Sensitivity of atmospheric models to rate terms within complex chemical mechanisms

The decoupled direct method for sensitivity analysis was extended to the rate terms for the loss of reactants and formation of products to provide a detailed understanding of the predictions of chemical mechanisms used in atmospheric models. A box-model version and the full three-dimensional (3-D) version of the Comprehensive Air Quality Model with Extensions were used to calculate sensitivities to rate terms in the Carbon Bond 2005 chemical mechanism. The processes studied were formation of products from oxidation of the more reactive organic compounds, removal of NOx by organic nitrate formation, re-formation of NOx from organic nitrates, and loss of OH and NO2 and formation of HNO3 in the reaction OH + NO2 → HNO3. Simulations of the box model for four scenarios representing urban and rural atmospheres indicated that the largest sensitivities of ozone, formaldehyde and HOx were to rate terms for certain products of the oxidation of isoprene and to the reactants in OH + NO2 → HNO3. The 3-D model was then applied to calculate sensitivities to these rate terms for an eastern U.S. ozone episode in July, 2008. The 3-D simulation results were better correlated with the more NOx-limited rural scenario of the box model and showed that the isoprene oxidation products with the greatest impact on O3 formation are methacrolein, methyl vinyl ketone, HO2 radical, and species converting NO to NO2 and organic nitrates. The 3-D results also showed that loss of NO2 was more important than loss of OH for O3 formation in most areas of the eastern U.S. This work emphasizes the need for continued experimental work to understand the oxidation of isoprene because the isoprene chemistry influenced modeled ozone predictions for both urban as well as rural areas.