Comparison of bottom-up and satellite-based emission estimates in a prescribed burn: Evaluation with airborne smoke measurements

Forest fire emissions are a significant source of air pollutants and in certain parts of the world they can lead to major air quality problems. In addition to the regional impacts, emissions from wildfires and controlled burns also have global climate impacts. Proper assessment of these impacts requires accurate estimates of the forest fire emissions. Forest fire emissions can be estimated with a bottom-up approach using ground-based information or from satellite observations. When there is sufficient local information about the area burned, the types of forest fuels and their consumption amounts, and the progression of the fire, bottom-up estimation is preferred. For controlled burns, a.k.a. prescribed burns, in places where a reasonable amount of land management is practiced there is typically sufficient ground-based information for emissions estimation. However, for remote regions where no ground-based information is available on the size, intensity, or the spread of the forest fire, estimates based on satellite observations are preferred. For example, burn location, size and timing information can be obtained from satellite retrievals of thermal anomalies and fuel loading information can be obtained from satellite products of vegetation cover. In both cases, emission factors derived from field or laboratory studies are used to quantify the amounts of emitted pollutants. It is important to determine the level of uncertainty in the resulting emission estimates.

Here, emissions from a prescribed burn are estimated using both ground-based information and satellite observations. The burn was conducted on 17 November 2009 near Santa Barbara, California over 80 ha of land covered with chaparral. An aircraft tracked the smoke plume and measured CO2 and light scattering along with meteorological parameters during the burn (Akagi et al., 2011). We simulated the burn using the Daysmoke plume rise and dispersion model (Achtemeier et al., 2011). This model is specifically designed to predict smoke concentrations downwind of prescribed burns. Wind fields generated by a weather prediction model (WRF) were adjusted locally to match the aircraft measurements of wind speed and direction as much as possible. Concentrations of downwind smoke predicted by using both bottom-up and satellite-based emission estimates are compared to corresponding aircraft measurements. The Daysmoke model along with wind adjustments reduces several of the uncertainties inherent to dispersion modeling of forest fire plumes. This allows for a more reliable investigation of the uncertainties in the magnitudes and timings of emissions. The levels of these uncertainties are evaluated and compared for the bottom-up and satellite-based emission estimates.