Effects of uncertainty in the reaction of the hydroxyl radical with nitrogen dioxide on model-simulated ozone control strategies

We evaluated the effect of a 20% reduction in the rate constant of the reaction of the hydroxyl radical with nitrogen dioxide to produce nitric acid (OH+NO2→HNO3) on model predictions of ozone mixing ratios ([O3]) and the effectiveness of reductions in emissions of volatile organic compounds (VOC) and nitrogen oxides (NOx) for reducing [O3]. By comparing a model simulation with the new rate constant to a base case scenario, we found that the [O3] increase was between 2 and 6% for typical rural conditions and between 6 and 16% for typical urban conditions. The increases in [O3] were less than proportional to the reduction in the OH+NO2 rate constant because of negative feedbacks in the photochemical mechanism. Next, we used two different approaches to evaluate how the new OH+NO2 rate constant changed the effectiveness of reductions in emissions of VOC and NOx: first, we evaluated the effect on [O3] sensitivity to small changes in emissions of VOC (d[O3]/dEVOC) and NOx (d[O3]/dENOx); and secondly, we used the empirical kinetic modeling approach to evaluate the effect on the level of emissions reduction necessary to reduce [O3] to a specified level. Both methods showed that reducing the OH+NO2 rate constant caused control strategies for VOC to become less effective relative to NOx control strategies. We found, however, that d[O3]/dEVOC and d[O3]/dENOx did not quantitatively predict the magnitude of the change in the control strategy because the [O3] response was nonlinear with respect to the size of the emissions reduction. We conclude that model sensitivity analyses calculated using small emissions changes do not accurately characterize the effect of uncertainty in model inputs (in this case, the OH+NO2 rate constant) on O3 attainment strategies. Instead, the effects of changes in model inputs should be studied using large changes in precursor emissions to approximate realistic attainment scenarios