Evaluation of α- and β-pinene degradation in the detailed tropospheric chemistry mechanism, MCM v3.1, using environmental chamber data

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.