Glossary

aerosol  A mist of very fine particles, either liquid or solid, suspended in air or another gas. Volatile organic compounds can contribute to the formation of aerosols.

biogenic  Produced by living things; in other words, produced by organisms.

compound  A chemical substance formed from two or more elements in specific proportions. The elements lose their original chemical properties; the compound has new properties.

concentration  The amount of a substance that is in a certain volume.

emissions  The release of a substance into the environment as the result of a process. Emissions usually refer to the release of gases into the atmosphere as a byproduct of combustion.

flux  The amount of a substance that moves through a unit of area per unit of time.

gas chromatograph An instrument that separates components and measures the relative concentration of each in a mixture of gases.

hydrocarbon  An organic compound consisting mostly of hydrogen and carbon.

isoprene  A hydrocarbon formed naturally in plants and animals. Isoprene can be an important precursor of ozone, and units of isoprene make up many biogenic volatile organic compounds.

model  A tool, usually mathematically-based, used to represent a complex or large-scale process in simpler terms. Models are typically used to understand or predict the behavior of a system that cannot be directly observed in its entirety.

monoterpene  One of several types of terpenes made up of two isoprene units. It has the molecular formula C10H16.

nursery  A place where plants are germinated and grown for planting elsewhere.

organic  Of, derived from, or about living organisms. Organic chemistry, specifically, studies the structures, synthesis, and reactions of compounds that contain carbon, the building block of life on Earth.

oxidizing  A chemical process in which molecules lose electrons and therefore change their composition. For example, rust is an oxidation of iron.

ozone  A form of oxygen that has three atoms, rather than the more common oxygen molecule’s two atoms.  Ozone is produced by the energy in sunlight acting on nitrogen oxide molecules and hydrocarbons in the air, which can be released by both industrial and automotive processes and from natural sources.

pollutant  A substance introduced into an environment that changes the environment’s physical, chemical, or biological properties in a negative way. Pollutants can be liquid, solid, or gas and may be found in soils, air, water, and other environments.

sesquiterpene    One of several types of terpenes made up of three isoprene units. It has the molecular formula C15H24.

shrub  A bush; a low, spreading, woody, perennial plant. A shrub is generally shorter than a tree and has many branching stems instead of a central trunk.

species  A group of plants, animals, or other living creatures that have common characteristics and are capable of interbreeding.

terpene  A type of hydrocarbon made up of two units of isoprene, mainly produced by plants, especially conifers like pine trees.

tree  A large, woody, perennial plant. Trees are generally taller than shrubs and have a single main stem or trunk that supports secondary branches. They are generally long-lived compared to other plants.

urban forest  The total collection of trees and associated plants growing in a city or town. It includes trees in parks and yards, along roadways and paths, and in other areas.

volatile  Evaporating easily at room temperature.

volatile organic compound  An organic compound that readily evaporates into the air, and that reacts with other chemicals in the presence of sunlight. VOCs contribute to ozone production.

Willow catkins emerge in spring. Photo: Andre Karwath, obtained from WikiMedia Commons under the Creative Commons Attribution ShareAlike 2.5 License.

A blooming forsythia bush is an example of a shrub. Photo: Dan Smith, obtained from WikiMedia Commons under the terms of the GNU Free Documentation License.

Autumn leaves. Photo: R. Neil Marshman, obtained from WikiMedia Commons under the terms of the GNU Free Documentation License.