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 arid 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 σ 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 1σ imprecision in boundary layer OH and HO₂ is 70%, and H₂O₂ is 90%. The 1σ imprecision in computed O₃ is ∼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 ∼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.