# Isoprene Emissions, Oxidation Chemistry and Environmental Impacts
## 4 Isoprene HO\({}_{\mathrm{x}}\) Recycling and Updated Isoprene Oxidation Chemistry  
The increased understanding of isoprene HO\({}_{\mathrm{x}}\) recycling at low NO\({}_{\mathrm{x}}\) has occurred in the past 15 years, mostly due to the realisation that H shift reactions in peroxy radicals formed in the initial oxidation of isoprene can occur quickly enough to compete with, or even dominate, their reactions via pathways I (attachment to the 1-carbon) and II (attachment to the 4-carbon atom) [53; 63]. A theoretical study conducted by [PERSON] et al. [53] proposed that unimolecular isomerization reactions of specific RO\({}_{2}\) radical isomers by 1,6-H- and 1,5-H-shifts are sufficiently rapid to be competitive with bimolecular reactions with other peroxy radicals to form organic hydroperoxides (ROOH). The new RO\({}_{2}\) radicals formed from unimolecular isomerization generate additional ROOH, which can be photolyzed, leading to a significant source of OH. These HO\({}_{\mathrm{x}}\) regenerating reactions have subsequently been included in many atmospheric models and were estimated to yield \(\sim\)0.7 HO\({}_{2}\) and \(\sim\)0.03 OH radicals per isoprene oxidized [53]. A chamber study with conditions relevant to the remote forested regions undertaken by [PERSON] et al. [63] found the formation of high concentrations of dihydroxyepoxides via an OH neutral pathway. These dihydroxyepoxides are thought to be more stable (lifetime with respect to OH addition is of the order of 18-22 h) than the precursor hydroperoxides, and their reactive uptake to aerosol surfaces may lead to a significant source of isoprene-derived secondary organic aerosol (SOA). The stability of the hydroxyepoxides could also help to explain the remarkable stability of OH in remote regions, as it is a stable chain-terminating reaction formed by an OH neutral mechanism. Due to the presence of allyl resonance in both the 1-OH and 4-OH adducts as well as the proposed isoprene oxidation mechanism [53], initial OH isoprene adducts can quickly add O\({}_{2}\) molecule at two additional sites, as depicted in Figure 3. The Z-8-OH-peroxy (i.e., Z-1-OH-4-OO. and Z-4-OH-1-OO.) is demonstrated to be the most reactive in both cases. In this compound, the weakly bonded \(\alpha\)-OH hydrogen undergoes a 1,6-H shift to the peroxy radical site to produce a resonant hydroxy hydroperoxylenyl radical. These radical products can react quickly with O\({}_{2}\) generating HO\({}_{2}\) along with photolabile hydroperoxyl-methylbutenals (HPALDs), which quickly produce OH radicals. In addition to the 1,6-H shift channel, the 1,5-H shift reaction of the \(\beta\)-OH peroxy radicals produces HO\({}_{\mathrm{x}}\) radicals along with oxygenated organic products metachrolein (MACR), methyl vinyl ketone (MVK), and formaldehyde [53; 64]. However, the OH radical generation through 1,5-H shift is found to be very low compared with 1,6-H shift isomerization [61]. [PERSON] et al. [65] reported experimental evidence for the formation of HPALDs from 1,6-H shift isomerization for \(\delta\)-isoprene peroxy radicals with k\({}_{\mathrm{isom}}\)(295 K) = 0.002 s\({}^{-1}\). They estimated that the fraction of isoprene peroxy radicals reacting through this mechanism could be 8-11% globally, up to 20% in the tropics. This rate is substantially slower than predicted in the theoretical calculations [53; 66; 67]. The direct comparison to the theoretical rate is not straightforward, as the experimental study [65] only observed the end products of interconversion between the \(\beta\)-OH-OO, Z-8-OH-OO, and E-8-OH-OO radicals and the 1,6-H shift reactions of the Z-8-OH-OO radicals. From the isomerization rates determined, [PERSON] et al. [65] concluded that isomerization reactions of isoprene peroxy radicals were unlikely to explain the differences in HO\({}_{\mathrm{x}}\) concentrations between model measurements [68]. [PERSON] et al.