Formulation and evaluation of IMS, an interactive three-dimensional tropospheric chemical transport model 2. Model chemistry and comparison of modelled CH4, CO, and O-3 with surface measurements

In part two of this series of papers on the IMS model, we present the chemi stry reaction mechanism used and compare modelled CH4, CO, and O-3 with a d ataset of annual surface measurements. The modelled monthly and 24-hour mea n tropospheric OH concentrations range between 5-22 x 10(5) molecules cm(-) 3, indicating an annual averaged OH concentration of about 10 x 10(5) molec ules cm(-3). This value is close to the estimated 9.7 +/- 0.6 x 10(5) molec ules cm(-3) calculated from the reaction of CH3CCl3 with OH radicals. Compa rison with CH4 generally shows good agreement between model and measurement s, except for the site at Barrow where modelled wetland emission in the sum mer could be a factor 3 too high. For CO, the pronounced seasonality shown in the measurements is generally reproduced by the model; however, the mode lled concentrations are lower than the measurements. This discrepancy may d ue to lower the CO emission, especially from biomass burning, used in the m odel compared with other studies. For O-3, good agreement between the model and measurements is seen at locations which are away from industrial regio ns. The maximum discrepancies between modelled results and measurements at tropical and remote marine sites is about 5-10 ppbv, while the discrepancie s can exceed 30 ppbv in the industrial regions. Comparisons in rural areas at European and American continental sites are highly influenced by the loc al photochemical production, which is difficult to model with a coarse glob al CTM. The very large variations of O-3 at these locations vary from about 15-25 ppbv in January to 55-65 ppbv in July-August. The observed annual O- 3 amplitude is about 40 ppbv compared with about 20 ppbv in the model. An o verall comparison of modelled O-3 with measurements shows that the O-3 seas onal surface cycle is generally governed by the relative importance of two key mechanisms that drive a springtime ozone maximum and a summertime ozone maximum.