Components of Smoke

The chemistry of the fuel as well as the efficiency of combustion governs the physical and chemical properties of the resulting smoke from fire. Although smoke from different sources may look similar to the eye, it is often quite different in terms of its chemical and physical properties. Generally, the emissions we cannot see are gas emissions and the emissions we can see are particulate emissions.

Carbon dioxide and water

Two products of complete combustion during fires are carbon dioxide (CO₂) and water (H₂O) and generally make up over 90 percent of the total emissions from wildland fire. Under ideal conditions complete combustion of one ton of forest fuels requires 3.5 tons of air and yields 1.84 tons of CO₂ and 0.54 tons of water (Prescribed Fire Effects Working Team 1985). Under wildland conditions, however, inefficient combustion produces different yields. Neither carbon dioxide nor water vapor are considered air pollutants in the usual sense, even though carbon dioxide is considered a greenhouse gas and the water vapor will sometimes condense into liquid droplets and form a visible white smoke near the fire. This fog/smoke mixture can dramatically reduce visibility and create hazardous driving conditions. As combustion efficiency decreases, less carbon is converted to CO₂ and more carbon is available to form other combustion products such as carbon monoxide (CO), hydrocarbons (HC), nitrogen oxides (NO_x), and sulfur oxides (SO_x), all of which are considered pollutants.

Carbon Monoxide

Carbon monoxide (CO) is the most abundant emission product from wildland fires. Its negative effect on human health depends on the duration of exposure, CO concentration, and level of physical activity during the exposure. Generally, dilution occurs rapidly enough from the source of the fire that carbon monoxide will not be a problem for local citizens unless a large fire occurs and inversion conditions trap the carbon monoxide near rural communities. Carbon monoxide is always a concern for wildland firefighters however, both on the fire line at prescribed fires and wildfires, and at fire camps (Reinhardt and Ottmar 2000, Reinhardt and others 2000).

Hydrocarbons

Hydrocarbons (HC) are an extremely diverse class of compounds containing hydrogen, carbon and sometimes oxygen. Usually, the classes of hydrocarbon compounds are identified according to the number of carbon atoms per molecule. Emission inventories often lump all gaseous hydrocarbons together. Although a majority of the HC pollutants may have no harmful effects, there are a few that are toxic. More research is needed to characterize hydrocarbon production from fires.

Nitrogen Oxides

In wildland fires, small amounts of nitrogen oxides (NO_x) are produced, primarily from oxidation of the nitrogen contained in the fuel. Thus the highest emissions of NO_x occur from fuels burning with a high nitrogen content. Most fuels contain less than 1 percent nitrogen. Of that about 20 percent is converted to NO_x when burned. Hydrocarbons and possibly nitrogen oxides from large wildland fires contribute to increased ozone formation under certain conditions.

Particulate Matter

Particulate matter produced from wildland fires limits visibility, absorbs harmful gases, and aggravates respiratory conditions in susceptible individuals. Over 90 percent of the mass of particulate matter produced by wildland is less than 10 microns in diameter and over 80-90 percent is less than 2.5 microns in diameter. These small particles are inhalable and respirable. Respirable suspended particulate matter is that proportion of the total particulate matter that, because of its small size has an especially long residence time in the atmosphere and penetrates deeply into the lungs. Small smoke particles also scatter visible light and thus reduce visibility.