What Do the Hydrocarbons from Trees Contribute to Air Pollution?

Plant species release appreciable quantities of volatile organic substances to the atmosphere. The major compounds emitted are monoterpenes (C10) like α-pinene, β-pinene, and limonene and the hemiterpene (C5) isoprene. The rate of emission of isoprene is light dependent and ranges between 0.04 to 2.4 ppb/cm2/min/l for oak, cottonwood, and eucalyptus foliage. The rate of emission of a- and/3-pinene and limonene is dependent on the rate of transpiration, structural integrity of the oil cells and resin glands, and temperature of the foliage. Rates of emission for conifer foliage range from 0.4 to 3.5 ppb/g/min/l. An inventory of North American forest regions for the frequency of occurrence of these chemicals released by different tree species showed that 15% was the lowest value for a specific forest-type that emitted terpenes to the atmosphere. More commonly 100% of the trees of a given forest-type emitted terpenes to the atmosphere. An average of 70% is typical of the United States forested regions as a whole. The annual contribution of forest hydrocarbon emissions to the air pollution problem on a global basis is reflected in the 175 × 106 tons of reactive hydrocarbons from tree foliage sources compared to the 27 × 106 tons from man’s activities; in other words, there is a 6.2-fold greater emission level from natural sources than from man made sources. The fate of these gaseous olefins in the atmosphere is undetermined.