As discussed briefly in Chapter 1, gaseous air pollutants have either biogenic or anthropogenic origins. The major sources over the last century have been dominated by anthropogenic fossil fuel combustion for energy and transportation applications. However, non-combustion sources are particularly important for volatile organic compounds and methane. Whilst we tend to be familiar with anthropogenic air pollutants, we largely overlook the natural sources of airborne volatile organic compounds (VOCs), such as terpenes and isoprenes from trees, and noxious gases from wildfires. The Great Smoky Mountains in the United States are so-called because of chemical emissions from trees.