Temperature-dependent kinetics of the gas-phase reactions of Cl atoms with nopinone, ketolimonene, and myrtenal

In this study, the gas phase reaction of chlorine atoms with three first-generation oxidation products of monoterpene: (myrtenal C₁₀H₁₄O, nopinone C₉H₁₄O, and ketolimonene C₉H₁₄O) were investigated using a relative technique method. These compounds result from the atmospheric oxidation of monoterpene compounds such as α/β – pinene and limonene. Experiments were performed at temperature range 298–353 K and atmospheric pressure in synthetic air bath gas. Cl atoms were generated by UV photolysis of dichloride (Cl₂). The reaction was followed using a proton-transfer reaction mass spectrometer (PTR-MS) and/or Fourier-transform infrared spectroscopy (FTIR) to monitor the concentrations of the investigated species simultaneous with several reference compounds as a function of time. The rate constants were obtained and the Arrhenius expressions (cm³ molecule⁻¹ s⁻¹) obtained were established over the temperature range of 298–353 K:

k_{nopinone + Cl} =  (5.0  ±  1.2) ×  10⁻¹⁰ exp ( − (406  ±  78) /T)

k_{ketolimonene + Cl} =  (8.88 ±  1.3) × 10⁻¹⁰ exp( − (246 ± 46)/T)

k_{myrtenal + Cl} =  (13.5 ±  6.4) × 10⁻¹⁰ exp( − (535 ±  153)/T)

Based on rate constants, the atmospheric lifetimes (τ) of targeted compounds with respect to reaction with chlorine atoms were estimated and expected to be less than 1 day. There results led to conclude that the reaction with chlorine atoms can be an effective tropospheric loss pathway mainly in regions presenting relatively high chlorine concentrations.