Comparison of isoprene chemical mechanisms at atmospheric night-time conditions in chamber experiments: Evidence of hydroperoxy aldehydes and epoxy products from NO₃ oxidation

Abstract

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.

Daytime Isoprene Nitrates Under Changing NOx and O3

“…(Reeves et al, 2021;Mayhew et al, 2022) Recent work has highlighted species with the formula C4H7NO5 as potentially major oxidation products of isoprene. (Tsiligiannis et al, 2022) Consistent with previous modelling studies (Mayhew et al, 2022), these models largely form C4H7NO5 from OH-initiated oxidation, meaning the concentration isopleth is similar to that of OH and IHN. In contrast, INCE shows reasonably high concentrations under high-O3 mixing ratios at a range of NOx mixing ratios.…”

“…Comparison of the concentrations of species in these so-called steady-state models then allows for conclusions to be drawn as to the preferred oxidation products under various conditions. (Jenkin et al, 2015;Wennberg et al, 2018;Vereecken et al, 2021;Carlsson et al, 2022) The steady-state models are sets of models run at a range of fixed NOx and O3 mixing ratios. Models were run for NOx mixing ratios up to 45 ppb and O3 mixing ratios of 140 ppb, corresponding to the upper limit of measurements made in the Beijing 2017 campaign.…”

“…(Hallquist et al, 2009;Schwantes et al, 2019;Palmer et al, 2022) Isoprene organonitrates have been widely studied due to the large emissions of isoprene resulting in the relevance of isoprene chemistry to a range of environments around the globe. (Guenther et al, 2006;Pye et al, 2015;Reeves et al, 2021;Tsiligiannis et al, 2022) Isoprene hydroxynitrate (IHN) is widely studied due to its formation from OH oxidation in the presence of NO resulting in high concentrations during the daytime (Figure 1). (Xiong et al, 2015;Wennberg et al, 2018) IHN also has formation routes from oxidation with the nitrate radical (NO3).…”

“…(Schwantes et al, 2015) Recent work aiming to improve the representation of isoprene NO3 chemistry in chemical mechanisms highlighted a previously unrepresented reaction pathway to forming nitrated epoxides from alkoxy radicals (RO). (Vereecken et al, 2021;Carlsson et al, 2022) This alkoxyepoxidation pathway provides an alternative formation route to INHE that doesn't rely on a stable intermediate or the presence of OH (Figure 3). Additionally, three more nitrated epoxides can result from this pathway: isoprene nitrooxycarbonyl epoxide (INCE), isoprene nitrooxyhydroperoxy epoxide (INPE), and isoprene dinitrooxy epoxide (IDNE).…”