# Isoprene Emissions, Oxidation Chemistry and Environmental Impacts
## 2 Biological Function of Isoprene Emission
Isoprene emissions are strongly temperature sensitive. The evaporation temperature of isoprene is very low (34 \({}^{\circ}\)C), which explains the exponential growth of isoprene emissions at physiological temperatures [48]. Due to the great temperature sensitivity of isoprene synthesis, the temperature is a crucial factor affecting the rate of isoprene emission. Young leaves do not release isoprene, and isoprene emission normally  
Figure 1: Spatial distribution of mean isoprene emissions (mg m\({}^{-2}\) day\({}^{-1}\)) for January (**left**) and July (**right**) averaged over the years 1980–2010 calculated by the MEGAN model. The figure is adapted from [PERSON] et al. [43]. occurs several weeks after the start of photosynthesis at the start of the growing season. The leaves must attain complete maturity before the maximum isoprene emission rates may be reached, which in certain species may not happen until the beginning of the summer. When dealing with isoprene emissions, it is important to specify the regions, seasons, and types of plants. In addition to temperature and light, isoprene emissions are controlled by the levels of carbon dioxide (CO\({}_{2}\)). It was found from laboratory and field studies that a direct inhibition of isoprene production was caused by an increasing concentration of atmospheric CO\({}_{2}\) which can have opposite responses to surface ozone concentrations, e.g., decreases in polluted regions and increases in less polluted regions [49]. The tropical forests may have the ability to maintain a strong atmospheric oxidation capacity whilst emitting large quantities of isoprene, which may to a degree be explained by the CO\({}_{2}\) inhibition effect. Climate change had the potential to increase levels of isoprene emissions due to higher temperatures; however, increased concentrations of CO\({}_{2}\) would oppose this effect. In future climate change scenarios, including the CO\({}_{2}\) inhibition effectively cancelled out climate-induced increases in isoprene emissions, and modeled global isoprene emissions in 2100 could be very similar to present-day emissions [46]. A similar conclusion was reached in a study by [PERSON] et al. [50] who also suggested that future isoprene emissions would be, to a large degree, buffered by competing influences (CO\({}_{2}\) inhibition effect), with the projected global isoprene emissions in 2100 dropping from 696 to 479 TgC yr\({}^{-1}\). [PERSON] et al. [51] showed that the isoprene emission rates from several species of trees (eucalyptus, sweetgum, cottonwood, and aspen) were reduced when trees were exposed to a higher concentration of CO\({}_{2}\) whilst growing. They presented a new model [51] for isoprene emission accounting for its response to CO\({}_{2}\), consistent with [PERSON] and co-workers' [1] algorithms, which can be used for the effect of long-term changes in CO\({}_{2}\) levels on isoprene emission rates. However, [PERSON] et al. [52] showed that the CO\({}_{2}\) inhibition effect may not take effect in a warmer climate, as the isoprene could protect leaf photosynthetic function against heat stress more effectively under increased CO\({}_{2}\) levels. Furthermore, [PERSON] et al. [50] discussed the possibility of the CO\({}_{2}\) inhibition of isoprene emissions being counteracted by the fertilization effect of CO\({}_{2}\), which enhances photosynthesis and water use efficiency, hence also increasing vegetation productivity. Isoprene is rapidly oxidized once it is emitted into the atmosphere, leading to its breakdown and a lower overall concentration despite a high initial emission rate. Thus, an accurate representation of isoprene oxidation chemistry is of vital importance to global atmospheric models.