Preprints
https://doi.org/10.5194/egusphere-2022-587
https://doi.org/10.5194/egusphere-2022-587
 
13 Jul 2022
13 Jul 2022
Status: this preprint is open for discussion.

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

Philip T. M. Carlsson1, Luc Vereecken1, Anna Novelli1, François Bernard2, Steven S. Brown3,4, Bellamy Brownwood5, Changmin Cho1,a, John N. Crowley6, Patrick Dewald6, Peter M. Edwards7, Nils Friedrich6, Juliane L. Fry5,b, Mattias Hallquist8, Luisa Hantschke1, Thorsten Hohaus1, Sungah Kang1, Jonathan Liebmann6, Alfred W. Mayhew7, Thomas Mentel1, David Reimer1, Franz Rohrer1, Justin Shenolikar6, Ralf Tillmann1, Epameinondas Tsiligiannis8, Rongrong Wu1, Andreas Wahner1, Astrid Kiendler-Scharr1,9, and Hendrik Fuchs1,9 Philip T. M. Carlsson et al.
  • 1Institute of Energy and Climate Research, IEK-8: Troposphere, Forschungszentrum Jülich GmbH, 52428 Jülich, Germany
  • 2Institut de Combustion, Aérothermique, Réactivité et Environnement (ICARE), UPR CNRS, 45071 Orléans, France
  • 3NOAA Chemical Sciences Laboratory, 80309 Boulder, USA
  • 4Department of Chemistry, University of Colorado, 80309 Boulder, USA
  • 5Department of Chemistry, Reed College, 97202 Portland, USA
  • 6Atmospheric Chemistry Department, Max Planck Institut für Chemie, 55128 Mainz, Germany
  • 7Wolfson Atmospheric Chemistry Laboratories, Department of Chemistry, University of York, Heslington, York, UK
  • 8Department of Chemistry and Molecular Biology, University of Gothenburg, 41296 Gothenburg, Sweden
  • 9I. Physikalisches Institut, Universität zu Köln, 50932 Köln, Germany
  • anow at: School of Environmental Sciences and Environmental Engineering, Gwangju Institute of Science and Technology, Gwangju, South Korea
  • bnow at: Environmental Sciences Group, Wageningen University, 6708 HB Wageningen, the Netherlands

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.

Philip T. M. Carlsson et al.

Status: open (until 02 Sep 2022)

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Philip T. M. Carlsson et al.

Philip T. M. Carlsson et al.

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Short summary
The investigation of the bight-time oxidation of the most abundant hydrocarbon, isoprene in chamber experiments shows the importance of so far unaccounted reaction pathways leading to epoxy products, which could enhance particle formation. The chemical lifetime of organic nitrates from isoprene is long enough that the majority will be further oxidized on the next by daytime oxidants.