Gas-phase reactions of NO3 and N2O5 with (Z)-hex-4-en-1-ol, (Z)-hex-3-en-1-ol (‘leaf alcohol’), (E)-hex-3-en-1-ol, (Z)-hex-2-en-1-ol and (E)-hex-2-en-1-ol

Abstract

The night-time atmospheric chemistry of the biogenic volatile organic compounds (Z)-hex-4-en-1-ol, (Z)-hex-3-en-1-ol (‘leaf alcohol’), (E)-hex-3-en-1-ol, (Z)-hex-2-en-1-ol and (E)-hex-2-en-1-ol, has been studied at room temperature. Rate coefficients for reactions of the nitrate radical (NO₃) with these stress-induced plant emissions were measured using the discharge–flow technique. We employed off-axis continuous-wave cavity-enhanced absorption spectroscopy (CEAS) for the detection of NO₃, which enabled us to work in excess of the hexenol compounds over NO₃. The rate coefficients determined were (2.93 ± 0.58) × 10⁻¹³ cm³ molecule⁻¹ s⁻¹, (2.67 ± 0.42) × 10⁻¹³ cm³ molecule⁻¹ s⁻¹, (4.43 ± 0.91) × 10⁻¹³ cm³ molecule⁻¹ s⁻¹, (1.56 ± 0.24) × 10⁻¹³ cm³ molecule⁻¹ s⁻¹, and (1.30 ± 0.24) × 10⁻¹³ cm³ molecule⁻¹ s⁻¹ for (Z)-hex-4-en-1-ol, (Z)-hex-3-en-1-ol, (E)-hex-3-en-1-ol, (Z)-hex-2-en-1-ol and (E)-hex-2-en-1-ol. The rate coefficient for the reaction of NO₃ with (Z)-hex-3-en-1-ol agrees with the single published determination of the rate coefficient using a relative method. The other rate coefficients have not been measured before and are compared to estimated values. Relative-rate studies were also performed, but required modification of the standard technique because N₂O₅ (used as the source of NO₃) itself reacts with the hexenols. We used varying excesses of NO₂ to determine simultaneously rate coefficients for reactions of NO₃ and N₂O₅ with (E)-hex-3-en-1-ol of (5.2 ± 1.8) × 10⁻¹³ cm³ molecule⁻¹ s⁻¹ and (3.1 ± 2.3) × 10⁻¹⁸ cm³ molecule⁻¹ s⁻¹. Our new determinations suggest atmospheric lifetimes with respect to NO₃-initiated oxidation of roughly 1–4 h for the hexenols, comparable with lifetimes estimated for the atmospheric degradation by OH and shorter lifetimes than for attack by O₃. Recent measurements of [N₂O₅] suggest that the gas-phase reactions of N₂O₅ with unsaturated alcohols will not be of importance under usual atmospheric conditions, but they certainly can be in laboratory systems when determining rate coefficients.