Analysis of radical propagation efficiency to assess ozone sensitivity to hydrocarbons and NO x : 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 NO x ‐limited and radical‐limited. We further examined these indicators in a three‐dimensional grid model. We proposed several new indicators including [H 2 O 2 ]/([O 3 ]+[NO 2 ]), [O 3 ]/[NO x ], and a measure using the OH rate constant weighted concentrations of NO 2 and hydrocarbons. Our analysis also supports the use of several indicators previously proposed by other researchers, including [O 3 ]/[HNO 3 ] and [H 2 O 2 ]/[HNO 3 ]. We found that [HCHO]/[NO 2 ] was more useful than the previously proposed [HCHO]/[NO y ]. We found that the indicators easily distinguished extremely NO x ‐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 NO x provides the most complete and useful description of [O 3 ] sensitivity. Time series of the indicators, at least from mid‐morning 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 HO 2 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 NO x or VOC emissions changes are needed to determine how reliably they distinguish NO x ‐and radical‐limited conditions.