Ozone production from wildfires: A critical review

Abstract Tropospheric ozone (O 3 ) negatively impacts human health and ecosystems, and is a greenhouse gas. Wildfires are a source of tropospheric O 3 , and studies show that wildfires are increasing in North America. In this study, we present a critical review of O 3 production from wildfires focusing on three key topics: the influence of wildfire emissions on O 3 production; the influence of photochemistry on wildfire O 3 production; and regulatory issues associated with wildfire O 3 production in the United States. Observations of ΔO 3 /ΔCO range from approximately −0.1 to 0.9, and are caused by the interplay of numerous factors including fire emissions, efficiency of combustion, chemical and photochemical reactions, aerosol effects on chemistry and radiation, and local and downwind meteorological patterns. Using average ΔO 3 /ΔCO ratios for major biomes, we estimate global wildfires produce approximately 170 Tg of O 3 per year, which is 3.5% of all global tropospheric O 3 production. Areas of uncertainty in wildfire O 3 production include the net effect of aerosols on chemical and photochemical reactions within a fire plume, the impact of oxygenated volatile organic compounds and nitrous acid on O 3 production, and the interplay of variables that lead to extreme ΔO 3 /ΔCO values. Because wildfire frequencies are likely increasing and have been shown to contribute to elevated O 3 at air quality monitoring sites, it is important to better understand the emissions, photochemistry and impacts of these fires.