In part two of this series of papers on the IMS model, we present the chemistry reaction mechanism used and compare modelled CH4, CO, and O3 with a dataset of annual surface measurements. The modelled monthly and 24-hour mean tropospheric OH concentrations range between 5-22 × 105 molecules cm-3, indicating an annual averaged OH concentration of about 10 × 105 molecules cm-3. This value is close to the estimated 9.7 ± 0.6 × 105 molecules cm-3 calculated from the reaction of CH3CC13 with OH radicals. Comparison with CH4 generally shows good agreement between model and measurements, except for the site at Barrow where modelled wetland emission in the summer could be a factor 3 too high. For CO, the pronounced seasonality shown in the measurements is generally reproduced by the model; however, the modelled concentrations are lower than the measurements. This discrepancy may due to lower the CO emission, especially from biomass burning, used in the model compared with other studies. For O3, good agreement between the model and measurements is seen at locations which are away from industrial regions. The maximum discrepancies between modelled results and measurements at tropical and remote marine sites is about 5-10 ppbv, while the discrepancies can exceed 30 ppbv in the industrial regions. Comparisons in rural areas at European and American continental sites are highly influenced by the local photochemical production, which is difficult to model with a coarse global CTM. The very large variations of O3 at these locations vary from about 15-25 ppbv in January to 55-65 ppbv in July-August. The observed annual O3 amplitude is about 40 ppbv compared with about 20 ppbv in the model. An overall comparison of modelled O3 with measurements shows that the O3 seasonal surface cycle is generally governed by the relative importance of two key mechanisms that drive a springtime ozone maximum and a summertime ozone maximum.
- Chemistry transport model