Analysis of radical propagation efficiency to assess ozone sensitivity to hydrocarbons and NOx 2.: Long-lived species as indicators of ozone concentration sensitivity

We used an analysis of radical propagation efficiency and OH chain length in a simple trajectory model to propose combinations of long-lived species that distinguish conditions in which O-3 concentration ([O-3]) is NOx-limited and radical-limited. We further examined these indicators in a three-dimensional grid model. We proposed several new indicators including [H2O2]/([O-3] + [NO2]), [O-3]/[NOx], and a measure using the OH rate constant weighted concentrations of NO2 and hydrocarbons. Our analysis also supports the use of several indicators previously proposed by other researchers, including [O-3]/[HNO3] and [H2O2]/[HNO3]. We found that [HCHO]/[NO2] was more useful than the previously proposed [HCHO]/[NOy]. We found that the indicators easily distinguished extremely NOx-, limited or extremely radical-limited regimes but did not reliably distinguish conditions closer to the transition between these two regimes. We propose that a combined analysis using photochemical model simulations and a large set of indicators of both [O-3] sensitivity and local odd oxygen production (P(O-x)) sensitivity to VOC and NOx provides the most complete and useful description of [O-3] sensitivity. Time series of the indicators, at least from midmorning to late afternoon, provide useful information about the evolution of [O-3] Sensitivity during the day. Values of the indicators change depending on the [O-3] level due in part to the effects of miscellaneous OH and HO2 termination reactions and to the effects of the composition of the HC mixture on P(O-x). Further evaluation of these indicators using modeling studies, measurements, and test cases with NOx or VOC emissions changes are needed to determine how reliably they distinguish NOx- and radical-limited conditions.