Radical Product Yields from the Ozonolysis of Short Chain Alkenes under Atmospheric Boundary Layer Conditions

Abstract

The gas-phase reaction of ozone with unsaturated volatile organic compounds (VOCs), alkenes, is an important source of the critical atmospheric oxidant OH, especially at night when other photolytic radical initiation routes cannot occur. Alkene ozonolysis is also known to directly form HO2 radicals, which may be readily converted to OH through reaction with NO, but whose formation is poorly understood. We report a study of the radical (OH, HO2, and RO2) production from a series of small alkenes (propene, 1-butene, cis-2-butene, trans-2-butene, 2-methylpropene, 2,3-dimethyl-2-butene (tetramethyl ethene, TME), and isoprene). Experiments were performed in the European Photoreactor (EUPHORE) atmospheric simulation chamber, with OH and HO2 levels directly measured by laser-induced fluorescence (LIF) and HO2 + ΣRO2 levels measured by peroxy-radical chemical amplification (PERCA). OH yields were found to be in good agreement with the majority of previous studies performed under comparable conditions (atmospheric pressure, long time scales) using tracer and scavenger approaches. HO2 yields ranged from 4% (trans-2-butene) to 34% (2-methylpropene), lower than previous experimental determinations. Increasing humidity further reduced the HO2 yields obtained, by typically 50% for an RH increase from 0.5 to 30%, suggesting that HOx production from alkene ozonolysis may be lower than current models suggest under (humid) ambient atmospheric boundary layer conditions. The mechanistic origin of the OH and HO2 production observed is discussed in the context of previous experimental and theoretical studies.

Atmospheric isoprene ozonolysis: impacts of stabilised Criegee intermediate reactions with SO<sub>2</sub>, H<sub>2</sub>O and dimethyl sulfide

“…Here, L accounts for the sum of any other chemical loss processes for SCIs in the chamber, after correction for dilution, and neglecting other (non-alkene) chemical sinks for O 3 , such as reaction with HO 2 (also produced directly during alkene ozonolysis; Alam et al, 2013;Malkin et al, 2010), which was indicated through model calculations to account for < 0.5 % of ozone loss under all the experimental conditions.…”

“…Previous studies have identified the hydroperoxide rearrangement as dominant for SCIs with a syn configuration, determining their overall unimolecular decomposition rate (Niki et al, 1987;Rickard et al, 1999;Martinez and Herron, 1987;Johnson and Marston, 2008). This route has been shown to be a substantial non-photolytic source of atmospheric oxidants (Niki et al, 1987;Alam et al, 2013). CIs formed in the anti configuration are thought to primarily undergo rearrangement and possibly decomposition via a dioxirane intermediate ("the acid/ester channel"), producing a range of daughter products and contributing to the observed overall HO x radical yield (Johnson and Marston, 2008;Alam et al, 2013).…”

“…This route has been shown to be a substantial non-photolytic source of atmospheric oxidants (Niki et al, 1987;Alam et al, 2013). CIs formed in the anti configuration are thought to primarily undergo rearrangement and possibly decomposition via a dioxirane intermediate ("the acid/ester channel"), producing a range of daughter products and contributing to the observed overall HO x radical yield (Johnson and Marston, 2008;Alam et al, 2013).…”

Rate coefficients for the reactions of OH radical and ozone with a series of unsaturated esters

“…The gas-phase reaction of O3 with unsaturated VOCs involves electrophilic addition of O3 on >C=C< followed by unimolecular decomposition of the 1,2,3-trioxolane adduct (Alam et al, 2013). Therefore, the reactivity of unsaturated esters toward O3 should reflect the electronic density at the C=C double bond.…”

Alkenyl and Aryl Peroxides

“…Also, tetramethyl ethylene consistently gave amongst the highest OH yields, in line with the model, because every carbonyl oxide can undergo the 1,4‐hydrogen shift. It should be noted that ethylene ozonation also generates OH radicals in the gas phase, although an alkenyl peroxide cannot form this way—obviously, different mechanisms can operate …”