Fine particulate matter, also called PM2.5, is an air pollutant with significant public health impacts that is regulated under the federal Clean Air Act and is the primary air pollutant of concern across much of the western United States. PM2.5 pollution has many sources: industrial and agricultural activities, power generation, transportation, and construction. In addition to these anthropogenic sources, wildfires are a major source of PM2.5. In contrast to anthropogenic sources, pollution from wildfires is sporadic, intense, and may impact urban areas 100’s to 1000’s of kilometers downwind. State agencies are tasked with developing emission control strategies to minimize public exposure to PM2.5 and maintain compliance with federal air quality standards. The development of effective and efficient emission controls for anthropogenic sources requires quantitative knowledge of the contribution of wildfires to air pollution in population centers.
A collaborative project between the RMRS Fire, Fuel, and Smoke Program’s Smoke Emission and Dispersion research team and researchers from the University of Utah sought to quantify the contribution of wildfires to pollution in Salt Lake City, Utah. A new, high-resolution wildfire emission inventory, the Missoula Fire Lab Emission Inventory (MFLEI), was combined with the Stochastic Time-Inverted Lagrangian Transport (STILT) – Weather Research and Forecasting (WRF) model, an advanced atmospheric model, to quantify the influence of western U.S. wildfires on PM2.5 concentrations in Salt Lake City. The study focused on the active wildfire seasons of 2007 and 2012. To determine the influences of wildfires, an ensemble of back trajectories at the Salt Lake City receptor within the WRF-STILT model were combined with the MFLEI pollutant emissions.
Initial results showed that the WRF-STILT model was able to replicate many periods of enhanced wildfire activity observed in PM2.5 and carbon monoxide (CO) measurements in Salt Lake City. Most of the contributions for the 2007 and 2012 wildfire seasons originated from fires located in Utah and central Idaho (Figure 1). The model results suggested that during intense episodes of upwind wildfires in 2007 and 2012, fires contributed as much as 15 μg/m3 of PM2.5 averaged over 24 h at Salt Lake City. The study also identified the treatment of smoke plume rise as a weakness in the model and a likely cause of pollution events that were poorly simulated. Based upon the initial study, the plume rise module of the WRF-STILT model is currently being improved. Following the model update, the study will be expanded to include additional urban areas in the west for 2007, 2012, and 2013. When completed, the simulations using the refine model should enable air quality regulators to more accurately account for wildfire contributions to PM2.5 pollution in urban areas.