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

摘要

In the US, wildfires and prescribed burning present significant challenges toair regulatory agencies attempting to achieve and maintain compliance withair quality regulations. Fire emission factors (EF) are essential input forthe emission models used to develop wildland fire emission inventories. Mostprevious studies quantifying wildland fire EF of temperate ecosystems havefocused on emissions from prescribed burning conducted outside of thewildfire season. Little information is available on EF for wildfires intemperate forests of the conterminous US. The goal of this work is toprovide information on emissions from wildfire-season forest fires in thenorthern Rocky Mountains, US.In August 2011, we deployed airborne chemistry instruments and sampledemissions over eight days from three wildfires and a prescribed fire thatoccurred in mixed conifer forests of the northern Rocky Mountains. Wemeasured the combustion efficiency, quantified as the modified combustionefficiency (MCE), and EF for CO, CO, and CH. Our study averagevalues for MCE, EFCO, EFCO, and EFCH were 0.883,1596 g kg, 135 g kg, 7.30 g kg, respectively.Compared with previous field studies of prescribed fires in temperateforests, the fires sampled in our study had significantly lower MCE andEFCO and significantly higher EFCO and EFCH.The fires sampled in this study burned in areas reported to have moderate toheavy components of standing dead trees and down dead wood due to insectactivity and previous fire, but fuel consumption data was not available.However, an analysis of MCE and fuel consumption data from 18 prescribedfires reported in the literature indicates that the availability of coarsefuels and conditions favorable for the combustion of these fuels favors lowMCE fires. This analysis suggests that fuel composition was an importantfactor contributing to the low MCE of the fires measured in this study.This study only measured EF for CO, CO, and CH; however, we usedour study average MCE to provide rough estimates of wildfire-season EF forPM and four non-methane organic compounds (NMOC) using MCE and EFdata reported in the literature. This analysis suggests the EFPM forwildfires that occur in forests of the northern Rocky Mountains may besignificantly larger than those reported for temperate forests in theliterature and that used in a recent national emission inventory. If the MCEof the fires sampled in this work are representative of the combustioncharacteristics of wildfire-season fires in similar forest types across thewestern US then the use of EF based on prescribed fires may result in anunderestimate of wildfire PM and NMOC emissions. Given the magnitudeof biomass consumed by western US wildfires, this may have importantimplications for the forecasting and management of regional air quality.