Rate Constant of the Reaction between CH3O2 Radicals and OH Radicals Revisited

Assaf, Emmanuel; Song, Bo; Tomás, A.; Schoemaecker, Coralie; Fittschen, C.

doi:10.1021/acs.jpca.6b07704

Cited by 47 publications

(88 citation statements)

References 39 publications

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“…S3 in the Supplement. Assumptions had to be made as to the individual values of k OH+RO 2,i , as only the rate constants of the reactions of OH with methyl (Assaf et al, 2016(Assaf et al, , 2017bBossolasco et al, 2014;Yan et al, 2016), ethyl (Assaf et al, 2017aFaragó et al, 2015), propyl ( 2017a) and i-and n-butyl (Assaf et al, 2017a) peroxy radicals have been measured in the laboratory to date. Reported experimental values for the rate constant of the reaction of the simplest peroxy radical, CH 3 O 2 , with OH disagree by more than a factor of 3.…”

Section: Overall Contribution Of Modelled Ro 2 To K Ohmentioning

confidence: 99%

Global modelling of the total OH reactivity: investigations on the “missing” OH sink and its atmospheric implications

et al. 2018

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Abstract. The hydroxyl radical (OH) plays a crucial role in the chemistry of the atmosphere as it initiates the removal of most trace gases. A number of field campaigns have observed the presence of a "missing" OH sink in a variety of regions across the planet. A comparison of direct measurements of the OH loss frequency, also known as total OH reactivity (k OH ), with the sum of individual known OH sinks (obtained via the simultaneous detection of species such as volatile organic compounds and nitrogen oxides) indicates that, in some cases, up to 80 % of k OH is unaccounted for. In this work, the UM-UKCA chemistry-climate model was used to investigate the wider implications of the missing reactivity on the oxidising capacity of the atmosphere. Simulations of the present-day atmosphere were performed and the model was evaluated against an array of field measurements to verify that the known OH sinks were reproduced well, with a resulting good agreement found for most species. Following this, an additional sink was introduced to simulate the missing OH reactivity as an emission of a hypothetical molecule, X, which undergoes rapid reaction with OH. The magnitude and spatial distribution of this sink were underpinned by observations of the missing reactivity. Model runs showed that the missing reactivity accounted for on average 6 % of the total OH loss flux at the surface and up to 50 % in regions where emissions of the additional sink were high. The lifetime of the hydroxyl radical was reduced by 3 % in the boundary layer, whilst tropospheric methane lifetime increased by 2 % when the additional OH sink was included. As no OH recycling was introduced following the initial oxidation of X, these results can be interpreted as an upper limit of the effects of the missing reactivity on the oxidising capacity of the troposphere. The UM-UKCA simulations also allowed us to establish the atmospheric implications of the newly characterised reactions of peroxy radicals (RO 2 ) with OH. Whilst the effects of this chemistry on k OH were minor, the reaction of the simplest peroxy radical, CH 3 O 2 , with OH was found to be a major sink for CH 3 O 2 and source of HO 2 over remote regions at the surface and in the free troposphere. Inclusion of this reaction in the model increased tropospheric methane lifetime by up to 3 %, depending on its product branching. Simulations based on the latest kinetic and product information showed that this reaction cannot reconcile models with observations of atmospheric methanol, in contrast to recent suggestions.

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Section: Overall Contribution Of Modelled Ro 2 To K Ohmentioning

confidence: 99%

Global modelling of the total OH reactivity: investigations on the “missing” OH sink and its atmospheric implications

et al. 2018

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“…In this work we use a chemistry scheme based on a subsection of the hydrocarbons (ethane, propane, iso-butane, n-butane, iso-pentane, n-pentane, hexane, ethene, propene, 1-butene, acetylene, isoprene, toluene, benzene, methanol, acetone, acetaldehyde and DMS) available from the Master Chemical Mechanism version 3.2 (MCM v3.2 http://mcm.leeds.ac.uk/MCM/home.htt) Saunders et al, 2003), with a halogen chemistry scheme described by Saiz-Lopez et al (2006), Whalley et al (2010) and Edwards et al (2011). We also include the reaction between OH and CH 3 O 2 (Bossolasco et al, 2014;Fittschen et al, 2014;Assaf et al, 2016;Yan et al, 2016), with a rate coefficient of 1.6 × 10 −10 cm 3 s −1 (Assaf et al, 2016) and products HO 2 + CH 3 O (Assaf et al, 2017), the impact of which on the HO 2 : OH ratio and CH 3 O 2 budget is described in the Supplement. The total number of species in the model is ∼ 1200, with ∼ 5000 reactions.…”

Section: Constrained Box Modelmentioning

confidence: 99%

Impacts of bromine and iodine chemistry on tropospheric OH and HO2: comparing observations with box and global model perspectives

et al. 2018

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Abstract. The chemistry of the halogen species bromine and iodine has a range of impacts on tropospheric composition, and can affect oxidising capacity in a number of ways. However, recent studies disagree on the overall sign of the impacts of halogens on the oxidising capacity of the troposphere. We present simulations of OH and HO 2 radicals for comparison with observations made in the remote tropical ocean boundary layer during the Seasonal Oxidant Study at the Cape Verde Atmospheric Observatory in 2009. We use both a constrained box model, using detailed chemistry derived from the Master Chemical Mechanism (v3.2), and the three-dimensional global chemistry transport model GEOSChem. Both model approaches reproduce the diurnal trends in OH and HO 2 . Absolute observed concentrations are well reproduced by the box model but are overpredicted by the global model, potentially owing to incomplete consideration of oceanic sourced radical sinks. The two models, however, differ in the impacts of halogen chemistry. In the box model, halogen chemistry acts to increase OH concentrations (by 9.8 % at midday at the Cape Verde Atmospheric Observatory), while the global model exhibits a small increase in OH at the Cape Verde Atmospheric Observatory (by 0.6 % at midday) but overall shows a decrease in the global annual mass-weighted mean OH of 4.5 %. These differences reflect the variety of timescales through which the halogens impact the chemical system. On short timescales, photolysis of HOBr and HOI, produced by reactions of HO 2 with BrO and IO, respectively, increases the OH concentration. On longer timescales, halogen-catalysed ozone destruction cycles lead to lower primary production of OH radicals through ozone photolysis, and thus to lower OH concentrations. The global model includes more of the longer timescale responses than the constrained box model, and overall the global impact of the longer timescale response (reduced primary production due to lower O 3 concentrations) overwhelms the shorter timescale response (enhanced cycling from HO 2 to OH), and thus the global OH concentration decreases. The Earth system contains many such responses on a large range of timescales. This work highlights the care that needs to be taken to understand the full impact of any one process on the system as a whole.

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“…has since been revised to a lower value (1.6 × 10 −10 cm 3 molecule −1 s −1 at T = 295 K) by the same group (Assaf et al, 2016): 10 the difference is thought to arise from complicating secondary chemistry due to the presence of electronically excited iodine atoms following the photolysis of the gaseous mixture used by Bossolasco et al (2014). However an even lower value has been reported by −11 cm 3 molecule −1 s −1 at T = 298 K), and the reason for the discrepancy between this study and that by Assaf et al is still unclear.…”

Section: Overall Contribution Of Modelled Ro2 To Kohmentioning

confidence: 99%

“…However these simulations used the high value of k4 reported by Bossolasco et al (2014). If the preferred lower value reported more recently by Assaf et al (2016) is used with γ = 0.4, we calculate methanol production via R4c to be 60 Tg year −1 and we estimate that a value of γ greater than 0.8 is needed to produce the amount of methanol necessary to reconcile models with observations. This is well beyond the uncertainty in the laboratory measurements of the branching of R4 and highlights that missing sources of methanol must come from direct emissions (or re-emissions) or as yet-undiscovered photochemical sources, rather than from the reaction between OH and CH3O2.…”

Section: The Ch3o2 + Oh Reaction and Product Branching Simulationsmentioning

confidence: 99%

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Global modelling of the total OH reactivity: investigations on the missing OH sink and its atmospheric implications

Ferracci¹,

Heimann²,

Abraham³

et al. 2018

Preprint

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Abstract. The hydroxyl radical (OH) plays a crucial role in the chemistry of the atmosphere as it initiates the removal of most trace gases. A number of field campaigns have observed the presence of a "missing" OH sink in a variety of regions across the planet. Comparison of direct measurements of the OH loss frequency, also known as total OH reactivity (kOH), with the 10 sum of individual known OH sinks (obtained via the simultaneous detection of species such as volatile organic compounds and nitrogen oxides) indicates that, in some cases, up to 80 % of kOH is unaccounted for. In this work, the UM-UKCA chemistry-climate model was used to investigate the wider implications of the missing reactivity on the oxidising capacity of the atmosphere. Simulations of the present-day atmosphere were performed and the model was evaluated against an array of field measurements to verify that the known OH sinks were reproduced well, with a resulting good agreement found for most 15 species. Following this, an additional sink was introduced to simulate the missing OH reactivity as an emission of a hypothetical molecule, X, which undergoes rapid reaction with OH. The magnitude and spatial distribution of this sink were underpinned by observations of the missing reactivity. Model runs showed that the missing reactivity accounted for on average 6 % of the total OH loss flux at the surface, and up to 50 % in regions where emissions of the additional sink were high. The lifetime of the hydroxyl radical was reduced by 3 % in the boundary layer, while tropospheric methane lifetime increased by 20 2 % when the additional OH sink was included. The UM-UKCA simulations also allowed us to establish the atmospheric implications of the newly characterised reactions of peroxy radicals (RO2) with OH. While the effects of this chemistry on kOH were minor, the reaction of the simplest peroxy radical, CH3O2, with OH was found to be a major sink for CH3O2 and source of HO2 over remote regions at the surface and in the free troposphere. Inclusion of this reaction in the model increased tropospheric methane lifetime by up to 3 %, depending on its product branching. Simulations based on the latest kinetic and 25 product information showed that this reaction cannot reconcile models with observations of atmospheric methanol, in contrast to recent suggestions.Atmos. Chem. Phys. Discuss., https://doi

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Rate Constant of the Reaction between CH₃O₂ Radicals and OH Radicals Revisited

Cited by 47 publications

References 39 publications

Global modelling of the total OH reactivity: investigations on the “missing” OH sink and its atmospheric implications

Global modelling of the total OH reactivity: investigations on the “missing” OH sink and its atmospheric implications

Impacts of bromine and iodine chemistry on tropospheric OH and HO<sub>2</sub>: comparing observations with box and global model perspectives

Global modelling of the total OH reactivity: investigations on the missing OH sink and its atmospheric implications

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Rate Constant of the Reaction between CH3O2 Radicals and OH Radicals Revisited

Cited by 47 publications

References 39 publications

Global modelling of the total OH reactivity: investigations on the “missing” OH sink and its atmospheric implications

Global modelling of the total OH reactivity: investigations on the “missing” OH sink and its atmospheric implications

Impacts of bromine and iodine chemistry on tropospheric OH and HO&lt;sub&gt;2&lt;/sub&gt;: comparing observations with box and global model perspectives

Global modelling of the total OH reactivity: investigations on the missing OH sink and its atmospheric implications

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Resources

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Rate Constant of the Reaction between CH₃O₂ Radicals and OH Radicals Revisited

Impacts of bromine and iodine chemistry on tropospheric OH and HO<sub>2</sub>: comparing observations with box and global model perspectives