How to Compare Air Quality and Health Impacts between Wildfires and Prescribed Fires: A Review and a Proposal

Prescribed fire is being used or considered for use as a tool for wildfire risk management in many places throughout the world. The smoke created by prescribed fire differs from wildfire smoke in many aspects but its potential to degrade air quality and negatively impact human health may be significant. Additionally, prescribed fire cannot completely eliminate the risk of wildfires. Therefore, when the tradeoffs between wildfire and prescribed fire are evaluated, the possibility of a post-burn wildfire should also be considered.

First, we will review different measurement and simulation approaches employed for tradeoff analysis. We will discuss the strengths and weaknesses of each approach. Then we will describe the approach that we prefer and illustrate it with an example. The case we studied is the 2016 Gatlinburg wildfires in Tennessee. We used a modeling framework consisting of WRF (Weather Research and Forecasting) model for meteorology, BlueSky for emissions and plume rise, and CMAQ (Community Multiscale Air Quality) model for transport and chemical transformations, to simulate the actual wildfires. We evaluated the simulation performance using PM_2.5, O₃, and NO₂ observations at air quality network monitors. Then, we designed counterfactual but realistic prescribed fires and simulated their air quality impacts using the same modeling framework. Finally, we simulated counterfactual post-burn wildfires, with reduced fuel loads, taking place over the same area during the same period as the actual wildfires. We calculated the impacted populations at various pollutant concentration levels for each case and compared the impacts of actual wildfires with counterfactual prescribed fires and post-burn wildfires.

For the case of Gatlinburg wildfires, we found that the total emissions from counterfactual prescribed fires and post-burn wildfires would be smaller than wildfire emissions, lead to lower daily average PM_2.5, maximum 8-hour average O₃, and hourly NO₂ concentrations over the smoke impacted areas, and reduce person-time exposures. These benefits were primarily due to preventing the long-distance transport of smoke and lowering its concentration. However, every other case would be different, and these results should not be construed as if prescribed fire would always be beneficial. We recommend the application of similar simulation approaches to other cases, especially at other places, for future tradeoff analyses.