J. Olson et al., RESULTS FROM THE INTERGOVERNMENTAL PANEL ON CLIMATIC-CHANGE PHOTOCHEMICAL MODEL INTERCOMPARISON (PHOTOCOMP), JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, 102(D5), 1997, pp. 5979-5991
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.