# Isoprene Emissions, Oxidation Chemistry and Environmental Impacts
## 4 Isoprene HO\({}_{\mathrm{x}}\) Recycling and Updated Isoprene Oxidation Chemistry
[68] used a box model to investigate the impacts of mechanistic changes on HO\({}_{\rm x}\) formation and recycling in the oxidation of isoprene in remote forested regions. The Master Chemical Mechanism version 3.1 (MCM v3.1) was used as a base reference mechanism with sensitivity studies conducted over a wide range of NOx levels. The radical propagating channels for the reactions of HO\({}_{2}\) with RO\({}_{2}\) radicals (namely acyl and \(\beta\)-oxoperoxy radicals) and the partial revision of the first generation hydroperoxides (representative of [PERSON] et al. [63] OH regeneration) in their study led to increases of OH concentrations by \(\sim\)5-7% over the sub-ppb NOx range. However, implementing the unimolecular isomerization reactions proposed by [PERSON] et al. [53] into the MCM mechanism yielded a factor of up to 3.3 increase in the OH concentration, with the highest increase at the low end of the NOx range. A representation of isoprene HO\({}_{\rm x}\) recycling for use with the Common Representative Intermediates version 2, reduction 5 (CRI v2-R5) mechanism was developed using the MCM v3.1 sensitivity studies as a reference benchmark [68]. The mechanism included a reduced representation of HO\({}_{\rm x}\) recycling resulting from isomerization of isoprene-derived peroxy radicals (based on [PERSON] et al. [53]) and the inclusion of propagating channels for the reactions of HO\({}_{2}\) with acyl peroxy radicals [69]. The inclusion of isoprene HO\({}_{\rm x}\) recycling caused an increase of up to 40% in the OH concentration over forested regions (the tropics in February and the tropics as well as forested boreal regions in August) with an increase of more than 20% simulated in the free troposphere [68]. Figure 3: Reaction scheme for the addition of O\({}_{2}\) to initial 1-OH and 4-OH adducts of isoprene, following the reaction of isoprene with an OH radical. Adapted from [PERSON] et al. [53]. The theoretical study [61] showed that 1,6-H shift isomerization occurred with large reaction rate constants (5.7 s\({}^{-1}\) and 0.52 s\({}^{-1}\) at 298 K for 4-OH and 1-OH adducts, respectively), which are in good agreement with the experimental study (3.6 s\({}^{-1}\) and 0.4 s\({}^{-1}\), respectively) [70]. However, 1,5-H shift isomerization occurred with small reaction rate constants (1.1 \(\times\) 10\({}^{-3}\) s\({}^{-1}\) and 0.7 \(\times\) 10\({}^{-3}\) s\({}^{-1}\) at 298 K for 4-OH and 1-OH adducts, respectively) [61], which makes the reaction competitive only in the presence of exceptionally low NO levels (<10 pptv). [PERSON] et al. [61] updated the isomerization mechanism (Figure 4) and suggested that the bulk isomerization of Z-\(\delta\)-OH-peroxy 1,6-H-shift rate k\({}_{\text{isom}}\) increases approximately linearly with the traditional bimolecular reaction rates, k\({}_{\text{tr}}\), because these bimolecular reactions reduce the abundances of majority \(\beta\)-OH peroxy radicals but have much less influence on the abundances of Z-\(\delta\)-OH-peroxy radicals. In the experiments of [PERSON] et al. [65], traditional bimolecular reaction rates, k\({}_{\text{tr}}\), were found to be \(\sim\)0.02 s\({}^{-1}\). However, [PERSON] et al. [71] reported that k\({}_{\text{tr}}\) is highly variable with NO\({}_{\text{x}}\) levels with a range of \(\sim\)0.02 s\({}^{-1}\) in the afternoon (low NO) to \(\sim\)0.1 s\({}^{-1}\) in the morning (high NO). The measured bulk isomerization rate reported by [PERSON] et al. [65] is at the lower end of atmospherically relevant k\({}_{\text{tr}}\), which leads to an underestimation of the actual isomerization rate in their study. The addition of O\({}_{2}\) to the hydroxy hydroperoxylkenyl radicals produced from 1,6-H shift isomerization of Z-\(\delta\)-OH-peroxy can proceed equally via \(\alpha\)-hydroxy H atom abstraction to yield HPALDs and HO\({}_{2}\) and via addition \(\gamma\) to the OH group, leading to the production of the complex dihydropyoxy-carbonyl peroxy radicals, di-HPCARPs (Figure 5) [61].