Experimental evidence for a non-OH oxidant produced from the reaction of isoprene with OH radical

The OH radical initiated oxidation of alkenes is of great importance to air quality and atmospheric chemistry. Although the related mechanism is well studied over several decades, several intermediate steps, such as the recycling of OH radical, the reaction of intermediates, and the formation of peroxides, are unresolved. As we known, the traditional mechanism cannot reproduce the high measured OH radical level in the rural forests. Currently, the recycling of OH radical in the isoprene-OH reaction is considered to be a potential candidate for the explanation. Here, alternatively, we intend to know if a non-OH oxidant leads to the discrepancy between the modeled and measured OH radical in the rural forest by reacting with plenty of oxygenated products of hydrocarbon compounds, sharing the 'oxidation responsibility' of OH radical, and consequently saving the OH radical. After mixing the products produced from the isoprene-OH reaction with formaldehyde and acetaldehyde in a flow reactor in the absent of light, we found an immediate increase of the peroxy formic acid and peroxy acetic acid. Control experiment results indicated that these peroxy acids were the products of aldehyde reaction with some non-OH oxidant, which was produced from the isoprene-OH reaction. Unfortunately, we have not identified this unknown oxidant. However, based on the decrement of aldehyde during the process of mixing with isoprene products, we estimate the OH-equivalent concentration of this oxidant to be ~0.2 pptv, which is one fifth of the OH radical in the isoprene-OH reaction. This mechanism may contribute to explaining the maintenance of the oxidation capacity of the troposphere. Additionally, this mechanism might involve in the functionalization of oxygenated organic compounds and the formation of secondary organic aerosols.