Significant OH regeneration from the photooxidation of several atmospherically important alkenes

The reaction of RO2 with HO2 has long been thought to yield almost exclusively hydroperoxides. Recent experimental studies have provided evidence that for certain systems a significant amount of OH can also be produced. Here we present results collected during the low-NOx photooxidation of methacrolein and methylvinylketone, two main products of the atmospheric oxidation of isoprene, and 2-methyl-3-buten-2-ol, a volatile organic compound emitted by coniferous trees. All show significant OH recycling. Oxidation products were monitored using chemical ionization mass spectrometry (CIMS) methods with the CF3O- reagent ion. The sensitivity of this technique to organic hydroperoxides and the products of the alkoxy channel allows for the estimation of the OH recycling. Additional experiments were conducted using labeled 18-OH. As the recycled OH is not labeled, we obtain a second independent estimate of the OH recycling efficiency using the ratio of unlabeled to labeled products. Currently, most chemical models (eg. MCM 3.1) do not include OH recycling for any of these systems. Inclusion of this recycling in the models will certainly improve the modeled-to-measured OH discrepancies at high isoprene levels which have been pointed out in several recent studies. Experimental details as well as the best estimates for the OH recycling efficiencies for each system will be presented.