SMOKE MANAGEMENT IN PRESCRIBED BURNING
Robert S. Stanton
Robert Stanton is the owner of Prescribed Fire Consulting (PFC). PFC provides services in the area of ecological management burning including crew and burn supervisor training.
Public concerns about smoke released from prescribed fires has led to the development of a wide range of strategies. While many of these strategies are accepted practice in other parts of the country; they are not currently in wide use in northeastern Illinois.
Woodland smoke is comprised of a variety of byproducts of burning. Carbon dioxide and water vapor make up over 90 % of the total emissions from prescribed fires. Other byproducts include particulates of various sizes, carbon monoxide, hydrocarbons such as acrogens and formaldehyde, nitrogen oxides, and. sulfur oxides (NPS 1992). Particulates make up most of the visible component of woodland smoke.
There are two smoke producing phases of burning. Active burning is characterized by open flames and relatively efficient burning. During the smoldering phase, the rate of combustion is such that it can not support an open flame. Emissions from a smoldering fire are at least twice that for flaming fire (NWCG 1985).
Atmospheric stability is a measure of the tendency for vertical mixing to take place in the atmosphere. An unstable air mass will tend to rise, stable air will not.
The stability of an air mass can be quantified through measuring air temperature at various altitudes. By comparing the temperature change over change in altitude to the normal change in dry air over a given change in altitude (adiabatic lapse rate) atmospheric stability can be measured. An inversion is a condition where temperature will rise with an increase in altitude. This condition prohibits any vertical mixing of smoke.
With a stable air mass or inversion, smoke will rise to a height which can be predicted. and the smoke plume will flatten out and be carried down wind. The altitude the smoke plume will rise to is known as the mixing height. The mixing height can range from 15 m (50 ft.) to above 3,660 m (12,000 ft). Once the smoke plume has risen to the mixing height it will be carried down wind by the winds aloft at that altitude (transport winds). Taking mixing height and multiplying it by transport winds will yield a value known as ventilation rate.
A basic concept in smoke management is not to release more smoke at any one time than can be carried away. Some smoke immediately adjacent to a burn unit can be dealt with. Yet, when this basic concept is ignored, major smoke impacts to 12-15 km2 (5-6 square mile) areas can result from relatively small 6-8 ha (15-20 A.) fires. The SASEM (Simplified Approach to Smoke Emissions Modeling) and other computer programs are designed to model smoke releases and compute down wind particulate concentrations. The downwind particulate concentrations are evaluated in comparison the National Ambient Air Quality Standards mandated in the Federal Clean Air Act. Use of SASEM predictions are required in several states.
In most northeastern Illinois situations, a 1-hr time lag fuel moisture of below 17% is needed to avoid major smoke impacts. Live fuel moisture is also a factor. Often fire objectives such as late spring burning for exotic species control will contraindicate burning with low live fuel moisture conditions.
A backing fire (fire burning into the wind) will produce considerably (as much as 50%) less smoke per unit of fuel burned than a head fire moving with the wind ( NWCG 1985; NWCG 1989) This is because the backing fire with its slower rate of spread and longer residence time consumes a higher percentage of the fuel in the active flaming phase of combustion.
Closely spaced lines of flanking fire or concentric center firing patterns can be used to attempt to create convective columns to carry smoke away. Rarely are these techniques effective. If created, the convective column win carry large quantities of smoke aloft during the active burning phase. Yet after the flaming front has passed a large area of smoldering fuel will remain. This smoldering material will not generate enough heat to maintain a convective column to carry it away.
During marginal smoke dispersal conditions or with smoke sensitive areas the most effective ignition pattern is a strip head fire. The width and timing of the strips is regulated by information from down wind smoke monitors. With this procedure you regulate the rate of smoke production to keep it below the ventilation rate. Creating "puffs' of smoke and allowing them to disperse prior to additional ignition is used to keep downwind smoke density within acceptable concentrations.
There are a variety of screening procedures available. (NWCG 1985; NWCG 1989; NPS 1992; USDA 1976)They all start with identifying critical smoke management targets within a specified range of the prescribed fire. This range is based on & fuel type, fuel loading, and size of the prescribed fire. Screening for 16-32 km (10-20) miles is recommended for certain fuel types and slash pile burning. In northeastern Illinois a 1,000 ft range has worked well.
The first, step is to identify all areas or targets that will not tolerate incursions of smoke. Examples are roads, airports, schools, high tension lines, or residences of smoke sensitive asthmatics. Much of this information will have already been prepared for the Illinois Environmental Protection Agency Open Burning permit application.
Once identified the targets should be classified as able to be mitigated or not. A school adjacent to the burn might be mitigated by only burning when the building is unoccupied; a small country road could be mitigated by arranging for a temporary closure. Some targets, such as O'Hare International Airport or an interstate highway, can not tolerate any smoke incursions and can not be mitigated.
After plotting targets, a range of wind directions that will blow smoke away from all targets may be available. A 30 degree spread vector should be applied to the wind direction to calculate smoke impact areas. If it also happens to be a wind direction conceivably available during bum season (i.e., not due east) you are finished. In metropolitan areas there often will not be any wind directions with no impacts to critical targets. For some sites road closures or other extraordinary measures may be needed.
Acceptable visibility distances are based on the speed limit and design of the road. They are derived from a formula that accounts for reaction time and stopping distance.
Speed Limit (MPH
|Acceptable Visibility (ft)|
Acceptable visibility should be doubled when the road is not divided due to the possibility of head on collisions (NPS 1992). A smoke monitor should monitor visibility in relation to objects of a known distance apart. Telephone poles work well.
Current research (USDA 1991) is centering on carbon monoxide (CO) and particulate inhalation concerns. The National Institute of Occupation Safety and Health and OSHA mandate maximum allowable 1 and 8 hour exposure levels for CO. Several agencies (Will County Forest Preserve, Illinois Department of Conservation, National Park Service) have done dosimeter sampling of crew member CO exposure on prescribed fires. They have found some exposures to be above allowable levels. Using Class A foam or wet line control techniques and rotating crew assignments will decrease individual exposure.
Small (under 2.5 micromillimeter) smoke particulates will be respired. They lodge in the lungs and can lead to long term health concerns. A problem is the only equipment currently available to protect from both CO and particulate exposures would be a self contained breathing apparatus (SCBA) as used by structural fire fighters. Due to the weight and short time of protection, SCBA are not a realistic option for prescribed fire crews. Work is continuing on a full face powered air purifying respirator suitable for prescribe fire crews (USDA 1991).
Certain individuals have had documented allergic reactions to smoke from prescribed fires. These pose real and politically sensitive challenges to a prescribed fire program. Based on 4 Illinois deaths from leaf burning smoke, smoke sensitive asthmatics have been lobbying for a Illinois state wide ban on leaf burning. At this time the lobbying effort has retained an exception for ecological management burning. A single tragic incident could easily prompt a state wide ban on ecological management burning. A common sense precaution is to mail notices to all residences within your smoke impact zone with a reply card to alert you to their health concerns. Your IEPA permit application process requires a notice anyway.
Mitigation has included burning when children are away at school, parents are at work, or even offering reimbursement for hotel lodging if needed. Often taking a sincere Concerned approach with the initial contact will solve most problems.
Several states (e.g., Oregon, Virginia, Arizona and Florida), have either mandatory or voluntary standards for smoke management. Minimum mixing heights, ventilation rates, visibility distances, and particulate concentration rates are set for all prescribed fires. Coordination between adjacent open burning permit holders is also required in the Arizona system to avoid impacting adjacent air sheds. Voluntary systems developed in conjunction with air quality regulators seem to be much more workable than mandated systems resulting from tragic incidents.
A Pana, Illinois, incident (Hoffman 1993) resulted in a multiple car accident with five injuries and criminal charges filed against the volunteers who ignited the prescribed fire. A contributing factor may have been an inversion during the burn. Three fatalities resulted from a prescribed fire smoke caused chain reaction accident in Florida (Alexandria Daily Newspaper 1983). This accident may, in part, have prompted Florida strict training and licensing requirements for prescribed fire practitioners (Brenner and Wade 1992).
Alexandria Daily Newspaper, 1983. Chain reaction collision kills 8 on Florida Interstate. November 19.
Brenner, J. and Wade, D. 1992. Florida 1990 prescribed burning act. Journal of Forestry 90:5 p27-30.
Hoffman E. L. 1993 Illinois Department of Conservation. personal communication regarding May 9, 1993 burn between Pana and Marseilles , IL.
National Park Service. 1992. RX-90 Prescribed fire for burn bosses. Student text. National Park Service, Harrisonburg, VG.
NWCG. 1985. Prescribed Fire Smoke Management Guide. National Wildfire Coordinating Group. NFES 1279. Interagency Fire Center. Boise Idaho February.
USDA. 1976. Southern Forestry Smoke Management Guidebook. General Technical Report. SE-10. Frest Service. Southeastern Forest Experiment Station, Asheville, NC.