Modelled OH reactivity

Abstract

The gas-phase reaction of isoprene with the nitrate radical (NO3) was investigated in experiments in the outdoor SAPHIR chamber at atmospherically relevant conditions specifically with respect to the chemical lifetime and fate of nitrato-organic peroxy radicals (RO2). Observations of organic products were compared to concentrations expected from different chemical mechanisms: (1) The Master Chemical Mechanism, which simplifies the NO3 isoprene chemistry by only considering one RO2 conformer. (2) The chemical mechanism derived from experiments in the CalTech chamber, which considers different RO2 conformers. (3) The FZJ-NO3 isoprene mechanism derived from quantum chemical calculations, which in addition to the CalTech mechanism includes equilibrium reactions of RO2 conformers, unimolecular reactions of nitrate RO2 radicals and epoxidation reactions of nitrate alkoxy radicals. Measurements using mass spectrometer instruments give evidence that the new reactions pathways predicted by quantum chemical calculations play a role in the NO3 oxidation of isoprene. Hydroperoxy aldehydes (HPALD), which are specific for unimolecular reactions of nitrate RO2, were detected even in the presence of an OH scavenger excluding the possibility that concurrent oxidation by hydroxyl radicals (OH) is responsible for their formation. In addition, epoxy compounds, which are specific for the epoxidation reaction of nitrate alkoxy radicals, were detected. Measurements of methyl vinyl ketone (MVK) and methacrolein (MACR) concentrations confirm that the decomposition of nitrate alkoxy radicals implemented in the CalTech mechanism cannot compete with the ring-closure reactions predicted by quantum-chemical calculations. The validity of the FZJ-NO3 isoprene mechanism is further supported by an accurate simulation of the measured hydroxyl radical (OH) reactivity. Nevertheless, the FZJ-NO3 isoprene mechanism needs further investigations with respect to the absolute importance of unimolecular reactions of nitrate RO2 and epoxidation reactions of nitrate alkoxy radicals. Absolute concentrations of specific organic nitrates such as nitrate hydroperoxides would be required to experimentally determine product yields and branching ratios of reactions but could not be measured in the chamber experiments due to the lack of calibration standards for these compounds. The temporal evolution of mass traces attributed to products species such as nitrate hydroperoxides, nitrate carbonyl, nitrate alcohols as well as hydroperoxy aldehydes observed by the mass spectrometer instruments demonstrates that further oxidation by the nitrate radical and ozone at atmospheric concentrations is not relevant on the typical time scale of one night (12 hours). However, oxidation by hydroxyl radicals present at night and potentially also produced from the decomposition of nitrate alkoxy radicals can contribute to their nocturnal chemical loss.