Atmospheric Chemistry and Physics, vol. 13, issue 14 (2013) pp. 7241-7262 Published by Copernicus GmbH
In the US, wildfires and prescribed burning present significant
challenges to air regulatory agencies attempting to achieve and maintain
compliance with air quality regulations. Fire emission factors (EF) are
essential input for the emission models used to develop wildland fire
emission inventories. 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 temperate 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 instruments 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
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 temperate forests, the
fires sampled in our study had significantly lower MCE and EFCO2 and
significantly higher EFCO and EFCH4.
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 prescribed fires reported in the literature indicates that the
availability of coarse fuels and conditions favorable for the combustion
of these fuels favors low MCE fires. This analysis suggests that fuel
composition was an important factor contributing to the low MCE of the
fires measured in this study.
This study only measured EF for CO2, CO, and CH4; however, we used our
study average MCE to provide rough estimates of wildfire-season EF for
PM2.5 and four non-methane organic compounds (NMOC) using MCE and EF
data reported in the literature. This analysis suggests the EFPM2.5 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 characteristics 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 PM2.5 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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