Gas-Phase Reactions of Isoprene and Its Major Oxidation Products

Abstract

Isoprene carries approximately half of the flux of non-methane volatile organic carbon emitted to the atmosphere by the biosphere. Accurate representation of its oxidation rate and products is essential for quantifying its influence on the abundance of the hydroxyl radical (OH), nitrogen oxide free radicals (NO ), ozone (O), and, via the formation of highly oxygenated compounds, aerosol. We present a review of recent laboratory and theoretical studies of the oxidation pathways of isoprene initiated by addition of OH, O, the nitrate radical (NO), and the chlorine atom. From this review, a recommendation for a nearly complete gas-phase oxidation mechanism of isoprene and its major products is developed. The mechanism is compiled with the aims of providing an accurate representation of the flow of carbon while allowing quantification of the impact of isoprene emissions on HO and NO free radical concentrations and of the yields of products known to be involved in condensed-phase processes. Finally, a simplified (reduced) mechanism is developed for use in chemical transport models that retains the essential chemistry required to accurately simulate isoprene oxidation under conditions where it occurs in the atmosphere-above forested regions remote from large NO emissions.

Low-pressure gas chromatography with chemical ionization mass spectrometry for quantification of multifunctional organic compounds in the atmosphere

“…A prime example of this can be seen during the OH oxidation of isoprene, a highly abundant and reactive biogenic VOC, which produces six isomeric peroxy radicals (RO 2 ). Changes in the relative abundance of these radicals can result in vastly different ratios of its OVOC products (Orlando and Tyndall, 2012;Teng et al, 2017;Wennberg et al, 2018), allowing isoprene to have a profound effect on ozone or SOA through its bimolecular reaction products -isoprene hydroxy nitrates (IHNs) and isoprene hydroxy hydroperoxides (ISOPOOHs), respectively -or the OH radical that is recycled during the subsequent chemistry of products formed from the unimolecular RO 2 reaction channel (e.g., hydroperoxy aldehydes or HPALDs; Peeters et al, 2014). These structural effects are also apparent throughout the later-generation chemistry of isoprene and other NMHCs and the outputs of global chemistry transport models can be quite sensitive to this isomer-specific chemistry.…”

Estimating the saturation vapor pressures of isoprene oxidation products C<sub>5</sub>H<sub>12</sub>O<sub>6</sub> and C<sub>5</sub>H<sub>10</sub>O<sub>6</sub> using COSMO-RS

“…Two of the strongest signals detected in the particle phase from isoprene + OH (for both high and low NO x conditions) are C 5 H 12 O 6 and C 5 H 10 O 6 , tentatively identified as a dihydroxy dihydroperoxide (denoted ISOP(OOH) 2 ) and a dihydroperoxy hydroxy aldehyde, with estimated saturation vapor pressures at 298 K of 9.87 × 10 −5 and 6.70 × 10 −4 Pa, respectively (D'Ambro et al, 2017). While technically not a "pure" autoxidation product, as its formation mechanism likely involves two OH oxidation steps (see D'Ambro et al, 2017 andWennberg et al, 2018 for details), ISOP(OOH) 2 shares many features of the proposed autoxidation products of larger alkenes such as monoterpenes; it has an O : C ratio above one, contains two OOH groups and two other functional groups, and potentially forms up to four intramolecular hydrogen bonds. At the same time, ISOP(OOH) 2 is small enough that the uncertainty related to its chemical structure is relatively small (i.e., C 5 H 12 O 6 is highly likely to be a dihydroxy dihydroperoxide, though multiple structural isomers may coexist); also, the computational cost of treating all possible structural isomers and conformers of ISOP(OOH) 2 using quantum chemical methods is not prohibitive.…”

Low-pressure gas chromatography with chemical ionization mass spectrometry for quantification of multifunctional organic compounds in the atmosphere

“…For the ISOPOOH isomers, an average daytime 1,2-ISOPOOH to 4,3-ISOPOOH ratio of~7.6 was observed. The ISOPOOH isomer ratio is much higher than expected accounting only for the isomer-specific bimolecular reaction rates of the isoprene peroxy radicals (Wennberg et al, 2018). The higher ratio is consistent with a large sink of the 4-OH RO 2 isomers via RO 2 isomerization (Peeters et al, 2009;Crounse et al, 2011;Teng et al, 2017).…”