Air quality impact of slash pile burns: Simulated geo-spatial impact assessment for Washington State

In the Western U.S., the prescribed burning of woody biomass in forests, mainly harvest slash, is the prevailing practice for in-woods fuel reduction and wildfire mitigation. Though these prescribed burns play an essential role in mitigating the wildfire risks, the resultant emission is a major air pollutants source that adversely affects air quality, negatively impacting human health. With an increased need for fire hazard reduction thinning, coupled with shrinking regional demand for lower quality biomass (pulpwood, hog-fuel, etc.), the volumes of unused biomass left on the forest floor as ‘waste’ will continue to grow. Reducing prescribed burns by utilizing this ‘waste’ biomass for alternate bio-based solutions (like bio-energy or bio-char) will enhance the economic feasibility of much-needed thinning operations and reduce uncontrolled emissions and related environmental and local health impacts. In this study, we simulate the increase in air pollutants due to additional prescribed fires in the Southwestern part of Washington State. Using the ‘BlueSky smoke modeling system,’ the study estimated the emissions associated with burning additional 726,000 dry t of residual biomass, which corresponds to a 30% increment from 2011. The burn was simulated over 29 days of the fall quarter and subsequently incorporated into the AIRPACT pollution transportation modeling system using the 2011 air quality and meteorological data as the baseline. The results showed that the ambient PM₂.₅ concentrations, due to the simulated pile burns, exceeded EPA's air quality standards on multiple days and in various locations across the Western part of the state, with two days reaching “very unhealthy” levels and one day reaching “hazardous” levels. By layering the census data on top of the pollution concentration data, the model estimated that, over the 29-day burn period, approximately 440,000 additional human days would be affected by higher than the EPA air-quality standards for ambient PM₂.₅ levels.