Impact of temperature-dependent non-PAN peroxynitrate formation, RO 2 NO 2 , on nighttime atmospheric chemistry.

The formation of peroxynitrates (RO 2 NO 2 ) from the reaction of peroxy radicals (RO 2 ) and nitrogen dioxide (NO 2 ) 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 RO 2 + NO 2 reaction for most conditions. Exceptions are the reactions of the methyl peroxy radical due to its abundance in the atmosphere and of acyl-RO 2 radicals due to the long lifetime of peroxyacyl nitrates RO 2 NO 2 (PANs). In this study, the nighttime oxidation of cis -2-butene and trans -2-hexene in the presence of NO 2 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, NO 2 , 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 RO 2 + NO 2 reactions are included for all RO 2 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 RO 2 , can be applied to a large range of substituted RO 2 radicals, simplifying generalised implementation in chemical models. RO 2 NO 2 concentrations from non-acyl RO 2 radicals of up to 2 × 10 cm -3 are predicted at 276 K, impacting effectively the kinetics of RO 2 radicals. Under these conditions, peroxy radicals are slowly regenerated downwind of the pollution source and may be lost in the atmosphere through deposition of RO 2 NO 2 . Based on this study, 60% of RO 2 radicals would be stored as RO 2 NO 2 at a temperature of 10 °C and in the presence of a few ppbv of NO 2 . The fraction increases further at colder temperatures and/or higher NO 2 mixing ratios. This does not only affect the expected concentrations of RO 2 radicals but, as the peroxynitrates can react with OH radicals or photolyse, they could comprise a net sink for RO 2 radicals as well as increase the production of NO x (= NO + NO 2 ) 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.