# Isoprene Emissions, Oxidation Chemistry and Environmental Impacts
## 4 Isoprene HO\({}_{\mathrm{x}}\) Recycling and Updated Isoprene Oxidation Chemistry
Under low NO\({}_{\text{x}}\) conditions, di-HPCARPs react significantly via a 1,4-H-shift isomerization reaction to form OH, carbon monoxide (CO), and dihydropyoxy-carbonyl compound, which also photolyzes rapidly to produce additional OH. Figure 4: Updated reaction scheme of OH-initiated oxidation of isoprene under low NO\({}_{\text{x}}\) conditions (adapted from [61]). There are large uncertainties on the yields of HPALD and di-HPCARPs. [PERSON] et al. [61] proposed a theoretical yield of \(\sim\)0.5 for HPALDs and di-HPCARPs. The experimental studies conducted by [PERSON] et al. [70] and [PERSON] et al. [72] obtained a total yield of 0.4 (\(\beta\)-HPALD \(\sim\)0.15, \(\delta\)-HPALD \(\sim\)0.25). Some other experimental and theoretical [73; 74; 75] studies suggested the formation of HPALDs with very high yield of \(\sim\)0.75. A recent theoretical study by [PERSON] et al. [76] reported a \(\delta\)-HPALD yield of \(\sim\)1 with negligible contribution from \(\beta\)-HPALD. However, another recent experimental study performed by [PERSON] et al. [77] indirectly determined very low HPALD yield of 0.19 without distinction between the \(\beta\)- and \(\delta\)-isomers. The chemistry of isoprene degradation in the MCM was refined and updated to reflect advances in understanding in the latest version, MCM v3.3.1, consisting of 1926 reactions of 602 species [78]. The updates mainly related to the OH-initiated chemistry, which has large impacts on HO\({}_{\mathrm{x}}\) recycling at lower NO\({}_{\mathrm{x}}\) levels, with particularly important simulated contributions resulting from the chemistry initiated by the 1,6-H shift isomerization reactions of the \(Z\)-\(\delta\)-OH-peroxy and from the higher-generation 1,4-H shift isomerization reactions of the \(\alpha\)-formyl peroxy radicals. The rate coefficient for the 1,6-H shift isomerization reaction is based on those reported by [PERSON] et al. [61] but scaled (slower by a factor of \(\sim\)5) to recreate the phenomenological bulk isomerization rate (\(k_{\mathrm{bulk}}\)) to form the HPALDs for the conditions of the experiments reported by [PERSON] et al. [65]. The revised oxidation chemistry of isoprene was also incorporated into the CRI v2-R5 mechanism [79] using the MCM v3.1.1 sensitivity studies as a reference benchmark. Figure 5: Mixing ratios of isoprene simulated by STOCHEM-CRI (**a**) surface distribution for June-July-August, (**b**) surface distribution for December-January-February, (**c**) zonal distribution for June-July-August, (**d**) zonal distribution for December-January-February. [PERSON] et al. [74] implemented a new chemical mechanism for the oxidation of BVOCs in the Model of Atmospheric Composition at Global and Regional Scales using Inversion Techniques for Trace Gas Emissions (MAGRITTE v1.1), incorporating the representation of isoprene's oxidation scheme with major advances in recent theoretical and laboratory studies. The isomerization reactions in the study of [PERSON] et al. [74] controlled the fate of about one-fifth of the total flux of hydroperoxy radicals formed from the reaction between isoprene and OH (16.5% for the 1,6-H shift and 3% for the 1,5-H shift). Accurate modeling of isoprene emissions, involving the most up-to-date understanding of isoprene oxidation chemistry, would increase the understanding of the global environmental impacts of isoprene oxidation.