Articles | Volume 17, issue 24
Research article
21 Dec 2017
Research article |  | 21 Dec 2017

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

Terry J. Dillon, Katrin Dulitz, Christoph B. M. Groß, and John N. Crowley

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.

Short summary
A great quantity and variety of organic compounds is released to the atmosphere annually. These compounds greatly impact air chemistry, quality and climate. Laser-based experiments were used to study the atmospheric breakdown of three organics: isoprene, alpha-pinene, and delta-3-carene. Results provided important missing information for low-temperature atmospheric conditions, resolved discrepancies from previous work, and allowed estimation of lifetimes of a few hours for each compound.
Final-revised paper