On Rates and Mechanisms of OH and O_3 Reactions with Isoprene-Derived Hydroxy Nitrates

Eight distinct hydroxy nitrates are stable products of the first step in the atmospheric oxidation of isoprene by OH. The subsequent chemical fate of these molecules affects global and regional production of ozone and aerosol as well as the location of nitrogen deposition. We synthesized and purified 3 of the 8 isoprene hydroxy nitrate isomers: (E/Z)-2-methyl-4-nitrooxybut-2-ene-1-ol and 3-methyl-2-nitrooxybut-3-ene-1-ol. Oxidation of these molecules by OH and ozone was studied using both chemical ionization mass spectrometry and thermo-dissociation laser induced fluorescence. The OH reaction rate constants at 300 K measured relative to propene at 745 Torr are (1.1 ± 0.2) × 10^(−10) cm^3 molecule^(−1)s^(−1) for both the E and Z isomers and (4.2 ± 0.7) × 10^(−11) cm^3 molecule^(−1)s^(−1) for the third isomer. The ozone reaction rate constants for (E/Z)-2-methyl-4-nitrooxybut-2-ene-1-ol are (2.7 ± 0.5) × 10^(−17) and (2.9 ± 0.5) × 10^(−17) cm^3 molecule^(−1)^s(−1), respectively. 3-Methyl-2-nitrooxybut-3-ene-1-ol reacts with ozone very slowly, within the range of (2.5−5) × 10^(−19) cm^3 molecule^(−1)s^(−1). Reaction pathways, product yields, and implications for atmospheric chemistry are discussed. A condensed mechanism suitable for use in atmospheric chemistry models is presented.