EFFECT OF CHEMICAL-KINETICS UNCERTAINTIES ON CALCULATED CONSTITUENTS IN A TROPOSPHERIC PHOTOCHEMICAL MODEL

The imprecision of photochemical reaction rates as measured in the laboratory introduces significant uncertainty into trace species concentration calculated in a photochemical model. We have evaluated uncertainties in tropospheric concentrations calculated with a one-dimensional photochemical model, using a Monte Carlo technique to introduce random uncertainty into the model rate coefficients. Correlations between rate coefficients and species and between species and species have been determined to answer the following questions. Which are the most critical kinetic processes in determining constituent distributions? Which are the strongest species-species correlations? The most critical reactions turn out to be the primary photodissociations of O3 and NO2, which initiate ozone destruction and formation, respectively. The reaction between OH and methane is critical, as is the rate of nitric acid formation, which removes both odd nitrogen (NO(x)) and odd hydrogen (HO(x)). Species-species correlations reveal anticorrelation between HO(x) and NO(x) and positive correlation between OH and peroxides, acids, and aldehydes. Particular attention is given to ozone and to the transient OH, which is difficult to measure and is therefore frequently calculated using photochemical models and observations of more stable trace gases. A set of Monte Carlo computations is performed for conditions simulating several distinct chemical environments because imprecision in computed species are nonlinear and depend strongly on mean chemical composition. For low NO(x), low hydrocarbon, low O3 levels, as in the remote troposphere, the 1-sigma imprecision in computed boundary layer OH may be as low as 20%, with that for HO2 at 15% and H2O2 at 25%. At higher NO(x) and O3 levels, the 1-sigma imprecision in boundary layer OH and HO2 is 70%, and H2O2 is 90%. The 1-sigma imprecision in computed O3 is approximately 15% (7-10 ppbv) in both cases. The implications of model computed OH imprecision for predictive and diagnostic calculations are explored. Averaging over the regionally differing results suggests that a typical estimate of global OH is approximately 25% uncertain due to kinetics imprecision. This limits the certainty with which lifetimes for numerous natural and anthropogenic trace gases can be calculated with a photochemical model. The imprecision in a given determination of computed OH can be cut to 20% or less with simultaneous high-precision measurements of O3, CO, CH4, and NO2. Several experimental strategies for optimizing the deduction of OH are described, including one based on measurement of the HO2 radical.