Daytime isoprene nitrates under changing NO_<i>x</i> and O₃

Abstract

Organonitrates are important species in the atmosphere due to their impacts on NOx, HOx, and O3 budgets, and their potential to contribute to secondary organic aerosol (SOA) mass. This work presents a steady-state modelling approach to assess the impacts of changes in NOx and O3 concentrations on the organonitrates produced from isoprene oxidation. The diverse formation pathways to isoprene organonitrates dictate the responses of different groups of organonitrates to changes in O3 and NOx. For example, organonitrates predominantly formed from the OH-initiated oxidation of isoprene favour formation under lower-ozone and moderate-NOx concentrations, whereas organonitrates formed via daytime NO3 oxidation show the highest formation under high-O3 concentrations with little dependence on NOx concentrations. Investigating the response of total organonitrates reveals complex and nonlinear behaviour with implications that could inform expectations of changes to organonitrate concentrations as efforts are made to reduce NOx and O3 concentrations, including a region of NOx–O3 space where total organonitrate concentration is relatively insensitive to changes in NOx and O3. These conclusions are further contextualised by estimating the volatility of the isoprene organonitrates revealing the potential for high concentrations of low-volatility species under high-ozone conditions.