Summary and analysis of approaches linking visual range, PM2.5 concentrations, and air quality health impact indices for wildfires

Abstract

Several U.S. state and tribal agencies and other countries have implemented a methodology developed in the arid intermountain western U.S. where short-term (1- to 3-hr) particulate matter (PM) with aerodynamic diameters less than 2.5 µm (PM2.5) concentrations are estimated from an observed visual range (VR) measurement. This PM2.5 concentration estimate is then linked to a public health warning scale to inform the public about potential health impacts from smoke from wildfire. This methodology is often used where monitoring data do not exist (such as many rural areas). This work summarizes the various approaches, highlights the potential for wildfire smoke impact messaging conflicts at state and international borders, and highlights the need to define consistent short-term health impact category breakpoint categories. Is air quality “unhealthy” when 1- to 3-hr PM2.5 is ≥ 139 µg/m3 as specified in the Wildfire Smoke, A Guide for Public Health Officials? Or is air quality unhealthy when 1- to 3-hr PM2.5 is ≥ 88.6 μg/m3 as specified in the Montana categorizations? This work then examines the relationship between visual range and PM2.5 concentrations using data from the Interagency Monitoring of PROtected Visual Environments (IMPROVE) program and the IMPROVE extinction coefficient (βext) equation to simulate an atmosphere dominated by smoke for sites in the arid intermountain western U.S. and great plains. This was accomplished by rearranging the βext equation to solve for organic mass as a function of VR. The results show that PM2.5 and VR are related by PM2.5 = 622 * VR−0.98 with a correlation of 0.99 and that at low VR values (<10 km) a small change in VR results in a large change in PM2.5 concentrations. The results also show that relative humidity and the presence of hygroscopic pollutants from sources other than fire can change the VR/PM2.5 relationships, especially at PM2.5 concentrations less than approximately 90 µg/m3. Implications: This work illustrates the limitations of current approaches linking VR, PM2.5 concentrations, and air quality health impact indices. It is the first published summary of known VR/PM2.5/health impact approaches nationally and internationally and the first application of the IMPROVE data and extinction coefficient equation to investigate these approaches scientifically for smoke-filled atmospheres. These VR/PM2.5/health impact indices are used to estimate PM2.5 concentrations when monitoring data are not available in order to inform the public during periods of wildfire smoke impacts. This work also offers discussion points to inform future work.