Atmos. Chem. Phys, vol. 13 (2013) pp. 7241-7262
In the US, wildfires and prescribed burning present significant challenges to air regulatory agencies attempting to achieve and maintain compliance with air quality regula-tions. Fire emission factors (EF) are essential input for the emission models used to develop wildland fire emission in-ventories. Most previous studies quantifying wildland fire EF of temperate ecosystems have focused on emissions from prescribed burning conducted outside of the wildfire season. Little information is available on EF for wildfires in temper-ate forests of the conterminous US. The goal of this work is to provide information on emissions from wildfire-season forest fires in the northern Rocky Mountains, US. In August 2011, we deployed airborne chemistry instru-ments and sampled emissions over eight days from three 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 combus-tion efficiency (MCE), and EF for CO 2 , CO, and CH 4 . Our study average values for MCE, EFCO 2 , EFCO, and EFCH 4 were 0.883, 1596 g kg −1 , 135 g kg −1 , 7.30 g kg −1 , respec-tively. Compared with previous field studies of prescribed fires in temperate forests, the fires sampled in our study had significantly lower MCE and EFCO 2 and significantly higher EFCO and EFCH 4 . The fires sampled in this study burned in areas reported to have moderate to heavy components of standing dead trees and down dead wood due to insect activity and previous fire, but fuel consumption data was not available. However, an analysis of MCE and fuel consumption data from 18 pre-scribed fires reported in the literature indicates that the avail-ability of coarse fuels and conditions favorable for the com-bustion of these fuels favors low MCE fires. This analysis suggests that fuel composition was an important factor con-tributing to the low MCE of the fires measured in this study. This study only measured EF for CO 2 , CO, and CH 4 ; how-ever, we used our study average MCE to provide rough esti-mates of wildfire-season EF for PM 2.5 and four non-methane organic compounds (NMOC) using MCE and EF data re-ported in the literature. This analysis suggests the EFPM 2.5 for wildfires that occur in forests of the northern Rocky Mountains may be significantly larger than those reported for temperate forests in the literature and that used in a recent national emission inventory. If the MCE of the fires sampled in this work are representative of the combustion character-istics of wildfire-season fires in similar forest types across the western US then the use of EF based on prescribed fires may result in an underestimate of wildfire PM 2.5 and NMOC emissions. Given the magnitude of biomass consumed by western US wildfires, this may have important implications for the forecasting and management of regional air quality.
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