Kinetics of the Reaction of OH with Isoprene over a Wide Range of Temperature and Pressure Including Direct Observation of Equilibrium with the OH Adducts

Abstract

The reaction of the OH radical with isoprene, CH (R1), has been studied over the temperature range 298-794 K and bath gas pressures of nitrogen from 50 to 1670 Torr using laser flash photolysis (LFP) to generate OH and laser-induced fluorescence (LIF) to observe OH removal. Measurements have been made using both a conventional LFP/LIF apparatus and a new high pressure system. The measured rate coefficient at 298 K ( k = (9.90 ± 0.09) × 10 cm molecule s) in the high pressure apparatus is in excellent agreement with the literature, confirming the accuracy of measurements made with this instrument. Above 700 K, the OH decays were no longer single exponentials due to regeneration of OH from adduct decomposition and the establishment of the OH + CH ⇌ HOCH equilibrium (R1a, R-1a). This equilibrium was analyzed by comparison to a master equation model of reaction R1 and determined the well depth for OH addition to carbon C and C to be equal to 153.5 ± 6.2 and 143.4 ± 6.2 kJ mol, respectively. These well depths are in excellent agreement with the present ab initio-CCSD(T)/CBS//M062X/6-311++G(3df,2p)-calculations (154.1 kJ mol for the C adduct). Addition to the less stable C and C adducts is not important. The data above 700 K also indicated that a minor but significant direct abstraction channel, R1b, was also operating with k = (1.3 ± 0.3) × 10 exp(-3.61 kJ mol/ RT) cm molecule s. Additional support for the presence of this abstraction channel comes from our ab initio calculations and from room-temperature proton transfer mass spectrometry product analysis. The literature data on reaction R1 together with the present data were assessed using master equation analysis, using the MESMER package. This analysis produced a refined data set that generates our recommended k( T, [ M]). An analytical representation of k( T, [ M]) and k( T, [ M]) is provided via a Troe expression. The reported experimental data (the sum of addition and abstraction), k = (9.5 ± 0.2) × 10( T/298 K) + (1.3 ± 0.3) × 10 exp(-3.61 kJ mol/ RT) cm molecule s, significantly extend the measured temperature range of R1.

Remote Sensing Retrieval of Isoprene Concentrations in the Southern Ocean

“…Where ISO w is isoprene concentration in sea water, ISO a is isoprene concentration in the air, K H is the Henry's Law constant for isoprene, and k AS is the gas exchange velocity (cmh −1 ). Air‐side isoprene can be considered near zero and neglected for flux calculations because (a) isoprene is typically largely supersaturated in the surface ocean (>700% during ANDREXII Wohl et al., 2020), and (b) its short atmospheric lifetime (Medeiros et al., 2018) prevents that high airborne isoprene concentrations of continental origin occurred during our sampling. For k AS we used the Wanninkhof (2014) parameterization: …”

“…Ocean‐emitted isoprene (2‐methyl‐1,3‐butadiene, C 5 H 8 ) contributes between 5% and 25% of the natural SOA sources in the global marine atmosphere (Claeys et al., 2004; Gantt et al., 2009; Luo & Yu, 2010; Liao et al., 2007). Owing to its high reactivity (1–2 hr lifetime) (Medeiros et al., 2018), isoprene can impact the chemistry of the marine boundary layer (Lewis et al., 2005; Palmer & Shaw, 2005) and cool the climate (Carslaw et al., 2013: Claeys et al., 2004). This impact is thought to be stronger in the Southern Ocean (SO), away from land vegetation emissions, where isoprene can act synergistically with marine dimethylsulfide to form CCN (Booge et al., 2016; Dani & Loreto, 2017; Meskhidze & Nenes, 2006; Vallina et al., 2007).…”

First oxidation products from the reaction of hydroxyl radicals with isoprene for pristine environmental conditions

“…1. OH attack in 2-and 3-position accounts for ≤ 5% of the initial OH adduct distribution each and will not be further considered here 8,9,11,16 . Possible bimolecular RO 2 radical reactions with NO, HO 2 , or other RO 2 radicals have been omitted to keep the schemes as lucid as possible, drawing the attention to the unimolecular RO 2 reactions.…”

Time-Resolved Measurements and Master Equation Modelling of the Unimolecular Decomposition of CH3OCH2

“…This effect on ΔE 0,1 of using hindered rotors in the fit is typical. [35] A fit to the high pressure limit from the MESMER analysis over the temperature range 300 -1100 K gives 1 ∞ (T) = 2.9 × 10 12 (T/300) 2.5 exp(-106.8 kJ mol -1 /RT) s -1 .…”