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 (NO x = NO + NO 2 ). To perform the evaluation, we augmented the model to include new diagnostic outputs such as rates of P (O x ), rates of conversion of NO x 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 NO x emissions. We found that the ozone ridgeline, which distinguishes between NO x ‐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,NO 2 ) which approximates the fraction of OH that reacts with hydrocarbons; I (NO,RO 2 ) which approximates the fraction of HO 2 that reacts with NO; the ratio of [O 3 ]/[NO x ] which affects the balance between radical initiation and propagation; and [HO 2 ] which is the dominant term in P (H 2 O 2 )/ P (HNO 3 ). Each of these indicators distinguishes conditions in which instantaneous P (O x ) is primarily sensitive to either NO x emissions reductions or VOC emissions reductions.