Total radical yields from tropospheric ethene ozonolysis

Abstract

The gas-phase reactions of ozone with alkenes can be significant sources of free radicals (OH, HO₂ and RO₂) in the Earth's atmosphere. In this study the total radical production and degradation products from ethene ozonolysis have been measured, under conditions relevant to the troposphere, during a series of detailed simulation chamber experiments. Experiments were carried out in the European photoreactor EUPHORE (Valencia, Spain), utilising various instrumentation including a chemical-ionisation-reaction time-of-flight mass-spectrometer (CIR-TOF-MS) measuring volatile organic compounds/oxygenated volatile organic compounds (VOCs/OVOCs), a laser induced fluorescence (LIF) system for measuring HO₂ radical products and a peroxy radical chemical amplification (PERCA) instrument measuring HO₂ + ΣRO₂. The ethene + ozone reaction system was investigated with and without an OH radical scavenger, in order to suppress side reactions. Radical concentrations were measured under dry and humid conditions and interpreted through detailed chemical chamber box modelling, incorporating the Master Chemical Mechanism (MCMv3.1) degradation scheme for ethene, which was updated to include a more explicit representation of the ethene–ozone reaction mechanism.

The rate coefficient for the ethene + ozone reaction was measured to be (1.45 ± 0.25) × 10⁻¹⁸ cm³ molecules⁻¹ s⁻¹ at 298 K, and a stabilised Criegee intermediate yield of 0.54 ± 0.12 was determined from excess CO scavenger experiments. An OH radical yield of 0.17 ± 0.09 was determined using a cyclohexane scavenger approach, by monitoring the formation of the OH-initiated cyclohexane oxidation products and HO₂. The results highlight the importance of knowing the [HO₂] (particularly under alkene limited conditions and high [O₃]) and scavenger chemistry when deriving radical yields. An averaged HO₂ yield of 0.27 ± 0.07 was determined by LIF/model fitting. The observed yields are interpreted in terms of branching ratios for each channel within the postulated ethene ozonolysis mechanism.