Combustion efficiency and emission factors for US wildfires

In the US wildfires and prescribed burning present significant challenges to air regulatory agencies attempting to achieve and maintain compliance with National Ambient Air Quality Standards (NAAQS) and Regional Haze Regulations. Wildland fire emission inventories (EI) provide critical inputs for atmospheric chemical transport models used by air regulatory agencies to understand and to predict the impact of fires on air quality. Fire emission factors (EF), which quantify the amount of pollutants released per mass of biomass burned, are essential input for the emission models used to develop EI. Over the past decade substantial progress has been realized in characterizing the composition of fresh biomass burning (BB) smoke and in quantifying BB EF. However, most BB studies of temperate ecosystems have focused on emissions from prescribed burning. Little information is available on EF for wildfires in the temperate forests of the conterminous US. Current emission estimates for US wildfires rely largely on EF measurements from prescribed burns and it is unknown if these fires are a reasonable proxy for wildfires. Over 8 days in August of 2011 we deployed airborne chemistry instruments and sampled emissions from 3 wildfires and a prescribed fire that occurred in mixed conifer forests of the northern Rocky Mountains. We measured the combustion efficiency, quantified as the modified combustion efficiency (MCE), and EF for CO2, CO, and CH4. Our study average values for MCE, EFCO2, EFCO, and EFCH4 were 0.883, 1596 g kg 1, 135 g kg 1, 7.30 g kg 1, respectively. Compared with previous field studies of prescribed fires in similar forest types, the fires sampled in our study had significantly lower MCE and EFCO2 and significantly higher EFCO and EFCH4. An examination of our study and 47 temperate forest prescribed fires from previously published studies shows a clear trend in MCE across US region/fire type: southeast (MCE = 0.933) > southwest (MCE = 0.922) > northwest (MCE = 0.900) > northwest wildfires (MCE = 0.883). The fires sampled in this work burned in areas reported to have moderate to heavy components of standing dead trees and dead down wood due to insect activity and previous fire, but fuel consumption data was not available for any of the fires. However, fuel consumption data was available for 18 prescribed fires reported in the literature. For these 18 fires we found a significant negative correlation (r =-0.83, p-value = 1.7e-5) between MCE and the ratio of heavy fuel (large diameter dead wood and duff) consumption to total fuel consumption. This observ