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

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, σisoprene = (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.