# Isoprene Emissions, Oxidation Chemistry and Environmental Impacts
## 5 Environmental Impacts of isoprene
[PERSON] et al. [79] showed that the isoprene HO\({}_{\mathrm{x}}\) recycling increased model OH levels significantly over low NOx and high isoprene emission regions, i.e., the tropical forests (typically 15-30%) and the boreal regions over Russia and Canada (typically 5-10%) (Figure 6a). Furthermore, the updated isoprene oxidation chemistry in the STOCHEM-CRI model can reduce the lifetime of methane by 5%, increase the global burden of organic nitrates by 180%, and reduce the tropospheric ozone and methane mixing ratios by up to 10% and 0.8%, respectively, over the tropical forest region (Figure 6b,c) [87]. The simulation study considering the recycling of OH radicals in isoprene oxidation through the isomerization of \(Z\)-\(\delta\)-hydroxyperoxy radicals in the MAGRITTE model by [PERSON] et al. [74] showed enhanced OH concentrations by up to 40% over western Amazonia in the boundary layer and by 10-15% over the southeastern US and Siberia in July. Their study also reported that isomerization reactions led to increased HO\({}_{2}\) concentration by up to \(\sim\)20% and reduced isoprene nitrate formation by up to \(\sim\)40% over Amazonia. [PERSON] and [PERSON] [82] also discussed the hydrogen shift reactions throughout the isoprene oxidation mechanism and used the condensed version of the reduced Caltech isoprene mechanism (RCIM) [73]. Using a global GEOS-Chem model simulation with RCIM, they estimated that the fraction of isoprene reacting with OH globally (88%) and the resultant hydroxy-peroxy radicals react with HO\({}_{2}\) (44%), NO (28%), and RO\({}_{2}\) (9%) or undergo H shifts to regenerate  
Figure 6: The percentage changes of annual surface mixing ratios of (**a**) OH, (**b**) O\({}_{3}\), (**c**) CH\({}_{4}\), simulated by STOCHEM-CRI following implementation of the isoprene HO\({}_{\mathrm{x}}\) recycling. HO\({}_{\rm X}\) (22%). [PERSON] and [PERSON] [82] looked at the overall effect of including updated isoprene chemistry, noting an increase in OH recycling, which, under low NO\({}_{\rm X}\) conditions, resulted in a lower depletion of OH than reported in previous mechanisms. They calculated that the presence of isoprene in the atmosphere could be responsible for an 11% decrease in OH, 6.5% increase in HO\({}_{2}\), 4.2% decrease in NO\({}_{\rm X}\) over the troposphere, and the increased OH resulted in a 12% increase in the tropospheric lifetime of methane. They also found that the updated isoprene oxidation chemistry could increase tropospheric ozone by 1.9 ppbv but decrease ozone levels by as much as 3.4 ppbv over tropical forest regions. The study also estimated the total global source of SOA from isoprene of 61 TgC yr\({}^{-1}\) (136 Tg yr\({}^{-1}\)), with approximately equal contributions from IEPOX, organonitrates, and highly functionalized C\({}_{5}\) compounds. However, a recent study [104] developed a new mechanism, CRI-HOM, incorporating it into the global chemistry-aerosol-climate model to simulate the formation of highly oxygenated organic molecules (HOMs) from the oxidation of \(\alpha\)-pinene. The simulation with the inclusion of CRI-HOM showed a significant impact of isoprene-driven suppression of involatile biogenic-derived species and the consequences for SOA. Evaluating the models against atmospheric measurements of isoprene is required to determine the extent to which using an updated isoprene oxidation mechanism improves or reduces the accuracy of the model. The amount of isoprene measurement data from surface stations has increased enormously over the last thirty years, which is beneficial to evaluate the model simulations.