Gas-Phase Photolysis of Hg(I) Radical Species: A New Atmospheric Mercury Reduction Process

Saiz‐Lopez, Alfonso; Acuña, A. Ulises; Trabelsi, Tarek; Carmona‐García, Javier; Dávalos, Juan Z.; Rivero, Daniel; Cuevas, Carlos A.; Kinnison, Douglas E.; Sitkiewicz, Sebastian P.; Roca‐Sanjuán, Daniel; Francisco, Joseph S.

doi:10.1021/jacs.9b02890

Cited by 50 publications

(55 citation statements)

References 47 publications

(79 reference statements)

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“…The structure is rather sensitive to the method and basis set employed. Note that the CCSD(T)/AVQZ structure of Saiz-Lopez et al 26 is very close to that reported here (Hg-O distance shorter by 0.007 Å), but rather different from the geometry they reported using MRCISD+Q. The H-O bond distance is 0.0037 Å smaller in HOHg• than in the OH radical at the CCSD(T)/AVQZ + ΔCV level of theory.…”

Section: Methodssupporting

confidence: 75%

“…Globally, Hg + OH contributes 0.5% of Hg(II) formation. Recently, Saiz-Lopez et al 26 suggested that photolysis of HOHg• and BrHg• could be an important atmospheric reduction process, and that photolysis occurs somewhat more rapidly for BrHg• than HOHg•. Inclusion of this effect would not make OH important for global oxidation of GEM.…”

Section: Global Modeling Of Oxidation Of Gaseous Elemental Mercurymentioning

confidence: 99%

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Modeling the OH-Initiated Oxidation of Mercury in the Global Atmosphere without Violating Physical Laws

et al. 2019

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In 2005, Calvert and Lindberg wrote that the use of laboratory-derived rate constants for OH + Hg(0) "…to determine the extent of Hg removal by OH in the troposphere will greatly overestimate the importance of Hg removal by this reaction." The HOHg• intermediate formed from OH + Hg will mostly fall apart in the atmosphere before it can react. By contrast, in laboratory experiments, Calvert and Lindberg expected HOHg• to react with radicals (whose concentrations are much higher than in the atmosphere). Yet almost all models of oxidation of Hg(0) ignore the argument of Calvert and Lindberg. We present a way for modelers to include the OH + Hg reaction while accounting quantitatively for the dissociation of HOHg•. We use high levels of quantum chemistry to establish the HO-Hg bond energy as 11.0 kcal/mole, and calculate the equilibrium constant for OH + Hg = HOHg•. Using the measured rate constant for association of OH with Hg, we determine the rate constant for HOHg• dissociation. Theory is also used to demonstrate that HOHg• forms stable compounds, HOHgY, with atmospheric radicals (Y = NO2, HOO•, CH3OO•, and BrO). We then present rate constants for use in in modeling OH-initiated oxidation of Hg(0). We use this mechanism to model the global oxidation of Hg(0) in the period 2013-2015 using the GEOS-Chem 3D model of atmospheric chemistry. Because of the rapid dissociation of HOHg•, OH accounts for <1% of the global oxidation of Hg(0) to Hg(II), while Br atoms account for 97%.

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Section: Methodssupporting

confidence: 75%

Section: Global Modeling Of Oxidation Of Gaseous Elemental Mercurymentioning

confidence: 99%

Modeling the OH-Initiated Oxidation of Mercury in the Global Atmosphere without Violating Physical Laws

et al. 2019

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“…Concentrations of OH, HO 2 , NO 2 , and particulate matter (PM 2.5 ) are imported from MOZART (42). We have also included the gas-phase photoreduction of HgBr, HOHg, HgBr 2 , HgBrOH, HgBrOOH, HgBrONO, HOHgOOH, and HOHgONO using the photolysis rates calculated by CAM-Chem (20,21,43) (see SI Appendix, Supplementary Note 4 for HgBr 2 ), including the photolysis yields determined theoretically by multiconfigurational quantum chemistry (24), and an aqueous-phase photoreduction in cloud droplets with a photolysis rate constant 0.153 h −1 . We perform simulations for the period 2007-2013 using anthropogenic Hg emissions for 2010 (1) of 1,875 Mg/y.…”

Section: Methodsmentioning

confidence: 99%

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

Saiz‐Lopez

Travnikov

Sonke

et al. 2020

Proc. Natl. Acad. Sci. U.S.A.

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105

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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.

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“…HgBr, HgOH, and HgO species are generated in the present photodynamics. HgBr and HgOH were found in our previous work to further dissociate to Hg 0 upon absorption to the electronic states in the energy range 500–800 nm, while a clearly bound excited state was determined with an absorption band appearing at 300–400 nm . In the case of mercury oxide, HgO, our previous calculations of the absorption spectrum revealed absorption bands at 290–320 and 350–400 nm for the 1 Σ and 3 Π states, respectively.…”

Section: Resultsmentioning

confidence: 74%

“…Recently, we have shown that Hg II and Hg I species efficiently absorb solar radiation, which leads to fast photoreduction in the atmosphere. This photoreduction competes with surface deposition, thereby increasing the lifetime and mobility of the metal.…”

Section: Introductionmentioning

confidence: 99%

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

et al. 2020

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Mercury is ac ontaminant of global concern that is transported throughout the atmosphere as elemental mercury Hg 0 and its oxidized forms Hg I and Hg II .T he efficient gasphase photolysis of Hg II and Hg I has recently been reported. However,w hether the photolysis of Hg II leads to other stable Hg II species,toHg I ,ortoHg 0 and its competition with thermal reactivity remain unknown. Herein, we showt hat all oxidized forms of mercury rapidly revert directly and indirectly to Hg 0 by photolysis.R esults are based on non-adiabatic dynamics simulations,inwhich the photoproduct ratios were determined with maximum errors of 3%. We construct for the first time ac omplete quantitative mechanism of the photochemical and thermal conversion between atmospheric Hg II ,H g I ,a nd Hg 0 compounds.T hese results reveal new fundamental chemistry that has broad implications for the global atmospheric Hg cycle.T hus,p hotoreduction clearly competes with thermal oxidation, with Hg 0 being the main photoproduct of Hg II photolysis in the atmosphere,w hich significantly increases the lifetime of this metal in the environment.

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Gas-Phase Photolysis of Hg(I) Radical Species: A New Atmospheric Mercury Reduction Process

Cited by 50 publications

References 47 publications

Modeling the OH-Initiated Oxidation of Mercury in the Global Atmosphere without Violating Physical Laws

Modeling the OH-Initiated Oxidation of Mercury in the Global Atmosphere without Violating Physical Laws

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

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

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