Temperature-dependent rate coefficients for the reactions of the hydroxyl radical with the atmospheric biogenics isoprene, alpha-pinene and delta-3-carene

Abstract. Pulsed laser methods for OH generation and detection were used to study atmospheric degradation reactions for three important biogenic gases: OH + isoprene (Reaction R1), OH +α-pinene (Reaction R2) and OH + Δ-3-carene (Reaction R3). Gas-phase rate coefficients were characterized by non-Arrhenius kinetics for all three reactions. For (R1), k₁ (241–356 K)  = (1.93±0.08) × 10⁻¹¹exp{(466±12)∕T} cm³ molecule⁻¹ s⁻¹ was determined, with a room temperature value of k₁ (297 K)  = (9.3±0.4) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹, independent of bath-gas pressure (5–200 Torr) and composition (M  =  N₂ or air). Accuracy and precision were enhanced by online optical monitoring of isoprene, with absolute concentrations obtained via an absorption cross section, σ_isoprene = (1.28±0.06) × 10⁻¹⁷ cm² molecule⁻¹ at λ = 184.95 nm, determined in this work. These results indicate that significant discrepancies between previous absolute and relative-rate determinations of k₁ result in part from σ values used to derive the isoprene concentration in high-precision absolute determinations.

Similar methods were used to determine rate coefficients (in 10⁻¹¹ cm³ molecule⁻¹ s⁻¹) for (R2)–(R3): k₂ (238–357 K)  = (1.83±0.04) × exp{(330±6)∕T} and k₃ (235–357 K)  = (2.48±0.14) × exp{(357±17)∕T}. This is the first temperature-dependent dataset for (R3) and enables the calculation of reliable atmospheric lifetimes with respect to OH removal for e.g. boreal forest springtime conditions. Room temperature values of k₂ (296 K)  = (5.4±0.2) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹ and k₃ (297 K)  = (8.1±0.3) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹ were independent of bath-gas pressure (7–200 Torr, N₂ or air) and in good agreement with previously reported values. In the course of this work, 184.95 nm absorption cross sections were determined: σ = (1.54±0.08) × 10⁻¹⁷ cm² molecule⁻¹ for α-pinene and (2.40±0.12) × 10⁻¹⁷ cm² molecule⁻¹ for Δ-3-carene.