Browsing by Author "Jenkin, M. E."
Now showing 1 - 2 of 2
Results Per Page
Sort Options
- Evaluation of alkene degradation in the detailed tropospheric chemistry mechanism, MCM v3, using environmental chamber dataPublication . Pinho, P. G.; Pio, C. A.; Carter, W. P. L.; Jenkin, M. E.The representation of alkene degradation in version 3 of the Master Chemical Mechanism (MCM v3) has been evaluated, using environmental chamber data on the photo-oxidation of ethene, propene, 1-butene and 1-hexene in the presence of NOx, from up to five chambers at the Statewide Air Pollution Research Center (SAPRC) at the University of California. As part of this evaluation, it was necessary to include a representation of the reactions of the alkenes with O(3P), which are significant under chamber conditions but generally insignificant under atmospheric conditions. The simulations for the ethene and propene systems, in particular, were found to be sensitive to the branching ratios assigned to molecular and free radical forming pathways of the O(3P) reactions, with the extent of radical formation required for proper fitting of the model to the chamber data being substantially lower than the reported consensus. With this constraint, the MCM v3 mechanisms for ethene and propene generally performed well. The sensitivity of the simulations to the parameters applied to a series of other radical sources and sink reactions (radical formation from the alkene ozonolysis reactions and product carbonyl photolysis; radical removal from the reaction of OH with NO2 and β-hydroxynitrate formation) were also considered, and the implications of these results are discussed. Evaluation of the MCM v3 1-butene and 1-hexene degradation mechanisms, using a more limited dataset from only one chamber, was found to be inconclusive. The results of sensitivity studies demonstrate that it is impossible to reconcile the simulated and observed formation of ozone in these systems for ranges of parameter values which can currently be justified on the basis of the literature. As a result of this work, gaps and uncertainties in the kinetic, mechanistic and chamber database are identified and discussed, in relation to both tropospheric chemistry and chemistry important under chamber conditions which may compromise the evaluation procedure, and recommendations are made for future experimental studies. Throughout the study, the performance of the MCM v3 chemistry was also simultaneously compared with that of the corresponding chemistry in the SAPRC-99 mechanism, which was developed and optimized in conjunction with the chamber datasets.
- Evaluation of α- and β-pinene degradation in the detailed tropospheric chemistry mechanism, MCM v3.1, using environmental chamber dataPublication . Pinho, P. G.; Pio, C. A.; Carter, W. P. L.; Jenkin, M. E.The representation of the degradation of the monoterpenes, α- and β-pinene, in version 3.1 of the Master Chemical Mechanism (MCM v3.1) has been evaluated, using environmental chamber data from the Statewide Air Pollution Research Center (SAPRC) at the University of California. As part of this evaluation, a representation of the reactions of the monoterpenes with O(3P) has also been included, these reactions being significant under chamber conditions but generally insignificant under atmospheric conditions. The results demonstrate that MCM v3.1 provides a consistent description of the photo-oxidation of α-pinene/NOX mixtures for a range of initial VOC/NOX, but with the formation rate of ozone and decay rate of α-pinene generally being overestimated. Sensitivity of the system to parameter uncertainties and mechanistic variations proposed in the literature are described. The collective implementation of a number of refinements allows the simulations to be brought into good agreement with the experimental observations for the complete series of experiments, with each of the refinements being consistent with reported parameter uncertainty ranges or mechanistic adjustments. The system is particularly sensitive to the magnitudes of sources and sinks of free radicals. The impacts of several other reported mechanistic variations which potentially influence the first generation product distribution and the ozone formation chain length of the initial oxidation step are also described and assessed. MCM v3.1 is shown to provide a reasonable, but less consistent, description of photo-oxidation of β-pinene/NOX mixtures. The simulated magnitudes of the ozone formation rates and β-pinene removal rates are broadly comparable with the experimental observations, but the mechanism tends towards overestimation of ozone formation at low VOC/NOX and underestimation at high VOC/NOX. Implementation of a number of mechanistic variations reported in the literature does not allow the associated simulations to be brought into good agreement with the observations for the entire VOC/NOX range. The system is particularly sensitive to changes which influence the formation of HCHO (and resultant radical production upon its photolysis), and the impacts of the tested mechanistic variations are usually dominated by this effect. As a result of this work, gaps and uncertainties in the kinetic, mechanistic and chamber database for the monoterpenes are identified and discussed.