Direct detection of the nitrooxyperoxy radicals in the oxidation of VOCs by NO₃

The photooxidn. of volatile org. compds. (VOCs) is a significant source of secondary org. aerosols (SOA) in the troposphere. The nighttime chem. leading to SOA growth however remains less characterized. The nitrate radical NO₃ is one of the most important nighttime oxidants. In the presence of oxygen, the free radical addn. of NO₃ to unsatd. VOCs will lead to formation of nitrooxyalkyl peroxy radicals. While various studies have measured the further product yields and SOA formation from the reaction, the peroxy intermediate has yet to be directly obsd. We use cavity ringdown spectroscopy to directly detect the nitrooxyalkyl peroxy radicals via the A-X electronic absorption. The A-X electronic transition has enough structure to distinguish between different peroxy radicals as well as different conformers of the same peroxy radical. In order to elucidate the mechanism and formation of the intermediate, we present results from the addn. of NO₃ to one of the simplest unsatd. VOCs, cis and trans 2-butene.