Throughout the world, biomass burning is an important source of air pollution. The effects of smoke from wildfires, particularly in forested areas have been observed on a continental scale in North America. For example, smoke from forest fires in Canada have resulted in elevated levels of CO as far south as the Caribbean Islands. Geostationary satellite imagery revealed that smoke plumes from the 2004 wild fires in Alaska affected air quality in eastern Canada and Western Europe. In many areas in North America, prescribed burning of forest or rangeland is used as a management strategy. One such area is the Flint Hills of Eastern Kansas and Oklahoma (see Figure 1). The Flint Hills are the largest extant remnant of the tallgrass prairie ecosystem that once covered a large proportion of the interior plains of North America. Unlike the tallgrass prairie elsewhere, the Flint Hills escaped the plow because the terrain was too hilly and rocky for cultivation. However, the dense grass cover makes the Flint Hills a premier area for livestock pasturing. Prescribed burning of the tallgrass prairie in the Flints Hills of Kansas is critical for the sustainability and productivity of the ecosystem. A large body of research has shown that burning on an annual or biennial basis enhances the quality of forage in these pastures by selecting for native, warm season grasses that are palatable and promote weight gain for grazing cattle. Suppression of burning also results in woody encroachment of shrubs and juniperous trees that ultimately replace the native grasses. The tallgrass prairie represents some of the most productive ranching land in the world, and it follows that woody expansion caused by burn suppression would have a devastating economic and socioeconomic impact on the region. In short, prescribed burning is a necessity in the Flint Hills. Typically, prescribed management burns in the Flint Hills are done in mid to late spring, to insure adequate regrowth of pasture forage for grazing throughout the summer. Frequent prescribed burning, usually every 2-3 years, keeps Flint Hills pastures free of woody vegetation that would otherwise degrade forage quality. Land management records suggest that, in a typical year, about 50-60% (≈1.0-1.2 million hectares) of the Flint Hills prairie is burned, resulting in the combustion of approximately 6.0 Tg of biomass per growing season.
Figure 1. Study area, showing Flint Hills and surrounding metropolitan areas.
Smoke from biomass fires has serious effects on human health. This is especially true if atmospheric processes concentrate the smoke in an urban air shed where large numbers of people are exposed to emissions. Smoke from biomass burning contains a variety of particulate and chemical pollutants, many of which are known to adversely affect human health. Some pollutants in smoke include a number of gases such as carbon monoxide (CO), carbon dioxide (CO2), ozone (O3), oxides of nitrogen (NOx), and methane (CH4) that contribute to poor air quality. Although the risks associated with temporary exposure to elevated levels of these compounds do not appear to be especially high, chronic exposure carries an elevated risk of contracting certain cancers.
In terms of both amount and impact, particulates are the pollutant most commonly observed in elevated quantities in biomass smoke. The definition of particulate pollution is broad and encompasses particles distributed over a wide range of diameters. The particle size distribution generated by a combustion event depends on the nature of the fuel, meteorological conditions at the time of the burn, and the rate at which energy was released in the fire. The particulates of most concern as are those that are small enough to be inhaled. In general, these particles are less than 10 micrometers in diameter. The capability of these particles to penetrate the human pulmonary system varies with size. Particles with diameters less than 10 micrometers (PM10) can settle in the bronchies, but larger particles in this range cannot readily enter the lungs themselves. Major sources of PM10 include biomass fires, as well as motor vehicle exhaust, industrial burning, and certain forms of wind-blown dust. A smaller subset of PM10, with diameters of <2.5 μm and <1.0 μm (PM2.5 and PM1.0) can penetrate into the lungs and are therefore considered a greater inhalation hazard. Particles in all of these size ranges are produced in great quantities by combustion of organic materials. Most woodland and rangeland fires produce a bimodal distribution of particles sizes, with roughly equal production of PM2.5 and PM10 particles. Due to the mass differences of these particle types, the conditions under which they become entrained in the atmosphere and transported by local or regional winds varies somewhat. Thus the particle size distribution present in a smoke plume varies with distance from the site of the burning. The exact relationship between distance and particle distribution is complex and not well known, particularly for grass and range fires.
Particulates have been shown to be associated with a wide range of respiratory health problems. They can cause changes in lung function, alteration of mucociliary clearance, and pulmonary inflammation which can lead to increased permeability of the lungs. As noted above, larger particles (PM10) can cause irritation in the bronchial tracts, and under prolonged exposure might cause potentially dangerous inflammation of the bronchies. This increased pulmonary permeability can in turn precipitate fluid in the lungs of people with existing heart disease. In addition, mediators released during the inflammatory response can increase the risk of blood clot formation and strokes. Particulate exposure might also increase susceptibility to bacterial or viral respiratory infections, leading to an increased incidence of pneumonia in vulnerable members of the population. Since PM2.5 particles are capable of penetrating directly into the lungs, their potential impacts are more severe. These particles can actually penetrate into the bronchial alveoli and therefore present the greatest pulmonary risk. Severe exposure might lead to onset of bronchial emphysema, especially in older persons, persons with genetic risk factors, or persons with smoking habits. In the presence of pre-existing heart disease, acute bronchiolitis or pneumonia induced by air pollutants might precipitate congestive heart failure. Particulate air pollution might also aggravate the severity of underlying chronic lung disease, causing more frequent or severe exacerbation of airway's disease or more rapid loss of lung function. Studies of periodic exposure to extremely high levels of particulate pollution among wildland firefighters suggest that chronic effects of smoke inhalation may persist long after cessation of exposure. Other studies in developing countries (where exposure to wood smoke from cooking fires is common) show that women and children are especially susceptible to the effects of biomass burning. PM2.5 particles have also been associated with arterial plaque deposits, vascular inflammation, and arteriosclerosis, all risk factors for coronary artery disease and heart attack.
The smoke from springtime prairie fires in the Flint Hills can have a deleterious effect on air quality. If the smoke moves easterly or northeasterly into metropolitan areas bordering the Flint Hills (e.g. Kansas City, MO/KS, Omaha, NE, Tulsa, OK), chemical interactions with other forms of air pollution (e.g., automobile exhaust) can result in the elevation of ozone. The smoke can decrease air quality in other ways by contributing to atmospheric haze and the elevation of particulates. Normally, prescribed burning occurs between February and April. Spreading burning dates over a longer period helps dilute smoke concentrations and decreases the potential for a
large plume to drift into the airshed of a metropolitan area. However, meteorological conditions sometimes dictate that burning must be compressed into a small number of suitable days. Such a situation occurred in 2003, when spring weather conditions prohibited early season burning and a high number and density of burns occurred over a short period in April. These burns peaked over the weekend of April 11-13, resulting in numerous grass fires in the Flint Hills burning simultaneously and producing a cloud of combustion products that drifted east for considerable distance (see Figure 2). The fires coincided with atmospheric conditions that resulted in a smoke plume entering the Kansas City area and then continuing eastward. This episode caused a spike in ozone concentrations in Kansas City that exceeded the 8-hr ozone standard. Following this event, it was recognized that improved prescribed burning management techniques would be needed if future recurrences were to be avoided.
Figure 2. Fire locations and smoke plume extent for April 13th, 2003. This image was produced by combining data from a variety of remote sensing sources. Fire locations are shown by red dots. The smoke plume (in gray) was interpreted visually from the imagery. Note the extensive area of coverage of the smoke plume, covering the three metropolitan areas considered in this proposal (Kansas City, Omaha, Tulsa) and extending as far as Tennessee and Alabama.
In order to avoid air pollution events like the one in April of 2003, it is necessary to insure that concentrated episodes of burning do not occur during times when meteorological conditions might result in concentration of smoke in major urban areas. This means reconciling the often divergent needs of two groups; 1) land mangers in the Flint Hills who rely on burning for productive rangeland, and 2) air pollutant monitors in the metropolitan areas who must ensure that there cities do not violate air quality standards. Development of land management guidelines that reconcile the needs of these two interest groups must be based on scientific understanding of how the extent and timing of burning in the Flint Hills interacts with local and regional meteorological conditions to affect pollutant quantities in urban air sheds. Currently, this scientific understanding is lacking.