# Isoprene Emissions, Oxidation Chemistry and Environmental Impacts
## 2 Biological Function of Isoprene Emission
[43] showed that the dominant source of isoprene emissions is broadleaf tropical forests (84.5%), followed by temperate shrubs (10%), cool and warm grass (4%), and finally needleleaf forest in boreal regions (1.5%) (Table 1). Thus, the emitted isoprene is mostly visible in the tropical region (e.g., 56% in the southern tropics and 32% in the northern tropics), followed by temperate regions (e.g., 7% in the northern temperate and 4% in the southern temperate) with a minor contribution from the northern Arctic (<1%). \begin{table}
\begin{tabular}{l l l} \hline \hline
**Plant Species** & **Land Area (10\({}^{12}\) km\({}^{2}\))** & **Isoprene (Tg yr\({}^{-1}\))** \\ \hline Needleleaf Evergreen Temperate Tree & 5.46 & 1.61 \\ \hline Needleleaf Evergreen Boreal Tree & 10.6 & 5.9 \\ \hline Needleleaf deciduous Boreal Tree & 1.46 & 0.0002 \\ \hline Broadleaf Evergreen Tropical Tree & 15.6 & 244 \\ \hline Broadleaf Evergreen Temperate Tree & 2.64 & 21.9 \\ \hline Broadleaf deciduous Tropical Tree & 12.9 & 178 \\ \hline Broadleaf Deciduous Temperate Tree & 5.33 & 35.4 \\ \hline Broadleaf Deciduous Boreal Tree & 2.14 & 4.79 \\ \hline Broadleaf Evergreen Temperate Shrub & 0.18 & 0.23 \\ \hline Broadleaf Deciduous Temperate Shrub & 4.15 & 21.8 \\ \hline Broadleaf Deciduous Boreal Shrub & 9.33 & 2.93 \\ \hline Arctic C3 Grass & 4.94 & 0.97 \\ \hline Cool C3 Grass & 14.3 & 11.2 \\ \hline Warm C4 Grass & 13.2 & 5.93 \\ \hline Crop 1 & 16.3 & 0.02 \\ \hline Total & 118.5 & 535 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Global land area (10\({}^{12}\) km\({}^{2}\)) and isoprene emissions for individual plant functional types estimated using MEGAN2.1 algorithms for the year 2000. Adapted from [PERSON] et al. [2]. Amazonia is known to be a major source of isoprene emissions. Areas of elevated isoprene emissions can also be seen over Russia and Canada in a biome known as boreal forest. Regions of boreal forest, or Taiga, are characterized by coniferous trees situated in a highly seasonal climate, with cold seasons seeing extreme low temperatures and snow coverage. In spring, the melted snow reveals vast forests that can achieve high rates of photosynthesis and hence release a significant mass of isoprene contributing to the global budget [44]. [PERSON] et al. [45] estimated that over 70% of total emitted BVOCs originate from the tropics, and Figure 1 shows how the south tropical region dominates the spatial distribution of isoprene emissions globally [43]. Emissions of isoprene are also strongly species-dependent, leading to further spatial differences in the distribution of emissions between distinct tropical regions such as Amazonia and Southeast Asia. Numerous field and modeling studies have calculated a value for the atmospheric emission flux of isoprene and have shown remarkably consistent results. [PERSON] et al. [3] summarized several previous studies to investigate why estimates of global terrestrial isoprene emissions are so similar when the same is not true for monoterpenes (another prominent BVOC). They calculated an average annual global terrestrial isoprene emission rate of 516 TgC yr\({}^{-1}\), with a range of 460-570 TgC yr\({}^{-1}\). For perspective, this estimated flux of isoprene is roughly equal to the annual atmospheric flux of methane on a carbon mass basis [1]. Atmospheric modelling by [PERSON] et al. [46] estimated global annual isoprene emissions of 460 TgC yr\({}^{-1}\), and [PERSON] et al. [43] estimated emissions of 523 TgC yr\({}^{-1}\), both showing agreement with previous studies and the average range calculated by [PERSON] et al. [3]. The experimental data indicates that temperature and light are the key environmental factors affecting isoprene emission from plants [47]. The architecture, leaf size, and form of the existing plants all have an impact on many of these variables, which vary across the canopy. It is generally known that isoprene emission depends on light, which differentiates isoprene emissions from monoterpenes and other hydrocarbon emissions from plants that occur at night.