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Abstract. Pulsed laser methods for OH generation and detection were used to study atmospheric degradation reactions for three important biogenic gases: OH + isoprene (R1); OH + α-pinene (R2); and OH + Δ-3-carene (R3). Gas-phase rate coefficients were characterised by non-Arrhenius kinetics for all three reactions. For (R1), k1 (241–356 K) = (1.93 ± 0.08) × 10−11 exp (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.
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) × 10−17cm 2 molecule−1 for α-pinene and (2.40 ± 0.12) × 10−17 cm2 molecule−1 for Δ-3-carene.
