Rate Constants for the Gas-Phase Reactions of OH and O3with β-Ocimene, β-Myrcene, and α- and β-Farnesene as a Function of Temperature

Abstract

The rate constants for the gas-phase reactions of hydroxyl radicals and ozone with the biogenic hydrocarbons β-ocimene, β-myrcene, and α- and β-farnesene were measured using the relative rate technique over the temperature ranges 313-423 (for OH) and 298-318 K (for O₃) at about 1 atm total pressure. The OH radicals were generated by photolysis of H₂O₂, and O₃ was produced from the electrolysis of O₂. Helium was used as the diluent gas. The reactants were detected by online mass spectrometry, which resulted in high time resolution, allowing large amounts of data to be collected and used in the determination of the Arrhenius parameters. The following Arrhenius expressions have been determined for these reactions (in units of cm³ molecules⁻¹ s⁻¹): for β-ocimene + OH, k = (4.35(-0.66)(+0.78)) × 10⁻¹¹ exp[(579 ± 59)/T]; for β-ocimene + O₃, k = (3.15(-0.95)(+1.36)) × 10⁻¹⁵ exp[-(626 ± 110)/T]; for β-myrcene + O₃, k = (2.21(-0.66)(+0.94)) × 10⁻¹⁵ exp[-(520 ± 109)/T]; for α-farnesene + OH, k(OH) = (2.19 ± 0.11) × 10⁻¹⁰ for 23-413 K; for α-farnesene + O₃, k = (3.52(-2.54)(+9.09)) × 10⁻¹² exp[-(2589 ± 393)/T]; for β-farnesene + OH, k(OH) = (2.88 ± 0.15) × 10⁻¹⁰ for 323-423 K; for β-farnesene + O₃, k = (1.81(-1.19)(+3.46)) × 10⁻¹² exp[-(2347 ± 329)/T]. The Arrhenius parameters here are the first to be reported. The reactions of α- and β-farnesene with OH showed no significant temperature dependence. Atmospheric residence times due to reactions with OH and O₃ were also presented.