The formal management of smoke from wildfire and prescribed natural fire has a long history in the federal land management agencies. It remains a component of most fire planning activities. Substantial fractions of the particulate in wild land smoke are in the size ranges that are regulated by the US Environmental Protection Agency. Ambient atmospheric concentrations of particulate matter less than 2.5 micrometers in diameter (PM2.5) and particulate matter less than 10 micrometers in diameter are regulated under the US National Ambient Air Quality Standards (NAAQS) for fine particles. These standards require that concentrations over different averaging times must remain below numerical levels set to protect people who are especially sensitive to this form of air pollution.
Attainment or non-attainment of the NAAQS is determined by ambient air quality monitoring but has both scientific and practical/political components. Obviously, monitoring is a local activity; as officially approved monitors all collect an air sample, selection of locations, what area the sample represents, decisions about record length, and the like are all subjects of public debate. The US Clean Air Act gives the responsibility to develop monitoring and determine which areas attain and which do not attain the standards to States, Indian nations and other local air quality control districts. This responsibility is implemented through State (or Tribal) Implementation Plans (S/TTPS).
In many areas of the USA, the PM2.5 standard level is either being exceeded (non-attainment areas) or in danger of being exceeded. In such circumstances, the law (Clean Air Act) requires that S/TIPs include strategies to either reduce emissions to attain the NAAQS or to manage pollution increases such that attainment is maintained (Egan et al., 1981). As a result, smoke from forest burning is specifically regulated in some States and informally regulated in others. Almost everywhere in the USA where there are publicly managed wildlands, forest fire smoke is recognized as a potentially volatile public policy issue. The issue is confounded by recognition that forest wildfire is, to a large extent, an uncontrollable natural occurrence. As such, wildfire contributions to ambient concentrations are often exempt from attainment/non-attainment decisions. This conflicts with the concept that smoke from managed fire use, often done to diminish the wildfire threat and always resulting in lower net smoke emissions, is included as a contributor to attainment/non-attainment decisions.
Since the 1930s, the US Forest Service, along with academic co-operators and experts in other wildland management agencies, have conducted research on fire. Historically, this research was driven by the desire to understand wildland fire and, from that understanding, to predict fire ignition and fire behaviour. Eventually, fire research developed into a sophisticated effort to improve understanding of the ecological and associated environmental consequences of fire, including smoke production, transport and dispersion (Sandberg et al., 1979). These research activities led to an expanded knowledge base and an inventory of available smoke prediction technologies.
Prescribed fires usually lack on-site meteorological data. Although remote, deployable meteorological stations exist, much of the time they are not used for economic and logistical reasons. Most site meteorological information comes from the nearest stations, which may or may not represent the burn site, or from estimates made on-site by burn personnel. Current local or regional wind field patterns are usually only estimated intuitively, and not by either formal modelling or measurement.
Local scale weather forecasts are rarely available for the prescribed fire area, except as "spot" fire weather forecasts in the immediate area of a fire. In some regions, experimental mesoscale models generate useful local forecasts but these forecasts are not widely used by prescribed fire managers. At present prescribed fire managers do not have the resources to generate or use local scale meteorological information. Thus, as weather service offices begin to produce regional scale forecasts, prescribed fire management teams will need to develop the capacity to gather and interpret this data and generate useful information from them.
Predicting smoke behaviour from wildland fires is a difficult task. To be more tractable, the problem has been divided into discreet elements such as gas and particle chemistry, transport and dispersion, and emissions and heat production. A number of entrepreneurial contributions by various US research and operations groups have provided solutions to many of these elements (NWCG, 1985; Fox et al., 1986; Riebau et al., 1988; Southern
Forest Fire Laboratory, 1976; Peterson and Ottmar, 1991; Harrison, 1996; Hummel and Rafsneider, 1995; Lavdas, 1996). For each of these activities, products have been developed to suit particular needs and customers (e.g., Forest and Rangeland planners, operational fire fighters, or regulators). Validation demonstrations often have not been fully satisfactory for a number of reasons, among them, that valid observational data have been scarce. Consequently, "model validation" has often consisted of an assessment of user friendliness or applicability rather than a measure of precision and accuracy of model performance.
Predicting precisely where smoke from a forest burn is likely to go, and in what amount it will get to a selected receptor in the complex mountain topography of many forests cannot be confidently done at present in any deterministic way. This is because smoke prediction involves a wide variety of components that take on quite uncertain, effectively random, values. Strung together in a modelling system, these lead to assured failure of any prediction scheme in at least some significant circumstances. Smoke prediction cannot be improved without more and better observational data to try to capture these uncertainties and improve understanding of limits of predictability.
The fundamentals of natural vegetation combustion are poorly characterized because fire behaviour is physically complex. It depends on the location, distribution, and condition of the fuels, the topography and micrometeoro-logy of the vegetation/fuel complex, and the nature of the combustion. Experimentation has illustrated that different fuels burn differently, generate different chemicals at different stages of the combustion process and physically disperse those chemicals differently depending on all of the above variables. For example, predicting plume rise is not as straightforward an engineering calculation as it is for a stack emission because of complex turbulent interactions between the ambient airflow and the micrometeorolo-gy of the vegetation canopy. As the flame front progresses through the fuel bed, varying amounts of heat and momentum are released.
In the US, air quality regulators and land managers recently agreed on a non-binding Interim Air Quality Policy on Wildland and Prescribed Fires (EPA, 1998). In it, they described and agreed upon the need for both wildland fires and clean air. They also defined the roles of each entity in smoke and air quality management. Actions to minimise emissions were proposed along with ancillary tracking measures to be used to maintain accountability. Being a policy agreement, it did not include details of specific models or technologies for estimating smoke emissions or behaviour. It did, however, serve to set the stage for implementing advanced tools to help manage smoke from managed and wildfires.
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