Analysis of radical propagation efficiency to assess ozone sensitivity to hydrocarbons and NOx: 1. Local indicators of instantaneous odd oxygen production sensitivity

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Abstract
We used a simple trajectory model and a three‐dimensional grid model to evaluate several indicators that can be used to predict the sensitivity of odd oxygen production (P(Ox)) to changes in emissions of anthropogenic nonmethane hydrocarbons (VOC) and nitrogen oxides (NOx = NO + NO2). To perform the evaluation, we augmented the model to include new diagnostic outputs such as rates of P(Ox), rates of conversion of NOx to unreactive nitrogen, and radical chain length. We used the new diagnostic outputs to explain the model‐predicted sensitivity of P(Ox) to changes in VOC and NOx emissions. We found that the ozone ridgeline, which distinguishes between NOx‐limited and radical‐limited conditions, is determined by a ridgeline of maximum OH chain length. We examined the radical propagation reactions which affect OH chain length, and we developed four indicators related to radical propagation efficiency: I(HC,NO2) which approximates the fraction of OH that reacts with hydrocarbons; I(NO,RO2) which approximates the fraction of HO2 that reacts with NO; the ratio of [O3]/[NOx] which affects the balance between radical initiation and propagation; and [HO2] which is the dominant term in P(H2O2)/P(HNO3). Each of these indicators distinguishes conditions in which instantaneous P(Ox) is primarily sensitive to either NOx emissions reductions or VOC emissions reductions.