Air Quality Science

Clean Sierra skies

The following discussion is excerpted from the EPA's guidebook "AP 42", Compilation of Air Pollutant Emission Factors (1996).

The complete combustion of wildland fuels (forests, grasslands, wetlands) require a heat flux (temperature gradient), adequate oxygen supply, and sufficient burning time. The size and quantity of wildland fuels, meteorological conditions, and topographic features interact to modify the burning behavior as the fire spreads, and the wildfire will attain different degrees of combustion efficiency during its lifetime.

Smoke from prescribed fires is a complex mixture of carbon, tars, liquids, and different gases.
This open combustion source produces particles of widely ranging size, depending to some extent on the rate of energy release of the fire.

Burning methods differ with fire objectives and with fuel and weather conditions. For example, the various ignition techniques used to burn under standing trees include: (1) heading fire, a
line of fire that runs with the wind; (2) backing fire, a line of fire that moves into the wind; (3) spot
fires, which burn from a number of fires ignited along a line or in a pattern; and (4) flank fire, a line
of fire that is lit into the wind, to spread laterally to the direction of the wind. Methods of igniting the
fires depend on forest management objectives and the size of the area. Often, on areas of 50 or more
acres, helicopters with aerial ignition devices are used to light broadcast burns. Broadcast fires may
involve many lines of fire in a pattern that allows the strips of fire to burn together over a sizeable

In discussing prescribed burning, the combustion process is divided into preheating, flaming,
glowing, and smoldering phases. The different phases of combustion greatly affect the amount of
emissions produced. The preheating phase seldom releases significant quantities of material to the atmosphere. Glowing combustion is usually associated with burning of large concentrations of woody fuels such as logging residue piles. The smoldering combustion phase is a very inefficient and incomplete combustion process that emits pollutants at a much higher ratio to the quantity of fuel
consumed than does the flaming combustion of similar materials.

The amount of fuel consumed depends on the moisture content of the fuel. For most fuel
types, consumption during the smoldering phase is greatest when the fuel is driest. When lower layers of the fuel are moist, the fire usually is extinguished rapidly.

The major pollutants from wildland burning are particulate, carbon monoxide, and volatile
organics. Nitrogen oxides are emitted at rates of from 1 to 4 g/kg burned, depending on combustion
temperatures. Emissions of sulfur oxides are negligible.

Particulate emissions depend on the mix of combustion phase, the rate of energy release, and
the type of fuel consumed. All of these elements must be considered in selecting the appropriate
emission factor for a given fire and fuel situation. In some cases, models developed by the U. S.
Forest Service have been used to predict particulate emission factors and source strength. These
models address fire behavior, fuel chemistry, and ignition technique, and they predict the mix of
combustion products. There is insufficient knowledge at this time to describe the effect of fuel
chemistry on emissions.

More information can be downloaded under Resources, in the column at right, and on the EPA's website, at