Regional air quality simulations were performed to evaluate the contributions of wildland fires to inter-annual variability of black carbon (BC) concentrations and to assess the contributions of wildfires vs. prescribed fires to BC concentrations and deposition rates to glacier areas and snow-covered surfaces in the western US.

Simulations for June-September of 1997-2005 indicated that monthly mean BC concentrations over most of the western US were significantly elevated by wildland fires for at least one month during this period. Wildland fires contributed greater than 50% of the monthly mean BC concentrations in Idaho, Montana, and northern Wyoming during August and September. The central and north central Idaho regions were most affected by wildland fires. Modeling results and observations from the IMPROVE network indicated that wildland fire emissions contributed significantly to the inter-annual variations in August-mean BC concentrations in Idaho, Montana, northern Wyoming, Utah Colorado, and the eastern Dakotas.

Simulations for the full year of 2011 indicated that the seasonal trend in relatively contributions of wild and prescribed fires to BC concentrations followed closely to the seasonal trend in their emissions: wildfires had the largest contributions during the summer while prescribed fires contributions were the most significant during the fall and early winter. In contrast, their relative contributions to BC deposition reflected other factors, including specific relationships of fire locations and wind directions relative to snowpack locations and the relative lofting heights of wildfires vs. prescribed burns. Non-wildland fire sources dominated annual total BC deposition rates to glacial areas, but monthly contributions during summer from wildfires and during fall and winter from prescribed fires can be significant (> 40%).

For BC deposition to snow-covered surfaces, prescribed fires in Washington, Oregon, Idaho, Wyoming, and Colorado were significant contributors (40% to more than 90%) in significant portions of these states during November. Prescribed fire contributions were also significant in central Colorado during December. These were the months when high emissions from prescribed fires coincided with the snow season. Because wildfires occurred predominantly during warmer months when the snow-covered areas were small, contributions from wildfires were generally negligible except for some small regions in October and May. Because fire activities and snow amounts vary significantly from year to year, the analysis performed here should be extended to include more years.