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

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%.

Download Full-text

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

10.26434/chemrxiv.10062161.v2 ◽

2019 ◽

Author(s):

Theodore Dibble ◽

Hanna Tetu ◽

yuge jiao ◽

Colin Thackray ◽

Daniel J. Jacob

Keyword(s):

Quantum Chemistry ◽

Rate Constant ◽

Atmospheric Chemistry ◽

Rate Constants ◽

Bond Energy ◽

3D Model ◽

Kcal Mole ◽

Present Rate ◽

Physical Laws ◽

Atmospheric Radicals

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%.

Download Full-text

Impact of uncertainties in inorganic chemical rate constants on tropospheric composition and ozone radiative forcing

10.5194/acp-2017-12 ◽

2017 ◽

Author(s):

Ben Newsome ◽

Mat Evans

Keyword(s):

Rate Constant ◽

Atmospheric Chemistry ◽

Rate Constants ◽

Tropospheric Ozone ◽

Radiative Forcing ◽

Transport Model ◽

Surface Ozone ◽

Photolysis Rates ◽

The Impact ◽

Chemical Rate

Abstract. Chemical rate constants determine the composition of the atmosphere and how this composition has changed over time. They are central to our understanding of climate change and air quality degradation. Atmospheric chemistry models, whether online or offline, box, regional or global use these rate constants. Expert panels synthesise laboratory measurements, making recommendations for the rate constants that should be used. This results in very similar or identical rate constants being used by all models. The inherent uncertainties in these recommendations are, in general, therefore ignored. We explore the impact of these uncertainties on the composition of the troposphere using the GEOS-Chem chemistry transport model. Based on the JPL and IUPAC evaluations we assess 50 mainly inorganic rate constants and 10 photolysis rates, through simulations where we increase the rate of the reactions to the 1σ upper value recommended by the expert panels. We assess the impact on 4 standard metrics: annual mean tropospheric ozone burden, surface ozone and tropospheric OH concentrations, and tropospheric methane lifetime. Uncertainty in the rate constants for NO2 + OH M → HNO3, OH + CH4 → CH3O2 + H2O and O3 + NO → NO2 + O2 are the three largest source of uncertainty in these metrics. We investigate two methods of assessing these uncertainties, addition in quadrature and a Monte Carlo approach, and conclude they give similar outcomes. Combining the uncertainties across the 60 reactions, gives overall uncertainties on the annual mean tropospheric ozone burden, surface ozone and tropospheric OH concentrations, and tropospheric methane lifetime of 11, 12, 17 and 17 % respectively. These are larger than the spread between models in recent model inter-comparisons. Remote regions such as the tropics, poles, and upper troposphere are most uncertain. This chemical uncertainty is sufficiently large to suggest that rate constant uncertainty should be considered when model results disagree with measurement. Calculations for the pre-industrial allow a tropospheric ozone radiative forcing to be calculated of 0.412 ± 0.062 Wm−2. This uncertainty (15 %) is comparable to the inter-model spread in ozone radiative forcing found in previous model-model inter-comparison studies where the rate constants used in the models are all identical or very similar. Thus the uncertainty of tropospheric ozone radiative forcing should expanded to include this additional source of uncertainty. These rate constant uncertainties are significant and suggest that refinement of supposedly well known chemical rate constants should be considered alongside other improvements to enhance our understanding of atmospheric processes.

Download Full-text

Shock-wave observations of rate constants for atomic hydrogen recombination from 2500 to 7000 °K: collisional stabilization by exchange of hydrogen atoms

Proceedings of the Royal Society of London Series A - Mathematical and Physical Sciences ◽

10.1098/rspa.1969.0074 ◽

1969 ◽

Vol 310 (1501) ◽

pp. 253-276 ◽

Cited By ~ 34

Keyword(s):

Shock Waves ◽

Rate Constant ◽

Rate Constants ◽

Atomic Hydrogen ◽

Functional Form ◽

Kcal Mole ◽

Hydrogen Atoms ◽

Hydrogen Molecules ◽

Hydrogen Recombination ◽

Equilibrium Dissociation

Rate constants for the recombination of atomic hydrogen with hydrogen molecules, hydrogen atoms, and argon atoms as the third bodies are presented in functional form for the range of temperatures from about 2500 to 7000 °K and are critically compared with the results of other workers. The rate constants are evaluated from detailed analyses of spectrum-line reversal measurements of the fall in temperature accompanying dissociation behind shock waves in gas mixtures containing 20, 40, 50 and 60% of hydrogen in argon. The rate constants for recombination with hydrogen molecules ( k -1 ) and argon atoms ( k -3 ) fit the equations log 10 k -1 = 15.243 - 1.95 x 10 -4 T cm 6 mole -2 s -1 , log 10 k -3 = 15.787 - 2.75 x 10 -4 T cm 6 mole -2 s -1 , with a standard deviation of 0.193 in log 10 k -1 . The rate constant for recombination with hydrogen atoms is about ten times larger than these at 3000 °K and shows a steep inverse dependence on temperature ( ~ T -6 ) above 4000 °K. Below this temperature the power of this dependence decreases rapidly and there is strong evidence that the value of this rate constant has a maximum around 3000 °K. This behaviour is interpreted on the basis of a process of collisional stabilization by atom exchange, requiring an activation energy around 8 kcal mole -1 and taking place under conditions of vibrational adiabaticity. The over-all results indicate that the assumption of equality between the equilibrium constant and the ratio of the rate constants for dissociation and recombination is valid throughout the region of non-equilibrium dissociation and at all temperatures in the shock waves examined.

Download Full-text

BrHgO• + C2H4 and BrHgO• + HCHO in Atmospheric Oxidation of Mercury: Determining Rate Constants of Reactions with Pre-Reactive Complexes and a Bifurcation

10.26434/chemrxiv.8266733.v1 ◽

2019 ◽

Author(s):

Khoa T. Lam ◽

Curtis J. Wilhelmsen ◽

Theodore Dibble

Keyword(s):

Hydrogen Atom ◽

Quantum Chemistry ◽

Transition State ◽

Rate Constant ◽

Rate Constants ◽

Minimum Energy ◽

Atmospheric Oxidation ◽

Minimum Energy Path ◽

Oh Radical ◽

Hydrogen Atoms

Models suggest BrHgONO to be the major Hg(II) species formed in the global oxidation of Hg(0), and BrHgONO undergoes rapid photolysis to produce the thermally stable radical BrHgO•. We previously used quantum chemistry to demonstrate that BrHgO• can, like OH radical, readily can abstract hydrogen atoms from sp3-hybridized carbon atoms as well as add to NO and NO2. In the present work, we reveal that BrHgO• can also add to C2H4 to form BrHgOCH2CH2•, although this addition appears to proceed with a lower rate constant than the analogous addition of •OH to C2H4. Additionally, BrHgO• can readily react with HCHO in two different ways: either by addition to the carbon or by abstraction of a hydrogen atom. The minimum energy path for the BrHgO• + HCHO reaction bifurcates, forming two pre-reactive complexes, each of which passes over a separate transition state to form a different product.

Download Full-text

Impact of uncertainties in inorganic chemical rate constants on tropospheric composition and ozone radiative forcing

Atmospheric Chemistry and Physics ◽

10.5194/acp-17-14333-2017 ◽

2017 ◽

Vol 17 (23) ◽

pp. 14333-14352 ◽

Cited By ~ 16

Author(s):

Ben Newsome ◽

Mat Evans

Keyword(s):

Rate Constant ◽

Atmospheric Chemistry ◽

Rate Constants ◽

Tropospheric Ozone ◽

Radiative Forcing ◽

Transport Model ◽

Surface Ozone ◽

Chemistry Transport Model ◽

The Impact ◽

Chemical Rate

Abstract. Chemical rate constants determine the composition of the atmosphere and how this composition has changed over time. They are central to our understanding of climate change and air quality degradation. Atmospheric chemistry models, whether online or offline, box, regional or global, use these rate constants. Expert panels evaluate laboratory measurements, making recommendations for the rate constants that should be used. This results in very similar or identical rate constants being used by all models. The inherent uncertainties in these recommendations are, in general, therefore ignored. We explore the impact of these uncertainties on the composition of the troposphere using the GEOS-Chem chemistry transport model. Based on the Jet Propulsion Laboratory (JPL) and International Union of Pure and Applied Chemistry (IUPAC) evaluations we assess the influence of 50 mainly inorganic rate constants and 10 photolysis rates on tropospheric composition through the use of the GEOS-Chem chemistry transport model. We assess the impact on four standard metrics: annual mean tropospheric ozone burden, surface ozone and tropospheric OH concentrations, and tropospheric methane lifetime. Uncertainty in the rate constants for NO2 + OH →M  HNO3 and O3 + NO  →  NO2 + O2 are the two largest sources of uncertainty in these metrics. The absolute magnitude of the change in the metrics is similar if rate constants are increased or decreased by their σ values. We investigate two methods of assessing these uncertainties, addition in quadrature and a Monte Carlo approach, and conclude they give similar outcomes. Combining the uncertainties across the 60 reactions gives overall uncertainties on the annual mean tropospheric ozone burden, surface ozone and tropospheric OH concentrations, and tropospheric methane lifetime of 10, 11, 16 and 16 %, respectively. These are larger than the spread between models in recent model intercomparisons. Remote regions such as the tropics, poles and upper troposphere are most uncertain. This chemical uncertainty is sufficiently large to suggest that rate constant uncertainty should be considered alongside other processes when model results disagree with measurement. Calculations for the pre-industrial simulation allow a tropospheric ozone radiative forcing to be calculated of 0.412 ± 0.062 W m−2. This uncertainty (13 %) is comparable to the inter-model spread in ozone radiative forcing found in previous model–model intercomparison studies where the rate constants used in the models are all identical or very similar. Thus, the uncertainty of tropospheric ozone radiative forcing should expanded to include this additional source of uncertainty. These rate constant uncertainties are significant and suggest that refinement of supposedly well-known chemical rate constants should be considered alongside other improvements to enhance our understanding of atmospheric processes.

Download Full-text

THE PYROLYSIS OF TOLUENE

Canadian Journal of Chemistry ◽

10.1139/v62-201 ◽

1962 ◽

Vol 40 (7) ◽

pp. 1310-1317 ◽

Cited By ~ 24

Author(s):

S. J. Price

Keyword(s):

Activation Energy ◽

Rate Constant ◽

Rate Constants ◽

Contact Time ◽

Flow System ◽

Kcal Mole ◽

Homogeneous Process ◽

First Order ◽

Order Rate ◽

Toluene Concentration

The pyrolysis of toluene has been studied in a flow system from 913 to 1143 °K. First-order rate constants are independent of the toluene concentration but decrease approximately 9% when the contact time is reduced from 1.0 to 0.41 second. Increasing the contact time from 1.0 second to 2.07 seconds does not affect the rate constant. The overall rate has been resolved into homogeneous and heterogeneous components. It is suggested that the activation energy of the homogeneous process, 85 kcal/mole, may be associated with D(C6H5CH2—H).

Download Full-text

BrHgO• + C2H4 and BrHgO• + HCHO in Atmospheric Oxidation of Mercury: Determining Rate Constants of Reactions with Pre-Reactive Complexes and a Bifurcation

10.26434/chemrxiv.8266733 ◽

2019 ◽

Author(s):

Khoa T. Lam ◽

Curtis J. Wilhelmsen ◽

Theodore Dibble

Keyword(s):

Hydrogen Atom ◽

Quantum Chemistry ◽

Transition State ◽

Rate Constant ◽

Rate Constants ◽

Minimum Energy ◽

Atmospheric Oxidation ◽

Minimum Energy Path ◽

Oh Radical ◽

Hydrogen Atoms

Models suggest BrHgONO to be the major Hg(II) species formed in the global oxidation of Hg(0), and BrHgONO undergoes rapid photolysis to produce the thermally stable radical BrHgO•. We previously used quantum chemistry to demonstrate that BrHgO• can, like OH radical, readily can abstract hydrogen atoms from sp3-hybridized carbon atoms as well as add to NO and NO2. In the present work, we reveal that BrHgO• can also add to C2H4 to form BrHgOCH2CH2•, although this addition appears to proceed with a lower rate constant than the analogous addition of •OH to C2H4. Additionally, BrHgO• can readily react with HCHO in two different ways: either by addition to the carbon or by abstraction of a hydrogen atom. The minimum energy path for the BrHgO• + HCHO reaction bifurcates, forming two pre-reactive complexes, each of which passes over a separate transition state to form a different product.

Download Full-text

Determination of rate constants of redox reactions of second order from current-less potential-time curves by a simplified graphical-numerical method

Collection of Czechoslovak Chemical Communications ◽

10.1135/cccc19831358 ◽

1983 ◽

Vol 48 (5) ◽

pp. 1358-1367 ◽

Cited By ~ 4

Author(s):

Antonín Tockstein ◽

František Skopal

Keyword(s):

Numerical Method ◽

Explicit Form ◽

Rate Constant ◽

Optimization Algorithm ◽

Rate Constants ◽

Redox Reactions ◽

Second Order ◽

Wide Region ◽

Recurrent Formula

A method for constructing curves is proposed that are linear in a wide region and from whose slopes it is possible to determine the rate constant, if a parameter, θ, is calculated numerically from a rapidly converging recurrent formula or from its explicit form. The values of rate constants and parameter θ thus simply found are compared with those found by an optimization algorithm on a computer; the deviations do not exceed ±10%.

Download Full-text

The Effect of p-Toluidine on the Two-Step Electroreduction of Zn(II) in Water-Methanol and Water-Dimethylformamide

Collection of Czechoslovak Chemical Communications ◽

10.1135/cccc19990585 ◽

1999 ◽

Vol 64 (4) ◽

pp. 585-594 ◽

Cited By ~ 2

Author(s):

Barbara Marczewska

Keyword(s):

Electron Transfer ◽

Binary Mixtures ◽

Rate Constant ◽

Rate Constants ◽

Electrode Surface ◽

Mercury Electrode ◽

Forward Rate ◽

Solvent Molecules ◽

Activated Complex

The acceleration effect of p-toluidine on the electroreduction of Zn(II) on the mercury electrode surface in binary mixtures water-methanol and water-dimethylformamide is discussed. The obtained apparent and true forward rate constants of Zn(II) reduction indicate that the rate constant of the first electron transfer increases in the presence of p-toluidine. The acceleration effect may probably be accounted for by the concept of the formation on the mercury electrode an activated complex, presumably composed of p-toluidine and solvent molecules.

Download Full-text