Impact of temperature-dependent non-PAN peroxynitrate formation, RO₂NO₂, on nighttime atmospheric chemistry

The formation of peroxynitrates (RO₂NO₂) from the reaction of peroxy radicals (RO₂) and nitrogen dioxide (NO₂) and their subsequent redissociation are typically not included in chemical mechanisms. This is often done to save computational time as the assumption is that the equilibrium is strongly towards the RO₂ + NO₂ reaction for most conditions. Exceptions are the reactions of the methyl peroxy radical due to its abundance in the atmosphere and of acyl-RO₂ radicals due to the long lifetime of peroxyacyl nitrates RO₂NO₂ (PANs). In this study, the nighttime oxidation of cis-2-butene and trans-2-hexene in the presence of NO₂ is investigated in the atmospheric simulation chamber SAPHIR, Forschungszentrum Jülich, Germany, at atmospherically-relevant conditions at different temperatures (≈276 K, ≈293 K, ≈305 K). Measured concentrations of peroxy and hydroperoxy radicals as well as other trace gases (ozone, NO₂, volatile organic compounds) are compared to state-of-the-art zero-dimensional box model calculations. Good model-measurement agreement can only be achieved when reversible RO₂ + NO₂ reactions are included for all RO₂ species using literature values available from the latest SAR by [Jenkin et al., Atmos. Chem. Phys., 2019, 19, 7691]. The good agreement observed gives confidence that the SAR, derived originally for aliphatic RO₂, can be applied to a large range of substituted RO₂ radicals, simplifying generalised implementation in chemical models. RO₂NO₂ concentrations from non-acyl RO₂ radicals of up to 2 × 10 cm^-3 are predicted at 276 K, impacting effectively the kinetics of RO₂ radicals. Under these conditions, peroxy radicals are slowly regenerated downwind of the pollution source and may be lost in the atmosphere through deposition of RO₂NO₂. Based on this study, 60% of RO₂ radicals would be stored as RO₂NO₂ at a temperature of 10 °C and in the presence of a few ppbv of NO₂. The fraction increases further at colder temperatures and/or higher NO₂ mixing ratios. This does not only affect the expected concentrations of RO₂ radicals but, as the peroxynitrates can react with OH radicals or photolyse, they could comprise a net sink for RO₂ radicals as well as increase the production of NO (= NO + NO₂) in different locations depending on their lifetime. Omitting this chemistry from the kinetic model can lead to misinterpreted product formation and may prevent reconciling observations and model predictions.