# Isoprene Emissions, Oxidation Chemistry and Environmental Impacts
## 3 Initial Isoprene Oxidation Chemistry  
Oxidation by hydroxyl radicals (OH) is the main atmospheric removal process for isoprene. Hydroxyl radicals can add to any of the four carbons involved in double bonding, as shown labeled in Figure 2. [PERSON] et al. [53] found experimental evidence to support (within a few percent) their structure-activity relation predictions of 60% 1-OH-, 30% 4-OH-, 5% 2-OH-, and 5% 3-OH-isoprene adducts. Figure 2: Structure of isoprene with C-atom labelling. Whilst the more stable trans structure is the dominant conformer of isoprene, the addition of a hydroxyl radical releases around 38 kcal mol\({}^{-1}\) of energy. The trans- and cis-structures of the activated OH-isoprene adducts are thus able to readily interconvert as suggested in the Leuven isoprene mechanism (LIM), facing barriers of only 14-15 kcal mol\({}^{-1}\). After the initial reaction between isoprene and OH radicals, peroxy radicals are formed. Traditionally, chemical regimes have been divided into high NO\({}_{\mathrm{x}}\) (where the formation of peroxy radicals is the rate-limiting step for oxidation chemistry) and low NO\({}_{\mathrm{x}}\) (where the reactions of peroxy radicals with NO\({}_{\mathrm{x}}\) is the rate-limiting step for oxidation chemistry). The metric was used to assign photochemical conditions that ranged from urban/anthropologically impacted (high NO\({}_{\mathrm{x}}\)) to remote/pristine (low NO\({}_{\mathrm{x}}\)). However, as a result of emission controls, NO\({}_{\mathrm{x}}\) levels can be as low as a few hundred ppt [54], so one should use the term high NO\({}_{\mathrm{x}}\) with caution when referring to urban/remote chemical regimes. It was previously thought that at low NO\({}_{\mathrm{x}}\) conditions, peroxy radicals can recombine into peroxides (Pathway I), which are removed from the atmosphere through deposition processes, resulting in reduced tropospheric OH levels and thus depleting the atmospheric oxidation capacity. At high atmospheric NO\({}_{\mathrm{x}}\) concentrations, the alternate pathway (Pathway II) recycles OH through reactions with NO, producing secondary tropospheric pollutants such as ozone. This additional ozone can contribute to climate change as it is an effective radiative forcing agent in the tropical troposphere. The isoprene oxidation is directly linked to the concentrations of NO\({}_{\mathrm{x}}\) which is mostly released during fossil fuel combustion, with an approximate rate of 30 Tg N yr\({}^{-1}\)[55] However, in forest regions, soil-biogenic NO\({}_{\mathrm{x}}\) emissions are also important, which can globally contribute around 12.9 \(\pm\) 3.9 Tg N yr\({}^{-1}\) of the total source [56]. Soil-biogenic emissions of NO\({}_{\mathrm{x}}\) are influenced by vegetation type, use of nitrogen-containing fertilizer, and meteorological parameters such as moisture and temperature. In pathway II, the peroxy radicals formed from the initial oxidation reaction react with NO to form NO\({}_{2}\). The subsequent photolysis of NO\({}_{2}\) produces NO and ground state oxygen atoms, which can react with O\({}_{2}\) to form tropospheric O\({}_{3}\). [PERSON] et al. [57] showed that the oxidation of isoprene produced a significant amount of ozone with a rate of 1-5 ppb ozone per hour during daytime for the anthropogenically impacted rural environments (NO\({}_{\mathrm{x}}\) concentrations in the range of 0.1-10 ppb). However, the simulated impact of isoprene emissions on the global O\({}_{3}\) burden is small overall [46]. This is in agreement with previous studies where 91-92% of increased ozone concentrations were found to be related to direct effects of human activities [58]. The low-NO\({}_{\mathrm{x}}\) oxidation scheme, which is applicable to the forested and remote regions, highlights the historical controversy over isoprene oxidation mechanisms, as early mechanisms suggested a near-complete titration of isoprene.