Forest Emissions

Forests can act as sources of some of the trace gases in the atmosphere, such as hydrocarbons, hydrogen sulfide, NO, and NH3. Forests have been identified as emitters of terpene hydrocarbons. In 1960, Went (10) estimated that hydrocarbon releases to the atmosphere were on the order of 108 tons per year. Later work by Rasmussen (11) suggested that the release of terpenes from forest systems is 2 x 10 tons of reactive materials per year on a global basis. This is several times the anthropogenic input. Yet, it is important to remember that forest emissions are much more widely dispersed and less concentrated than anthropogenic emissions. Table 8-2 shows terpene emissions from different types of forest systems in the United States. 

Yokouchi et al., 1983). Micrometeorological conditions also are important in controlling the concentrations of forest emissions. For example, Isidorov et al. (1985) noted that a stable atmospheric stratification in the evening caused a considerable enhancement of terpene concentrations. 

The measurements refer to nine 600-liter canister samples, three collected south and six north of the ITCZ, analyzed according to the method of C.A.M. Brenninkmeijer [C.A.M. Brenninkmeijer, Journal of Geophysical Research, 98 10595 (1993) C.A.M. Brenninkmeijer et al., Chemosphere Global Change Science, 1 33 (1999)]. CO can result from biomass burning and die oxidation of natural hydrocarbons. Forest emissions of hydrocar-bons, however, are small, whereas hydrocarbon oxidation is minimal during winter. 

Since SO2 and NO2 are criteria pollutants, their emissions are regulated. In addition, for the purposes of abating acid deposition in the United States, the 1990 Clean Air Act Amendments require that nationwide SO2 and NO emissions be reduced by approximately 10 million and 2 million t/yr, respectively, by the year 2000. Reasons for these reductions are based on concerns which include acidification of lakes and streams, acidification of poorly buffered soils, and acid damage to materials. An additional major concern is that acid deposition is contributing to the die-back of forests at high elevations in the eastern United States and in Europe. 

Forest systems also act as sources of CO2 when controlled or uncontrolled burning and decay of litter occur. In addition, release of ethylene occurs during the flowering of various species. One additional form of emission to the atmosphere is the release of pollen grains. Pollen is essential to the reproductive cycle of most forest systems but becomes a human health hazard for individuals susceptible to hay fever. The contribution of sulfur from forests in the form of dimethyl sulfide is considered to be about 10-25% of the total amount released by soils and vegetation (12). 

Composition of U.S. Forest-Type Groups by Foliar Terpene Emissions... 

The third category for interactions is high dose (III). The effects produced by this level of interaction can be seen by the casual observer. The result of high-dose exposure is destruction or severe injury of the forest system. High-dose conditions are almost always associated with point source emissions. The pollutants most often involved are SO2 and hydrogen fluoride. Historically, the most harmful sources of pollution for surrounding forest ecosystems have been smelters and aluminum reduction plants. 

Acid deposition occurs when sulfur dioxide and nitrogen oxide emissions are transformed in the atmosphere and return to the earth in rain, fog or snow. Approximately 20 million tons of SOj are emitted annually in the United States, mostly from the burning of fossil fuels by electric utilities. Acid rain damages lakes, harms forests and buildings, contributes to reduced visibility, and is suspected of damaging health. 

Carlson, F.E., Phillips, E.K., Tenhaeff, S.C. and Detlefsen, W.D., Measuring and Controlling Volatile Organic Compound and Paniculate Emissions from Wood Processing Operations and Wood-Based Products. Forest Products Society, Madison WI, 1995, pp. 52-61. 

Some naturally occurring organohalogen compounds are produced in massive quantities. Forest fires, volcanoes, and marine kelp release up to 5 million tons of CH3CI per year, for example, while annual industrial emissions... 

Silicate dust Marine High temperature processes volcanic, rock and plant emissions, forest fires Anthropogenic... 

National initiatives in North America and Europe are designed to reduce pollution emissions from both stationary and mobile sources. Independently of whether they succeed in reducing pollutant loadings, the available evidence indicates that alterations in affected forests will continue. Obviously, no one knows what affected forests will be like in 50 years. There is little doubt that they will be different. And the sooner the anthropogenic causal factors - all of them - are reduced qualitatively and quantitatively, the better are the chances of retaining or regenerating forests that will have meaning and value for those who will want to use them. 

Tropical forests and savannas are the primary source of C emissions that originate from biomass burning (73, 75). However, temperate forests are also sources of atmospheric carbon. Harmon et al. (77) reported that conversion of primary temperate forests to younger, second-growth forests lead to increases in atmospheric CO2 levels, due to losses in long-term carbon storage within these forests. They ascertained that timber exploitation of 5 million hectares of primaiy forests in the Pacific Northwest of North America during the past century has resulted in the addition of 1,500 Tg of C to the atmosphere. 

Carbon monoxide emissions from the terrestrial biosphere are small, but forest fires produce 0.02 Pg C/yr. Degradation of chlorophyll is dying plant material seems to be the largest CO-producing mechanism at 0.04-0.2 Pg C/yr (Freyer, 1979). 

Acid precipitation, or acid rain, can causes significant impacts on freshwater, coastal, and forested ecosystems (e.g.. Likens et ai, 1996). Both NOi", from NO emissions, and SO from SO2 emissions contribute significantly to acid rain. The relative ratio of SO /NOf in precipitation will be substantially determined by the regional emissions of SO2/NO3. In developed countries, uncontrolled combustion of coal and high-sulfur fuel oil led to significant emissions of SO2, relative to NO Due to strict control of smokestack SO2 emissions in some regions and increasing NO emissions from automobiles, the relative contribution of NOi is expected to increase (Sirois, 1993 Mayewski et ai, 1990). 

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