# Isoprene Emissions, Oxidation Chemistry and Environmental Impacts
## 8 Conclusions  
Terrestrial vegetation accounts for a large portion of non-methane VOCs emitted into the atmosphere, with isoprene (C\({}_{5}\)H\({}_{8}\)) having the largest emission flux. The dominant source of isoprene emissions are broadleaf tropical forests (84.5%), followed by temperate shrubs (10%), cool and warm grass (4%), and finally needleleaf forest in boreal regions (1.5%). In both polluted and unpolluted environments, isoprene emission and oxidation chemistry can affect the climate in several ways. In the atmosphere, isoprene is oxidized by OH radicals, and isoprene can thus deplete the oxidizing capacity of the atmosphere. This causes concern as reaction with OH is also the main removal process for many other organic compounds, such as methane, which is a primary greenhouse gas (GHG). Furthermore, in polluted environments, isoprene can contribute to the production of tropospheric ozone, which is a pollutant and major component of smog. In unpolluted environments, certain isoprene oxidation products have been identified as precursors to the formation of secondary organic aerosol, which influences the number of climatically relevant particles in the atmosphere. Initial isoprene oxidation mechanisms were thrown into doubt, as far greater levels of HO\({}_{\text{x}}\) radicals were measured over pristine Amazon rainforests than predicted by models at the time. Following the observation of this model-measurement breakdown, it was realized that the peroxy radicals formed in the initial oxidation of isoprene could undergo isomerization reactions. These 1,6- and 1,5-H shifts were found to occur fast enough to compete with, or even outperform, other isoprene oxidation pathways and isoprene oxidation mechanisms were subsequently updated. Updated isoprene chemistry has since been included in numerous studies and has reduced the model-measurement breakdown seen in HO\({}_{\text{x}}\) concentrations. Due to the role of HO\({}_{\text{x}}\) radicals in the oxidizing chemistry of the atmosphere, these revisions have also affected the tropospheric burdens of important species such as carbon monoxide, secondary organic aerosols, PANs, methane, and ozone. With the updated isoprene oxidation chemistry scheme, the modelled-measured isoprene concentrations are still found to be a disagreement possibly due to the uncertainty of the MEGAN emission inventory used in the STOCHEM-CRI model. Laboratory studies have enabled us to develop a full reaction scheme for the oxidation of isoprene as a function of NO\({}_{\text{x}}\). Such laboratory studies have revealed previously unknown pathways, such as the isomerization reactions, which have been shown to have major impacts on the oxidizing capacity of the troposphere. Whilst these studies have provided a fundamental understanding of the reaction mechanism, there still remain many unknowns. For example, there have, as of yet, not been any isomer-specific studies on the 6 peroxy radicals shown in Figure 4, and thus their fate remains highly uncertain. Chemical synthesis of novel precursors to generate one peroxy radical at a time in the laboratory are needed or to be able to measure each isomer independently in a measurement system. This would enable new experimental studies, analogous to the use of 1,1-diodo compounds as photolytic precursors of Criegee intermediates, to study their fate for the first time [180; 181; 182]. Isoprene emissions and their response to deforestation and reforestation have been a major topic studied by atmospheric chemists over recent decades. This demonstrates the need for including a correct mapping of land covers by trees due to deforestation/reforestation in models. How isoprene emissions will change in response to climate change remains highly uncertain, and thus how altered isoprene emissions will impact air quality, human health, and climate change in the future remains unknown. Coordinated field studies are required both on a local and regional scale to investigate the impact of isoprene chemistry.