Abstract
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, 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.
Highlights
Large quantities and varieties of volatile organic compounds (VOC) are released into Earth’s atmosphere from vegetation; such biogenic emissions exceed those of anthropogenic origin by a factor of ∼ 10 (Goldstein and Galbally, 2007)
VOC lifetimes are constrained by OH, and its abundance is an important test of model reliability
The technique of pulsed laser photolysis (PLP) radical generation coupled to pulsed laser-induced fluorescence (LIF) detection of OH has been used in several absolute kinetic studies of OH + VOC reactions from this laboratory
Summary
Large quantities and varieties of volatile organic compounds (VOC) are released into Earth’s atmosphere from vegetation; such biogenic emissions exceed those of anthropogenic origin by a factor of ∼ 10 (Goldstein and Galbally, 2007). Dillon et al.: Temperature-dependent rate coefficients mechanisms Contrary to these predictions, results from field campaigns have demonstrated that a high OH concentration is maintained (Butler et al, 2008; Lelieveld et al, 2008; Martinez et al, 2010; Stone et al, 2011), with the greatest discrepancies (factor of ≈ 3 to 10) between measured and modelled [OH] being observed where isoprene mixing ratios were large (Kubistin et al, 2010). The techniques employed here allowed the first absolute kinetic studies of (R2 and R3) at the temperatures most representative of boreal forests, which can be as low as 253 K during late-autumn and early-spring months throughout which monoterpene emissions persist (Hakola et al, 2012)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
