One aspect of prescribed burning that people often overlook is smoke management. Smoke impacts not only the burn crew during a burn, but public safety, health and perception of prescribed burning as well. Even though weather conditions might be ideal for safe ignition, consideration must also be given to smoke produced by burning vegetation during and after a burn.
Smoke is produced when there is incomplete combustion due to a lack of oxygen to completely burn a fuel (vegetation). Carbon dioxide and water vapor are the largest components of smoke, comprising 90 percent of emissions. The remaining smoke is comprised of hydrocarbons, particulate matter and other compounds. Ways to decrease the amount of smoke produced are to burn smaller areas, utilize more backfires and burn when fuel moistures are relatively low. Burning smaller areas requires more burns, but reduces the amount of smoke each burn produces. Backfires are more efficient at consuming fuels, so they produce less smoke compared to a head fire. Since a large percentage of smoke is water vapor, burning with lower fine fuel moistures also decreases the amount of smoke produced.
Sensitive areas such as airports, roads, towns, hospitals, schools, nursing homes, private residences and public parks that are downwind need to be considered. It is the responsibility of the burn boss to keep smoke away from such areas by ensuring that suitable wind directions and conditions will lift smoke above the sensitive areas. Oklahoma's Prescribed Burning Notification Plan helps landowners ensure that neighbors and authorities are notified in order to mitigate potential health problems or hazards. When burning in other states, it is also a good idea to notify all neighbors and authorities of the planned burn.
Atmospheric dispersion and inversion are two environmental factors that influence smoke behavior and need to be monitored when planning a prescribed burn. OK-FIRE defines atmospheric dispersion as "the ability of the atmosphere to dilute and disperse a compound such as smoke as it travels downwind." The Oklahoma Dispersion Model breaks down atmospheric dispersion into six categories: 1 (very poor), 2 (poor), 3 (moderately poor), 4 (moderately good), 5 (good) and 5 (excellent).
It is best to burn when atmospheric dispersion conditions are 4 (moderately good) or higher. An inversion is when there is stable, warm air at higher altitudes, reducing atmospheric dispersion. An inversion can quickly be identified when it looks like smoke has hit a ceiling in the sky. Inversions are common at night, complicating smoke management. The distance from the ground to an inversion layer is called the mixing height. It is best to burn with a mixing height of at least 1,500 feet and even higher when smoke-sensitive areas exist downwind.
The Category Day system is another method used to predict smoke behavior. It is determined by the ventilation rate which takes into account mixing height and transport wind speed. The Category Day system has five categories. They are 1 (poor), 2 (fair), 3 (good), 4 (very good) and 5 (excellent). Avoid burning with a Category Day less than 3 (good). Two reference websites that provide this information are okfire.mesonet.org and radar.srh.noaa.gov/fire.
Air quality is a hot topic. As prescribed burners, we need to make sure we know where our smoke is going and lessen its impacts so we can continue to use prescribed burning as an effective land management tool.