Laboratory and modelling studies of the reactions of peroxy radicals with XO (where X = Cl, Br or I).
Lead Research Organisation:
University of Manchester
Department Name: Earth Atmospheric and Env Sciences
Abstract
Halogen chemistry can play a vital role in controlling the budgets of ozone, NOx, NOy and HOx throughout the troposphere and the importance of these families in determining atmospheric composition, the earth's radiative balance and any future change in climate can not be over-emphasised. However, there is a major gap in the kinetic database, in that there is little or no data for sub ambient temperatures and where data is available the picture is far from clear, and thus does not allow the full atmospheric importance of the coupling of halogen chemistry with peroxy radicals to be assessed. An integrated programme of laboratory and modelling studies is proposed to study a key set of peroxy radical reactions, namely RO2 + XO. A state of the art experimental system, a turbulent flow chemical ionisation mass spectrometer, that enables direct experimental acess to atmospherically relevant teperatures and pressures will be used to determine kinetic and mechanistic parameters. The experimental results will be raionalised using state of the art ab - intio calculations and then be incorporated into a global atmospheric model which allows the atmospheric importance of the interacttion of peroxy radicals with XO radicals to be assessed. The proposed work will be invaluable for the rationalisation of the effect of halogen chemistry on atmospheric pollution and climate change
Organisations
Publications
Archibald A
(2009)
On the importance of the reaction between OH and RO 2 radicals
in Atmospheric Science Letters
Leather KE
(2012)
Temperature and pressure dependence of the rate coefficient for the reaction between ClO and CH3O2 in the gas-phase.
in Physical chemistry chemical physics : PCCP
Muller J
(2010)
Sources of uncertainty in eddy covariance ozone flux measurements made by dry chemiluminescence fast response analysers
in Atmospheric Measurement Techniques
Shallcross DE
(2015)
Reaction between CH3O2 and BrO radicals: a new source of upper troposphere lower stratosphere hydroxyl radicals.
in The journal of physical chemistry. A
| Description | The temperature dependence of the rate coefficients for the reactions of RO2 + XO (where R = CH3 or C2H5) and X (where = I, Br or Cl) were investigated over the temperature range of 195 to 298 K and the pressure range 75 - 200 Torr using the turbulent flow technique with a chemical ionisation mass spectrometry detection system. An Arrhenius expression was obtained for the overall rate coefficient of k(T) = ( ) × 10-13 exp [(810 ± 127)/T] and cm-3 molecule-1 s-1 where the uncertainty associated with the rate coefficient is given at the two standard deviations level for CH3O2 +ClO, CH3O2 +BrO respectively. Our results are in good agreement with the recent evaluation of Enami et al., 2007 and would suggest that the reaction of CH3O2 with ClO has a negative temperature dependence, in agreement with Kukui et al. The room temperature rate coefficient of CH3O2 with IO was quantified to measured to be (1.0 ± 0.4) ×10-12 cm-3 molecule-1 s-1 which is in agreement, within experimental error, with the more recent work of Dillon et al. (2006). The reaction of peroxy radicals with XO radicals is thought to proceed via the general reaction scheme RO2 + XO _ RO + XOO (1a) RO2 + XO _ ROX + O2 (1b) RO2 + XO _ RO + X + O2 (1c) Attempts were made to quantify the branching ratio of channel 1c by building a resonance fluorescence lamp to detect halogen atoms. However, the limit of the detection of the RF lamp was three orders of magnitude worse than the values reported by Keyser et al., and thus given the low radical concentrations used, typically (0.5 - 20) × 1010 molecule cm-3, the branching ratio to channel c could not be assessed. The reactions of XO with RO2 have been investigated primarily using a marine boundary layer box model, BAMBO (Archibald et al., 2007; 2009; 2010). The model simulates clean atmospheric conditions considering the marine boundary layer (MBL) as a representative environment. The model receives input of chemical species through emissions from the surface and mixing in from the free troposphere. Free tropospheric emissions are provided for relatively long lived species such as O3, CO, CH3OOH, PAN and some NMHCs (C2H6, C2H4, C3H8, C3H6, C4H10) whilst surface emissions are provided for isoprene, ethene, propene, propane and dimethyl sulfide (DMS) (average mixing ratios and atmospheric parameters can be found in table 1). A comprehensive chemical mechanism is employed based on the MCMv3.1 (Saunders et al., 2003) which details near explicitly the gas phase degradation of methane; ethane; ethene; propane; propene; n-butane; isoprene and DMS. Sensitivity studies show that for the reaction between XO and CH3O2 for example, a range of impacts are observed. Simulating Antarctic marine boundary layer chemistry and using data from the CHABLIS campaign it emerges that IO could play a significant role in destroying ozone if the largest rate coefficient determined (Enami et al., 2006) is used (around 1.5 ppbv of ozone a day) but that this effect disappears to virtually no impact using the lower rate coefficients obrain in this study and that of Dillon et al., 2006. In the equatorial marine boundary layer the effect of this cycle is less but the same general conclusions hold (around 0.7 ppb of ozone a day using the largest rate coefficient). For BrO, addition of the reaction between BrO and CH3O2 enhances the loss of CH3O2 by 30% in the troposphere and by about 10% in the stratosphere. The reaction of ClO with RO2 becomes a non-negligible contribution in the polar marine boundary layer and in the upper troposphere lower stratosphere, but remains most potent inside the polar vortex in spring. |
| Exploitation Route | n/a n/a |
| Sectors | Environment |
| Description | The temperature dependence of the rate coefficients for the reactions of RO2 + XO (where R = CH3 or C2H5) and X (where = I, Br or Cl) were investigated over the temperature range of 195 to 298 K and the pressure range 75 - 200 Torr using the turbulent flow technique with a chemical ionisation mass spectrometry detection system. An Arrhenius expression was obtained for the overall rate coefficient of k(T) = ( ) × 10-13 exp [(810 ± 127)/T] and cm-3 molecule-1 s-1 where the uncertainty associated with the rate coefficient is given at the two standard deviations level for CH3O2 +ClO, CH3O2 +BrO respectively. Our results are in good agreement with the recent evaluation of Enami et al., 2007 and would suggest that the reaction of CH3O2 with ClO has a negative temperature dependence, in agreement with Kukui et al. The room temperature rate coefficient of CH3O2 with IO was quantified to measured to be (1.0 ± 0.4) ×10-12 cm-3 molecule-1 s-1 which is in agreement, within experimental error, with the more recent work of Dillon et al. (2006). The reaction of peroxy radicals with XO radicals is thought to proceed via the general reaction scheme RO2 + XO _ RO + XOO (1a) RO2 + XO _ ROX + O2 (1b) RO2 + XO _ RO + X + O2 (1c) Attempts were made to quantify the branching ratio of channel 1c by building a resonance fluorescence lamp to detect halogen atoms. However, the limit of the detection of the RF lamp was three orders of magnitude worse than the values reported by Keyser et al., and thus given the low radical concentrations used, typically (0.5 - 20) × 1010 molecule cm-3, the branching ratio to channel c could not be assessed. The reactions of XO with RO2 have been investigated primarily using a marine boundary layer box model, BAMBO (Archibald et al., 2007; 2009; 2010). The model simulates clean atmospheric conditions considering the marine boundary layer (MBL) as a representative environment. The model receives input of chemical species through emissions from the surface and mixing in from the free troposphere. Free tropospheric emissions are provided for relatively long lived species such as O3, CO, CH3OOH, PAN and some NMHCs (C2H6, C2H4, C3H8, C3H6, C4H10) whilst surface emissions are provided for isoprene, ethene, propene, propane and dimethyl sulfide (DMS) (average mixing ratios and atmospheric parameters can be found in table 1). A comprehensive chemical mechanism is employed based on the MCMv3.1 (Saunders et al., 2003) which details near explicitly the gas phase degradation of methane; ethane; ethene; propane; propene; n-butane; isoprene and DMS. Sensitivity studies show that for the reaction between XO and CH3O2 for example, a range of impacts are observed. Simulating Antarctic marine boundary layer chemistry and using data from the CHABLIS campaign it emerges that IO could play a significant role in destroying ozone if the largest rate coefficient determined (Enami et al., 2006) is used (around 1.5 ppbv of ozone a day) but that this effect disappears to virtually no impact using the lower rate coefficients obrain in this study and that of Dillon et al., 2006. In the equatorial marine boundary layer the effect of this cycle is less but the same general conclusions hold (around 0.7 ppb of ozone a day using the largest rate coefficient). For BrO, addition of the reaction between BrO and CH3O2 enhances the loss of CH3O2 by 30% in the troposphere and by about 10% in the stratosphere. The reaction of ClO with RO2 becomes a non-negligible contribution in the polar marine boundary layer and in the upper troposphere lower stratosphere, but remains most potent inside the polar vortex in spring. |