RESULTS FROM THE INTERGOVERNMENTAL PANEL ON CLIMATIC-CHANGE PHOTOCHEMICAL MODEL INTERCOMPARISON (PHOTOCOMP)

Results from the Intergovernmental Panel on Climatic Change (IPCC) tro pospheric photochemical model intercomparison (PhotoComp) are presente d with a brief discussion of the factors that may contribute to differ ences in the modeled behaviors of HOx cycling and the accompanying O-3 tendencies. PhotoComp was a tightly controlled model experiment in wh ich the IPCC 1994 assessment sought to determine the consistency among models that are used to predict changes in tropospheric ozone, an imp ortant greenhouse gas, Calculated tropospheric photodissociation rates displayed significant differences, with a root-mean-square (rms) erro r of the reported model results ranging from about +/-6-9% of the mean (for O-3 and NO2) to up to +/-15% (H2O2 and CH2O). Models using multi stream methods in radiative transfer calculations showed distinctly hi gher rates for photodissociation of NO2 and CH2O compared to models us ing two-stream methods, and this difference accounted for up to one th ird of the rms error for these two rates, In general, some small but s ystematic differences between models were noted for the predicted chem ical tendencies in cases that did not include reactions of nonmethane hydrocarbons (NMHC). These differences in modeled O-3 tendencies in so me 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. O-3 tendencies showed rms errors of a bout +/-10% in the moist, surface level cases with NOx concentrations equal to a few tens of parts per trillion by volume. Most of these mod el to model differences can be traced to differences in the destructio n of O-3 due to reaction with HO2. Differences in HO2, in turn, are li kely 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 NOx concentrations larger than a few parts per billion b y volume, O-3 tendencies showed rms errors of +/-10-30%. These model t o model differences most likely stem from differences in the calculate d rates of O-3 photolysis to O(D-1), which provides about 80% of the H Ox source under these conditions. The introduction of hydrocarbons dra matically increased both the rate of NOx loss and its model to model d ifferences, which, in turn, are reflected in an increased spread of pr edicted O-3. Including NMHC in the simulation approximately doubled th e rms error for O-3 concentration.