Impacts of upstream wildfire emissions on CO, CO2, and PM2.5 concentrations in Salt Lake City, Utah

Biomass burning is known to contribute large quantities of CO2, CO, and PM2.5 to the atmosphere. Biomass burning not only affects the area local to the fire, but may also impact the air quality far downwind from the fire. The 2007 and 2012 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 impact on a downstream urban system (Salt Lake City, which is a major urban center in the Intermountain West). Using an ensemble of stochastic back trajectories started at the Salt Lake City receptor, the Stochastic Time-Inverted Lagrangian Transport (STILT) model, driven by wind fields from the Weather Research and Forecasting (WRF) model was used to determine the influences of biomass burning emissions by combining the trajectories with a new, high-resolution biomass burning emissions inventory —the Wildfire Emissions Inventory. Various physical parameterization schemes and grid-nudging techniques were tested for WRF simulations centered over the Salt Lake valley in order to determine suitable settings that would reproduce atmospheric conditions over the western U.S. with fidelity. Remote sensing and speciated particulate matter were used to indicate periods of wildfire activity and to verify the WRF-STILT model results. 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 season originated from fires located in Utah and central Idaho. The model results suggested that during intense episodes of upstream 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 for Salt Lake City. Wildfires had a much smaller impact on CO2 concentrations in Salt Lake City with contributions rarely exceeding a 2 ppm increase above background values. While this work shows that anthropogenic emissions were the dominant source for elevated CO2, CO, and PM2.5 for an urban area like Salt Lake City, wildfires can have a substantial impact on CO and PM2.5 on an episodic basis during the wildfire season.