Naturally occurring hydrocarbons, emitted by many trees and plants. They mostly have very strong smells and are responsible for the aromas of the vegetation in which they are found. Terpenes can be thought of as being built from units of isoprene, C₅H₈, joined together into chains and rings. Monoterpenes, formula C₁₀H₁₆, constitute the major emissions from conifers and fruit trees. Sesquiterpenes, formula C₁₅H₂₄, are commonly found in citrus trees. Terpenes are very reactive and may contribute to diminution of air quality in forested areas. The name comes from turpentine, a liquid consisting of several terpene compounds distilled from the resin of pine trees.