Effects of Fire on Air Quality

Wildfires and prescribed burns have potentially negative effects on air quality. All forest fires emit carbon monoxide, sulfur dioxide, nitrogen dioxide, ozone, and particulate matter. Carbon monoxide is a poisonous gas but usually only reaches toxic levels above and adjacent to prescribed fires (McMahon and Ryan 1976). Carbon monoxide can be emitted in large amounts (approximately 140 lb/ton of fuel) during forest fires (EPA 1988).

Nitrogen oxides are released at temperatures >1,500^o C. Due to this high threshold temperature, nitrogen oxides are released in significant quantities only during the most severe fires (McMahon and Ryan 1976). Nitrogen gases may be volatilized at lower temperatures, but temperatures of 500^o C, are required to volatilize all of the nitrogen (Dunn and DeBano 1977). Nitrogen and volatile organic compounds are emitted at approximate rates of 4 and 24 lb/ton of fuel, respectively (EPA 1988). Although these amounts are insignificant in comparison with other natural and man-caused sources of these pollutants, both of these pollutants are precursors to the formation of ground-level ozone and may become significant where ozone problems already exist. Therefore, emissions of both nitrogen oxides and volatile organic compounds from open burning have been targeted for reduction in state plans to achieve the ozone standard in areas where ozone is persistently high (Georgia Environmental Protection Division 1994, SAMAB 1996).

Hydrocarbons, such as methane and ethylene, are compounds that contain hydrogen, carbon, and their oxygenated derivatives (Hall 1972). Hydrocarbons can have several deleterious effects. For example, some hydrocarbons are known to be carcinogenic to laboratory animals. Also, unsaturated hydrocarbons may form photochemical smog in the presence of sunlight and oxygen-donating compounds because of their high affinity for oxygen. However, there is little evidence to show that prescribed fire is increasing these compounds in the environment to dangerous levels (Van Lear and Waldrop 1988).

Sulfur dioxide emissions from prescribed fires are of minor importance because concentrations in most forest fuels are less than 0.2 percent. Sites with "peat" and "muck soil" may be exceptions (USDA Forest Service 1976, Van Lear and Waldrop 1988).

Particulates are one of the most important types of air pollution from forest fires and are of special concern to the prescribed burner because they reduce visibility (Dieterich 1971, Hall 1972, Sandberg and others 1978). Particulates are mixtures of soot, tars, and volatile organic substances, either solid or liquid; they average about 0.1 micron in diameter (McMahon and others 1976). Larger particles of ash and unburned fuel are carried aloft, but they usually settle to the ground within a distance of 1/2 to 1 mile of the burn. Approximately 80 percent of the particulate matter mass carried aloft from forest fires is in particles less than 1 micron in size; smoke particles larger than 10 microns are rare (USDA Forest Service 1976). Current levels of forest fire smoke contribute an estimated 1.5 percent to overall fine particle mass on an annual basis. The average emission rate for particulate matter is estimated at 17 lb/ton of fuel consumed. Since forest fires consume an average of approximately 9 tons/acre, such fires could yield as much as 153 lb/acre of particulate matter (EPA 1988, SAMAB 1996).

Particulate-matter emissions from forest fires vary widely depending on the type and amount of accumulated fuel, weather, fuel moisture, and the fires rate of spread. Rate of smoke dispersal depends mainly on atmospheric stability and windspeed. With low windspeed and high humidity, particulates serve as condensation nuclei and result in dense smoke or combinations of smoke and fog. Smoke often accumulates in depressions or along stream channels, roads, and other low-lying areas (Van Lear and Waldrop 1988).

Because of the potentially serious nature of the effects of fire on air quality, smoke management guidelines have been developed by the Forest Service to reduce the atmospheric impacts of prescribed fire (USDA Forest Service 1976). Due in part to smoke management, only a small proportion (<4 percent) of the total national emissions of particulates, carbon monoxide, and hydrocarbons are attributed to prescribed burning (Van Lear and Waldrop 1988).

However, land managers must be aware of the current standards of air quality because emissions from prescribed fires could contribute to violations of several NAAQS (National Ambient Air Quality Standards) for regulated substances. Although there is a potential to violate the 24-hour air quality standard within 1 mile of a prescribed fire, currently in the southern Appalachians, a small or moderate increase in prescribed fires should not cause a problem with the annual NAAQS for particulate matter. However, the EPAs ongoing review of NAAQS may result in regulating particulate matter to a level well below the current 24-hour standard of 150 g/m³. If tighter particulate matter standards are implemented, prescribed fires may cause a violation of air-quality standards because the prescribed fire emissions of particulate matter are predominately less than 1 micron in size. Therefore, forest managers will need to be cautious if either the size or number of acres burned in prescribed fires is increased (SAMAB 1996).