USE OF PHOTOCHEMICAL GRID MODELING TO QUANTIFY OZONE IMPACTS FROM FIRES IN SUPPORT OF EXCEPTIONAL EVENT DEMONSTRATIONS

Smoke from biomass burning contains a number of pollutants, including nitrogen oxides (NOx), volatile organic compounds (VOC), and fine particulate matter (PM2.5) (Jaffe et al., 2008; McKeen et al., 2002), which contribute to enhanced ozone and PM2.5 concentrations in the atmosphere under a variety of conditions (Pfister et al., 2006). Ozone enhancement due to biomass burning is highly variable and depends on fuel type, combustion efficiency, available solar radiation, and other factors (Jaffe and Wigder, 2012). Smoke and precursor emissions from fires can impact downwind cities, cause negative health effects, and trigger violations of National Ambient Air Quality Standards (NAAQS) for particulate matter and ozone. The U.S. Environmental Protection Agency (EPA) defines exceptional events as unusual or naturally occurring events that can affect air quality but are not reasonably controllable. State agencies can flag ambient data for exclusion from regulatory determinations if they can demonstrate to EPA’s satisfaction that the measurements were influenced by an exceptional event, such as wildfires. Technical evidence of such influences must be submitted to EPA in the form of a demonstration package, which must include analyses clearly showing no NAAQS exceedance would have occurred “but for” the exceptional event. Fires events can trigger violations of the NAAQS for ozone. Because ozone is a secondary pollutant formed by reactions involving precursor species emitted by fires and other sources, significant technical challenges are associated with meeting the “but for” criterion. Rather than simply establish a line of transport from the fire to the impacted ambient monitor, “but for” demonstrations must evaluate ozone levels that