Formation of organic nitrates from the oxidation of hydrocarbons in the troposphere is a significant loss pathway for nitrogen oxides (NOx) and HOx radicals (OH and HO2). Unsaturated hydrocarbons are emitted in large quantities into the atmosphere where they are predominantly oxidized by OH, leading to the production of β-hydroxyperoxy radicals. One of the major loss processes for β-hydroxyperoxy radicals is the reaction with NO, producing organic nitrates or NO2. Organic nitrates serve as a sink for NOx through deposition and the formation of secondary organic aerosols (SOA), whereas NO2 is responsible for the production of tropospheric ozone through photolysis. Isoprene-derived nitrates are particularly important as they could account for 25 to 50% of total organic nitrates in the troposphere. Formed from the oxidation of ethene, β-hydroxethylperoxy (β-HEP) is the simplest β-hydroxyperoxy radical and therefore serves as a model system for studying OH-initiated oxidation chemistry of more complex non-methane hydrocarbons, such as isoprene. A water vapor enhancement of the β-HEP self-reaction rate coefficient has been reported in the literature, increasing by a factor of 6 under 50% relative humidity conditions between 270 and 298 K. This observation is supported by theoretical works that attribute possible rate enhancement effects to the formation of an RO2:H2O complex. In the upper troposphere, where temperatures are low, up to 14% of RO2 radicals could be complexed with H2O. The structure, vibrations, and equilibrium constant for the formation of HEP:H2O complex has also been recently reported by theoretical works. Despite this, there are currently no chemical mechanisms or product yields predicted for reactions of RO2:H2O complex with NO in global models. Here we present results from an experimental study of the reaction of β-HEP + NO over a range of water concentrations and temperatures, using Multiplexed-Photoionization Mass Spectrometry (M-PIMS). Results will be discussed along with the mechanistic developments and atmospheric implications.

Effect of water complexation on the chemical kinetics of the β-hydroxyethylperoxy radical

2019 Fall National Meeting

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