Smoke Management for Controlled Burns

What are the components of smoke from wildland burning?

All wildland fires produce smoke plumes that contain embers, particulates, nitrous oxides, and volatile organic compounds. In grassland prescribed fires, embers generally drop out rapidly, frequently within the burned area. Particulates and gasses, however, can cause air quality problems downwind if not managed.

Particulates are tiny bits of solids or liquids, and are designated as PM10 or PM2.5, which refers to the diameter of the particle as measured by its ability to pass through 10 microns or 2.5 microns filters, respectively. Because of their larger size, PM10 particles tend to drop out of the air more rapidly than PM2.5 particles.

Both types of particulate matter can cause breathing problems, especially for those with respiratory or circulatory diseases. PM2.5 is especially dangerous, as its small size allows it to be drawn deeply into the lungs. Both types of particulate matter are cleaned from the air by rain.

Nitrous oxides (NOx) and volatile organic compounds (VOC) are gasses released during combustion. Although they are not ozone, they combine in the presence of sunshine to form ozone. Ozone is not cleaned from the air by rainfall. It can disappear during the night, only to reform the following day. Ozone plumes hang together and are not dissipated by rain. Because of these characteristics, these plumes can travel hundreds of miles from the source.

What are the effects of smoke?

Ozone can also cause health problems for those with respiratory and cardiac diseases. Ozone and particulate matter can combine and cause an air quality problem called regional haze. This is an aesthetic as well as a health problem. Regional haze is most likely to form during low wind conditions and weather inversions.

Smoke combined with fog can create a visually impenetrable condition that is sometimes known as superfog. It tends to settle in low-lying areas such as valleys and stream corridors. Superfog is often associated with multiple fatal collisions, as motorists encounter the fog without warning and road conditions change from excellent visibility to near total darkness in a matter of seconds.

How can I reduce smoke from controlled burns?

When conducting prescribed burns, it’s important to be aware of problems that may result from the smoke you produce if you are near people who will be impacted by your smoke. If there are many prescribed fires burning at the same time in your area, the combined smoke plume from these fires can travel hundreds of miles and cause air quality problems in downwind areas. Burning on days when smoke dispersal is high will improve air quality both locally and downwind. Burning when fuels are dry will also increase combustion and reduce smoke production.

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Reduce Smoke Through Ignition Techniques

Reducing the impacts of smoke should be considered when planning a prescribed fire. Smoke management is not really about the reduction of emissions, but the redistribution of emissions, which is done by choosing wind direction, atmospheric conditions (such as mixing height), and ignition techniques to manage the convection. Perhaps less discussed are ignition techniques themselves. Following are some basic guidelines when planning ignition techniques to reduce smoke emissions. We must be aware, however, of potential tradeoffs between reduced smoke emissions and achieving ecological goals with fire.

How to influence smoke

Smoke includes a mix of chemicals including bits of the unconsumed fuel. Unconsumed fuel on the ground leads to smoldering phase burning. Smoldering produces more smoke than burning in the flaming stage of ignition. Ignition techniques that lead to more complete combustion should produce less smoke than other techniques. Here are some examples.

Parts of a fire.

1. Backing fire. Backing fires (fires that burn against the wind) consume fuels more completely than head or flanking fires; thus, backing fires generally produce less smoke. Tradeoffs include increased time to complete burns because backing fires move more slowly, and flame lengths are relatively short so may not provide sufficient heat to kill some plants.

2. Headfire. Headfires (burn with the wind) move more quickly, produce more heat, generally consume fuel less completely, and produce taller flame lengths. Incomplete fuel combustion results in greater smoke production than backfires. Although headfires may produce more smoke, they may be used to complete a fire more quickly, thereby reducing the overall burn time. Furthermore, the speed and intensity of headfires releases more heat that can help the smoke to rise in a column, facilitating dispersal at higher levels in the atmosphere. Headfires can also be more challenging to conduct safely if used exclusively.

3. Ring headfire. This technique uses a combination of back, flank, and head fires. Backfires and flanking fires are used around the perimeter to create a safe black zone to send the headfire towards. This technique is often used because of its safety and compromise in completion time. The backfire portion of the fireline may be less smoky, but generates steady amounts of heat so when the headfire begins, smoke forms a column and is dispersed aloft. The placement of the head and backfire zones can be adjusted to avoid smoke in sensitive areas and apply the right flame length/residence time combination to treat fuels in different parts of the burn unit. Ring headfires are generally not recommended when wildlife is a consideration because animals may be entrapped.

4. Additional techniques, such as chevron, single point ignition, and strip head-firing, are often used to speed the development of a safe zone around a burn unit perimeter. These techniques can also be applied to create pulses of heat to lift smoke aloft. Keep in mind that head fires typically produce more smoke through incomplete ignition and subsequent smoldering, but generate high levels of heat. Size of the strips used with these techniques can make a huge difference. Large strips may produce more smoke, but because of increased heat production, the smoke is lofted higher more quickly than with smaller strips. A quick completion of a burn can reduce the time of potential exposure to smoke though generating more smoke in the short period.

When choosing an ignition technique, consider many factors, including smoke, and prioritize them with respect to objectives and risks.

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Resources: NWCG Fire Use Working Team. 2001. Smoke management guide for prescribed and wildland fire. NFES 1279, PMS 420-2 or an older version from Feb. 1985.

NWCG. 2012. Glossary of wildland fire terminology. PMS 205.