Products and mechanisms of the reactions of the nitrate radical (NO3) with isoprene, 1,3-butadiene and 2,3-dimethyl-1,3-butadiene in air

Products and mechanisms of the reaction of NO3 with isoprene have been studied under simulated atmospheric conditions with in situ FTIR spectroscopy as analytical technique. The study addressed also the reactions of NO3 with 1,3-butadiene and 2,3-dimethyl-1,3-butadiene as well as with the deuterated species 1,1,4,4-d4-1,3-butadiene, d6-1,3-butadiene and 4,4-d2-2-methyl-1,3-butadiene (d2-isoprene). The dienes examined apparently follow very similar reaction pathways. The decay of the intermediate peroxynitrates formed after the addition of NO3 to one of the methylene groups, leads to unsaturated ketone-nitrate, aldehyde-nitrate, alcohol-nitrate and perhaps also dinitrate species. The yield of unsubstituted carbonyl compounds was insignificant in the reactions of NO3 with 1,3-butadiene and isoprene. In the case of isoprene, NO3 reacts adding preferentially to the 1-position and 3-methyl-4-nitroxy-2-butenal appears to be the main product. 1,3-Butadiene predominantly reacts with NO3 via trans-1,4-addition and 1,2-addition while the cis-1,4-addition path is of minor importance. trans-4-Nitroxy-2-butenal and 1-nitroxy-3-buten-2-one were found as main products. Contrary to the daytime OH-initiated degradation of isoprene, the nighttime oxidation of isoprene by NO3 leads to formation of large quantities of organic nitrate compounds; this may have consequences for the tropospheric NOy budget as briefly discussed.