# Isoprene Emissions, Oxidation Chemistry and Environmental Impacts
## 5 Environmental Impacts of isoprene  
Isoprene is not seen to be a major threat to human health in its unreacted form, with only unrealistically high concentrations considered carcinogenic, though it possesses a greater risk when its subsequent atmospheric reaction products are considered [80]. As isoprene is highly reactive, its double bonds make it vulnerable to attack by the nitrate radical (NO\({}_{3}\)), OH, and O\({}_{3}\), with atmospheric lifetimes of 0.8 h, 1.7 h, and 1.3 days, respectively [81]. Isoprene's impact on climate, air quality, and human health can be recognized on both regional and global levels due to the persistence of long-lived multi-stage oxidation products. The oxidation normally occurs in the boundary layer in the vicinity of the emission regions, where it has a significant impact on the oxidizing capacity, the global carbon cycle and radiative forcing [82; 83; 84]. Isoprene emissions contribute to various atmospheric processes such as the formation of tropospheric O\({}_{3}\), affect the lifetime of methane (CH\({}_{4}\)), and the growth of SOA, which could have important climatic impacts [19; 63; 85]. Calculating the impact of isoprene emissions released by plants on the atmosphere is challenging because the reactions involve radicals, can take many alternative paths, and rely on the concentrations in a highly non-linear manner. Under high NO\({}_{\rm x}\) conditions ([NO\({}_{\rm x}\)] > 1 ppb), reactions between initial isoprene oxidation products and NO recycled HO\({}_{\rm x}\) led to the formation of tropospheric ozone, which is considered a major pollutant [83; 86]. Under low NO\({}_{\rm x}\) conditions, isoprene reacts directly with O\({}_{3}\) and thus reduces tropospheric O\({}_{3}\) levels. In addition, the isoprene oxidation with HO\({}_{\rm x}\) recycling led to a change in the oxidative capacity of the troposphere [87]. Peroxyacetyl nitrates (PANs) can also be produced when isoprene breaks down [88], which are recognized as phytotoxins, bacterial mutagens, and eye irritants (lachrymators). Its phytotoxic biological effects, which harm plants and vegetation, are the most detrimental [89]. Some isoprene oxidation products at both high and low NO\({}_{\rm x}\) conditions have been identified as precursors to the formation of SOA, which influences the number of climatically relevant particles in the atmosphere. SOA is the dominant contribution to atmospheric particulate organic matter (POM), which is highly variable with season and location, and in tropical forest regions, POM accounts for up to 90% of total atmospheric particulate mass [90]. The biogenic SOA can act as condensation nuclei for cloud droplet formation in the atmosphere, thus exerting a negative radiative effect on the climate [91]. Historically, it was thought that isoprene did not contribute to SOA formation because of the volatility of its key oxidation products, e.g., methacrolein, methyl vinyl ketone, glyoxal, methylglyoxal, glycolaldehyde, hydroxyacetone, and formaldehyde [92]. However, SOA formation from isoprene oxidation has been intensively explored during the last two decades and found that the overall contribution of isoprene to total atmospheric POM could be large (e.g., 6 Tg/yr) because of its large global emission flux [93]. The isoprene oxidation mechanisms identified specific SOA precursors such as isoprene epoxydiols (IEPOX) at low NO\({}_{\rm x}\) environments and methacryloyl perox-ynirrate (MPAN) at high NO\({}_{\mathrm{x}}\) environments [94]. In isoprene-dominating regions (e.g., the Amazon rainforest), a significant amount of IEPOX can be formed, and their reactive uptake on acidic sulfate aerosols may lead to a substantial production of SOA from isoprene. SOA can directly interact with incoming shortwave radiation (a direct radiative effect) and act as condensation nuclei for cloud droplet formation (an indirect radiative effect).