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Abstract:
Experiments to investigate the nitrate radical (NO3) initiated oxidation of isoprene under atmospherically relevant conditions were performed in the atmospheric simulation chamber SAPHIR at Forschungszentrum Jülich. By varying the reactant concentrations, the relative importance of the bimolecular reactions of peroxy radicals (RO2) with other RO2, with hydroperoxy radicals or NO3 as well as their unimolecular decomposition was changed.
The summed yield of methyl vinyl ketone (MVK) and methacrolein (MACR) was determined by high resolution proton mass spectrometry using the Vocus PTR-TOF MS. Time series of hydroperoxy aldehydes (HPALD) as a marker for unimolecular reaction, several of the nitrated first generation closed shell products resulting from the bimolecular reactions (nitrated alcohols, carbonyls and hydroperoxides) as well as epoxy-group containing molecules were also observed by Vocus PTR-TOF MS alongside Br-CIMS and I-CIMS.
These experimental results are compared to model calculations based on the MCM v3.3.1,1 the isoprene mechanism as published by Wennberg et al.2 and the FZJ-NO3-isoprene mechanism,3 which incorporates theory-based rate coefficients for a wider range of reactions.
Among other changes, the FZJ-NO3-isoprene mechanism contains a novel, fast oxidation route through the epoxidation of nitrate alkoxy radicals. This inhibits the formation of MVK and MACR from the NO3-initiated oxidation of isoprene to near zero, which agrees with the MVK, MACR and expoxy observations from our chamber experiments. In addition, the FZJ-NO3-isoprene mechanism introduces the unimolecular pathway to HPALD from nitrated RO2 and agrees qualitatively with the time series of the observed first-generation oxygenated nitrates.
1 M. E. Jenkin, J. C. Young and A. R. Rickard, The MCM v3.3.1 degradation scheme for isoprene, Atmos. Chem. Phys., 2015, 15, 11433–11459.
2 P. O. Wennberg et al., Gas-Phase Reactions of Isoprene and Its Major Oxidation Products, Chem. Rev., 2018, 118, 3337–3390.
3 L. Vereecken et al., Theoretical and experimental study of peroxy and alkoxy radicals in the NO3-initiated oxidation of isoprene, Phys. Chem. Chem. Phys., 2021, 23, 5496–5515.
