 K and 1 atm ( 24 )] to form HgBrOH.…”

“…It has been calculated that following gas-phase photolysis, syn -HgBrONO breaks down to HgBrO (90%) and HgBr (10%) ( 24 , 25 ), and HgBrOOH yields Hg 0 (66%), HgBrO (31%), and HgBr (3%) ( 25 ). The computed tropospheric photolysis rate of the HgBrO radical (2.95 × 10 −2 ⋅s −1 ) ( 25 ) is too slow to compete with the proposed reaction of the radical with methane [pseudo–first-order rate, k ′ = 11 s −1 at 298 K and 1 atm ( 24 )] to form HgBrOH. Note that the equivalent reaction of HgBrO with NO 2 has a k ′ = 2 × 10 −2 ⋅s −1 at 298 K and 1 atm ( 24 ).…”

“…The rapid photodissociation of syn -HgBrONO has been suggested to produce the HgBrO radical, which upon reaction with volatile organic compounds swiftly forms HgBrOH ( 24 ). Finally, a recent computational study has reported quantitative photodissociation channels and photoproducts of the likely major gaseous oxidized Hg II compounds ( syn -HgBrONO, HgBrOOH, HgBrO, and HgBrOH), and constructed a quantitative mechanism of the photochemical and thermal conversion between Hg 0 , Hg I , and Hg II species in the atmosphere ( 25 ).…”

On the Theoretical Determination of Photolysis Properties for Atmospheric Volatile Organic Compounds

“…The applications of some of the advanced techniques mentioned above in the atmospheric photochemistry of VOCs are still scarce 28 − 30 (see also ref ( 31 ) for a recent example of mercury-based compounds). More importantly, these strategies have not been combined to determine the components of the photolysis rate constant in a coherent manner.…”