# Isoprene Emissions, Oxidation Chemistry and Environmental Impacts
## 6 Measurements of Isoprene and Model Validation
holm oak Mediterranean forest site near the metropolitan area of Barcelona, and a French Mediterranean oak forest, respectively, during summer months due to the increased physiological activity and emission rates of the vegetation. Isoprene mixing ratios measured at US forests, e.g., Blodgett Forest, Nevada; Manitou Experimental Forest Observatory, Colorado; and PROPHET site, University of Michigan Biological Station, are impacted by local isoprene emissions from oak forests and woodlands, ponderosa pine, and quaking aspen, respectively [110; 111; 113]. In a recent study, [PERSON] et al. [143] used the iDirac, an autonomous portable gas chromatograph coupled with photoionization detection (GC-PID), and measured high isoprene concentrations (over 500 ppt) across the Southern Ocean during summertime, indicating the marginal ice zone is a significant source of isoprene at high latitudes. The measurements of isoprene at urban (summer and autumn), coastal suburban (summer), and inland forest (late spring) sites in southeast Australia showed 3-fold and 5-fold higher isoprene mixing ratios at urban sites compared with coastal suburban and inland forest sites, respectively [126]. Significant variations in isoprene mixing ratios across different environmental regions in China are observed, e.g., (a) mountain sites in Nanjing (summer: 0.38 ppbv) [131], Guangdong (annual: 0.76 ppbv) [134], Yunnan (spring: 0.08 ppbv), Hainan Island (spring: 0.40 ppbv), and Shandong (summer: 0.15 ppbv) [138], (b) urban sites in Shanghai (summer: 0.20 ppbv, autumn: 0.20 ppbv) [130], Shenzhen (summer: 0.75 ppbv) [132], Taipei (summer: 1.26 ppbv) [133], Tengchong (spring: 0.13 ppbv), (c) coastal sites in Xiamen (summer: 0.66 ppbv, autumn: 0.14 ppbv) [129] and Lantau Island (annual: 0.37 ppbv) [137], (d) rural site in Lin'an (spring: 0.55 ppbv) [136]. There are relatively few observational studies (Table 2), which makes it difficult to develop a completely accurate model representation of isoprene's emissions and its oxidation chemistry, especially when considering regional variability. An alternative indirect approach satellite-based constraints employ is isoprene's oxidation product formaldehyde (HCHO), and this has been used over the last decade to provide top-down constraints on regional and global isoprene emissions and assess their seasonal and inter-annual variability [144; 145; 146; 147; 148; 149; 150; 151]. However, this approach is affected due to myriad other sources of HCHO and spatial smearing in the isoprene-formaldehyde relationship. Stressed conditions, e.g., drought, can induce a large reduction in isoprene emissions, but the satellite HCHO columns did not reflect this reduction in isoprene emissions due to severe drought in the Missouri Ozarks region [152; 153]. The lack of HCHO column response can be attributed to a photochemical feedback whereby reduced isoprene emissions due to the drought increase the oxidative capacity of the atmosphere, which enhances the production of HCHO from other VOCs [153]. Furthermore, the chemical link between HCHO and isoprene is a strong nonlinear function of NO\({}_{\mathrm{x}}\), with the prompt yield increasing by a factor of 3 (from 0.3 to 0.9 ppbv ppbv\({}^{-1}\)) over a range of NO\({}_{\mathrm{x}}\) values (0.1 to 2 ppbv) and background HCHO increasing by a factor of 2 (from 1.6 to 3.3 ppbv) [154]. Inclusion of the relationship of HCHO column and isoprene emission into the chemistry transport model (CTM) produced large error of the isoprene emissions with the range of 40-90% under relatively low-NO\({}_{\mathrm{x}}\) conditions typical of the tropics [155]. In addition, the isoprene HO\({}_{\mathrm{x}}\) recycling led to a decrease in the lifetime of isoprene, resulting in an increase in the ratio [HCHO]/[isoprene] by up to 50% over the tropical forest region [82; 87]. Thus, using satellite measurements of HCHO as a proxy for isoprene emissions could lead to large uncertainties in isoprene concentration estimates, depending on the background conditions.