Department of Earth System Science

Results from the Intergovernmental Panel on Climatic Change (IPCC) tropospheric photochemical model intercomparison (PhotoComp) are presented with a brief discussion of the factors that may contribute to differences in the modeled behaviors of HO_x cycling and the accompanying O₃ tendencies. PhotoComp was a tightly controlled model experiment in which the IPCC 1994 assessment sought to determine the consistency among models that are used to predict changes in tropospheric ozone, an important greenhouse gas. Calculated tropospheric photodissociation rates displayed significant differences, with a root-mean-square (rms) error of the reported model results ranging from about ±6–9% of the mean (for O₃ and NO₂) to up to ±15% (H₂O₂ and CH₂O). Models using multistream methods in radiative transfer calculations showed distinctly higher rates for photodissociation of NO₂ and CH₂O compared to models using two-stream methods, and this difference accounted for up to one third of the rms error for these two rates. In general, some small but systematic differences between models were noted for the predicted chemical tendencies in cases that did not include reactions of nomnethane hydrocarbons (NMHC). These differences in modeled O₃ tendencies in some cases could be identified, for example, as being due to differences in photodissociation rates, but in others they could not and must be ascribed to unidentified errors. O₃tendencies showed rms errors of about ±10% in the moist, surface level cases with NO_x concentrations equal to a few tens of parts per trillion by volume. Most of these model to model differences can be traced to differences in the destruction of O₃ due to reaction with HO₂. Differences in HO₂, in turn, are likely due to (1) inconsistent reaction rates used by the models for the conversion of HO₂ to H₂O₂ and (2) differences in the model-calculated photolysis of H₂O₂ and CH₂O. In the middle tropospheric “polluted” scenario with NO_x concentrations larger than a few parts per billion by volume, O₃ tendencies showed rms errors of ±10–30%. These model to model differences most likely stem from differences in the calculated rates of O₃ photolysis to O(¹ D), which provides about 80% of the HO_x source under these conditions. The introduction of hydrocarbons dramatically increased both the rate of NO_x loss and its model to model differences, which, in turn, are reflected in an increased spread of predicted O₃. Including NMHC in the simulation approximately doubled the rms error for O₃concentration.