# Isoprene Emissions, Oxidation Chemistry and Environmental Impacts
## 5 Environmental Impacts of isoprene
Considering these direct and indirect radiative effects, it is thought that SOA produced from isoprene in the atmosphere exerts a negative radiative effect on the climate [90; 95]. Recent aircraft observations [96; 97] showed new particle formation (NPF) from extremely low volatile oxidation products of isoprene in the Amazonian upper troposphere, which can serve as cloud condensation nuclei (CCN) and influence the Earth's hydrological cycle, radiation budget, and climate. The incidence of the secondary products produced from isoprene oxidation processes in the troposphere increases the prevalence of respiratory diseases and damages the environment and infrastructure. [PERSON] et al. [98] have shown that across the United States, biogenic SOA mass was comparable with anthropogenic SOA mass and showed that biogenically derived SOA played a role in cardiorespiratory disease mortality. [PERSON] et al. [46] highlighted the importance of considering isoprene emissions and their impact on atmospheric chemistry and climate due to the complexity of competing influences on both emissions and the degradation mechanism of isoprene. Large emissions of isoprene, coupled with its short atmospheric lifetime, leads to difficulties in global atmospheric modelling, due to a corresponding reduction in the levels of OH over isoprene source regions. The model simulation of the initial isoprene HO\({}_{\mathrm{x}}\) recycling mechanism [53; 63; 66] incorporated into the global tropospheric chemistry-transport model, IMAGESv2 [99], showed an increase of OH and HO\({}_{2}\) concentration by a factor of 4 and 3, respectively, over the Amazon basin and an ~8% increase of ozone over tropical forests. [PERSON] et al. [100] showed the impacts of initial isoprene HO\({}_{\mathrm{x}}\) recycling for present-day, pre-industrial and future climate/emission scenarios using UKCA global chemistry-climate model. Their calculations showed that the inclusion of unimolecular isomerizations of the isoprene hydroxy-peroxy radicals increased the production of HO\({}_{\mathrm{x}}\) radicals in all scenarios. For the present-day climate scenario, they showed an enhanced OH and HO\({}_{2}\) concentration by as much as 410% and 225%, respectively, at the surface in tropical regions and up to 75% OH in the zonal within the boundary layer of the tropics, which could be important to reproduce the high concentrations of HO\({}_{\mathrm{x}}\) measured in the tropics by [PERSON] et al. [59]. The inclusion of HO\({}_{\mathrm{x}}\) regeneration and recycling in the oxidation of isoprene in their model increased the global burden of ozone by 25 Tg (8%), 36 Tg (18%), and 32 Tg (9%) and decreased the atmospheric lifetime of methane by 14%, 35%, and 11% for present, pre-industrial, and future climate/emission scenarios, respectively. The implementation of CRI-updated isoprene degradation chemistry into the STOCHEM-CRI chemistry transport model [79; 101; 102; 103] showed an increment of the global tropospheric burdens of OH and HO\({}_{2}\) by 6.4% and 0.8%, respectively. The increase in HO\({}_{\mathrm{x}}\) did in turn reduce isoprene's tropospheric burden by ~17% and shorten its tropospheric lifetime from 2.7 days to 2.2 days [87]. The peak isoprene is found over the Amazon rainforest, southern Africa, Southeast Asia, and North Australia with levels between 1 ppb to 4 ppb as a 24 h average (Figure 5a,b). There are also substantial emissions leading to levels up to 1 ppb over North America and Russia, and because of more sunlight during June-July-August (Figure 5b), leading to greater biological production in the northern hemisphere. The zonal distribution of isoprene shows the peak at the surface in the tropical region with levels going up to 0.35 ppb during both June-July-August and December-January-February seasons (Figure 5c,d). The peak at the surface in the boreal forested regions are seen with levels up to 0.2 ppb during summer (June-July-August) season. The concentrations of isoprene decrease significantly with altitude because of its short atmospheric lifetime.