Volatile organic compounds (VOCs) are substances consisting primarily of a carbon backbone, with most or all of the remaining bonds being occupied by hydrogen atoms. Organic compounds may also contain atoms such as sulfur, oxygen, nitrogen, or chlorine (Finlayson-Pitts, 2000). VOCs are volatile in that they evaporate under the conditions typically found at Earth's surface. As a result, VOCs generally include compounds with 12-14 or fewer carbon atoms. Compounds that evaporate more slowly are often referred to as semivolatile organic compounds, or SVOCs, and typically include 10-18 carbon atoms, with or without oxygen or other atoms.
Carbon dioxide (CO2) does not fit the definition of a VOC, because it is not organic; methane, by contrast, does. However, because methane is more abundant than almost all other VOCs (by a factor of ten or more), is removed from the atmosphere more slowly than other VOCs (by a factor of fifty or more), and is itself an important greenhouse gas (GHG), methane is generally not included in the category of VOCs. The term VOC is nevertheless more commonly used than is the more precise term "nonmethane organic compounds," or NMOCs. The bulk of VOCs that are released into the atmosphere are members of the subclass hydrocarbons and thus are frequently termed nonmethane hydrocarbons, or NMHCs.
VOCs are, like methane, very strong absorbers of infrared light and have the potential to contribute to greenhouse warming. They have short lifetimes, however, ranging from minutes to months, and thus have low concentrations except very close to their sources. As a result, their direct contribution to climate change is small. Their indirect contributions are significant, however. Ozone, the third ranking GHG, is not released into the atmosphere but is rather formed in the atmosphere from reactions of VOCs, oxides of nitrogen, and sunlight.
VOCs also affect the Earth's climate by forming secondary organic aerosols (SOAs). The formation of these aerosols is not understood well enough to determine whether the burden of SOAs has changed appreciably as a result of human activities. Because the organic aerosols are so prevalent, they make a large contribution to the Earth's energy balance and are thus critical to accurate predictions of changes in the Earth's climate.
VOCs generally enter the atmosphere as compounds containing carbon and hydrogen and are progressively oxidized, so they contain increasing amounts of oxygen and nitrogen, forming ozone as a by-product. As the reacting VOCs collect more and more oxygen and nitrogen, they become less and less volatile and eventually enter the condensed phase. The resulting tiny aerosol particles are clear and act to cool the Earth system by scattering and reflecting incoming sunlight.
About 1.3 quadrillion grams of VOCs are released annually by natural sources and human activities combined. Of this amount, about 1.2 quadrillion grams are natural in origin, and about 100 trillion grams are anthropogenic. In the temperate, developed nations, the contribution of human activities to VOC levels is larger; for example, in the United States, anthropogenic VOC emissions are roughly equal to those from natural sources: Each class of sources produces about 15 to 20 trillion grams per year (Atkinson, 2003).
At least 95 percent of the natural global VOC burden arises from vegetation, primarily trees. Deciduous trees release more VOCs than do evergreen trees, the former releasing roughly 1 percent of the CO2 fixed in photosynthesis as VOCs. These natural VOCs are dominated by species with structures common in plant biosynthetic pathways. Vegetation releases isoprene (2-methyl-1,3-butadiene); its C10H16 dimers, known as terpenes; and C15H24 trimers, known as sesqueterpenes; and a wide variety of closely related compounds, some with alcohol and carbonyl groups attached. Deciduous trees preferentially emit isoprene, and evergreen trees preferentially emit terpenes. Many of the terpenes and their derivatives are easily recognized by their characteristic odors of pine, turpentine, lemon, orange blossom, and many other plant-derived scents.
In the Earth's atmosphere, VOCs react primarily with the hydroxyl radical (OH). They react repeatedly until they are removed from the atmosphere-- whether by colliding with a surface such as a leaf or an aerosol particle, by being intercepted and removed by rain drops, or by being completely oxidized to become CO2 (oxidized VOCs are a negligible source of CO2). The speed at which molecules react determines both their concentration in the atmosphere and their ability to generate ozone and SOAs. The quantity of ozone formed depends on the concentration of its VOC and NOx precursors, but after being released VOCs are constantly diluted in the atmosphere, so species that react more rapidly can generate higher quantities of ozone. Each VOC has a different ability to generate ozone and SOA; however, several generalizations can be made. Alkanes, molecules consisting only of carbon-carbon single bonds, react relatively slowly and have lifetimes in the atmosphere of several hours to several days. The bulk of the anthropogenic VOCs are alkanes; examples include ethane, isobutane, and n-octane.
Alkenes are molecules with one or more double bonds. Double bonds render molecules much more reactive, and alkenes have lifetimes from minutes to hours. Most of the VOCs released from natural sources (primarily trees) are alkenes, including isoprene and the terpenes. A small fraction of anthropogenic VOCs have double bonds (generally less than 10 percent). Reducing the content of alkenes in motor vehicle fuel is a proven strategy for reducing ozone in urban areas. A very small fraction of VOCs emitted into the atmosphere have triple bonds, and these species react very slowly and are of little concern.
A fourth chemical class of VOCs is aromatics; species with conjugated double bonds arranged in one or more rings. Aromatics are almost entirely anthropogenic and have intermediate reactivities. SOA formation is highest from alkenes, followed by aromatics and then alkanes. A substantial fraction of VOCs from both natural and human sources contain one or more oxygen atoms, including aldehydes, ketones, alcohols, and organic acids. More of these oxygenates form as the VOCs are oxidized as they react in the atmosphere. Aldehydes react rapidly and speed ozone formation, while alcohols and acids frequently form SOA after undergoing one or two reactions.
1. Atkinson, R., and J. Arey. "Gas-Phase Tropospheric Chemistry of Biogenic Volatile Organic Compounds: A Review." Atmospheric Environment 37, suppl. 2 (2003): S197-S219.
2. Finlayson-Pitts, Barbara J., and James N. Pitts, Jr. Chemistry of the Upper and Lower Atmosphere: Theory, Experiments, and Applications. San Diego, Calif.: Academic Press, 2000.
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