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

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(O-x)) 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(O-x), 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(O-x) 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 [O-3]/[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(O-x) is primarily sensitive to either NOx emissions reductions or VOC emissions reductions.