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Combustion efficiency and emission factors for wildfire-season fires in mixed conifer forests of the northern Rocky Mountains, US

by S. P. Urbanski
Atmospheric Chemistry and Physics ()
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In the US, wildfires and prescribed burning present significant\nchallenges to air regulatory agencies attempting to achieve and maintain\ncompliance with air quality regulations. Fire emission factors (EF) are\nessential input for the emission models used to develop wildland fire\nemission inventories. Most previous studies quantifying wildland fire EF\nof temperate ecosystems have focused on emissions from prescribed\nburning conducted outside of the wildfire season. Little information is\navailable on EF for wildfires in temperate forests of the conterminous\nUS. The goal of this work is to provide information on emissions from\nwildfire-season forest fires in the northern Rocky Mountains, US.\nIn August 2011, we deployed airborne chemistry instruments and sampled\nemissions over eight days from three wildfires and a prescribed fire\nthat occurred in mixed conifer forests of the northern Rocky Mountains.\nWe measured the combustion efficiency, quantified as the modified\ncombustion efficiency (MCE), and EF for CO2, CO, and CH4. Our study\naverage values for MCE, EFCO2, EFCO, and EFCH4 were 0.883, 1596 g\nkg(-1), 135 g kg(-1), 7.30 g kg(-1), respectively. Compared with\nprevious field studies of prescribed fires in temperate forests, the\nfires sampled in our study had significantly lower MCE and EFCO2 and\nsignificantly higher EFCO and EFCH4.\nThe fires sampled in this study burned in areas reported to have\nmoderate to heavy components of standing dead trees and down dead wood\ndue to insect activity and previous fire, but fuel consumption data was\nnot available. However, an analysis of MCE and fuel consumption data\nfrom 18 prescribed fires reported in the literature indicates that the\navailability of coarse fuels and conditions favorable for the combustion\nof these fuels favors low MCE fires. This analysis suggests that fuel\ncomposition was an important factor contributing to the low MCE of the\nfires measured in this study.\nThis study only measured EF for CO2, CO, and CH4; however, we used our\nstudy average MCE to provide rough estimates of wildfire-season EF for\nPM2.5 and four non-methane organic compounds (NMOC) using MCE and EF\ndata reported in the literature. This analysis suggests the EFPM2.5 for\nwildfires that occur in forests of the northern Rocky Mountains may be\nsignificantly larger than those reported for temperate forests in the\nliterature and that used in a recent national emission inventory. If the\nMCE of the fires sampled in this work are representative of the\ncombustion characteristics of wildfire-season fires in similar forest\ntypes across the western US then the use of EF based on prescribed fires\nmay result in an underestimate of wildfire PM2.5 and NMOC emissions.\nGiven the magnitude of biomass consumed by western US wildfires, this\nmay have important implications for the forecasting and management of\nregional air quality.

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