Thermodynamics limits the reactivity of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.gif" overflow="scroll"><mml:mrow><mml:msup><mml:mrow><mml:mtext>BrHg</mml:mtext></mml:mrow><mml:mrow><mml:mtext/></mml:mrow></mml:msup></mml:mrow></mml:math> radical with volatile organic compounds

Dibble, Theodore S.; Schwid, Abraham C.

doi:10.1016/j.cplett.2016.07.065

Cited by 20 publications

(22 citation statements)

References 33 publications

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“…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 ). 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 ).…”

Section: Chemical Mechanisms and Global Model Simulationsmentioning

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.

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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Section: Chemical Mechanisms and Global Model Simulationsmentioning

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.

105

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“…As we showed, the weakness of the BrHg-CH2CH2• bond means that this radical will dissociate before undergoing any bimolecular collisions. 73 Given the higher barrier and lesser stability of the products in the HOHg• + CH2=CH2 reaction, we suggests that HOHg• addition to alkenes will be very inefficient.…”

Section: Y Pbe0 Ccsd(t)//pbe0mentioning

confidence: 88%

“…Our study 73 of the BrHg• + CH2=CH2 reaction showed that, like OH + CH2=CH2, this reaction proceeds by a barrierless formation of a van der Waals complex, which then passes over a transition state to form BrHgCH2CH2•. Figure 5 depicts the potential energy profile for the analogous HOHg• + ethylene system at PBE0/AVTZ.…”

Section: Y Pbe0 Ccsd(t)//pbe0mentioning

confidence: 96%

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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“…It has previously been suggested that syn ‐HgBrONO and HgBrOOH are the major gaseous oxidized mercury compounds from the reaction of Hg 0 with Br atoms . Previous computational studies have focused mainly on the thermal reactivity of closed‐shell molecules .…”

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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Thermodynamics limits the reactivity of BrHg radical with volatile organic compounds

Cited by 20 publications

References 33 publications

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

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

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

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

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