Wildfires are known to contribute large quantities of carbon dioxide (CO2), carbon monoxide (CO), and fine particulate matter (PM2.5) to the atmosphere. Wildfires affect not only the area in the vicinity of fire, but may also impact the air quality far downwind from the fire.
The 2007, 2012, and 2015 western U.S. wildfire seasons were characterized by significant wildfire activity across much of the Intermountain West and California. In this study, we determined the locations of wildfire-derived emissions and their aggregate impacts on Salt Lake City, Utah, a major urban center downwind of the fires.
The USFS Rocky Mountain Research Station’s new Wildland Fire Emission Inventory Version 2 model was used to determine the location and timing of wildfire emissions. The influence of wildfire emissions at Salt Lake City were assessed using the Stochastic Time-Inverted Lagrangian Transport (STILT) model driven by wind fields from the Weather Research and Forecasting (WRF) model. The STILT model framework generated an ensemble of stochastic back trajectories arriving at Salt Lake City. The trajectories were combined with the new, high-resolution wildfire emissions inventory. Initial results showed that the WRF-STILT model was able to replicate many periods of enhanced wildfire activity observed in the measurements.
Most of the contributions for the 2007 and 2012 wildfire seasons originated from fires located in Utah and central Idaho. The model results suggested that during intense episodes of upwind wildfires in 2007 and 2012, fires contributed as much as 250 ppb of CO during a 3-hour period and 15 μg/m3 of PM2.5 averaged over 24 hours at Salt Lake City, more than double ambient conditions.
Our study has demonstrated that combining the Wildland Fire Emission Inventory Version 2 with the STILT modeling framework provides a powerful tool for quantifying the contribution of wildfires to air pollution impact relative to anthropogenic sources. This is critical for air regulator efforts to develop successful strategies for improving air quality. The preliminary results also indicate that a real-time implementation of the fire emissions-model framework may provide more improved forecasting of wildfire smoke impacts on population centers in the western United States.