Summer PM<sub>2.5</sub> Pollution Extremes Caused by Wildfires Over the Western United States During 2017–2018
Yuanyu Xie, Meiyun Lin, Larry W. Horowitz
Abstract
Abstract Using observations and model simulations (ESM4.1) during 1988–2018, we show large year‐to‐year variability in western U.S. PM 2.5 pollution caused by regional and distant fires. Widespread wildfires, combined with stagnation, caused summer PM 2.5 pollution in 2017 and 2018 to exceed 2 standard deviations over long‐term averages. ESM4.1 with a fire emission inventory constrained by satellite‐derived fire radiative energy and aerosol optical depth captures the observed surface PM 2.5 means and extremes above the 35 μg/m 3 U.S. air quality standard. However, aerosol emissions from the widely used Global Fire Emissions Database (GFED) must be increased by 5 times for ESM4.1 to match observations. On days when observed PM 2.5 reached 35–175 μg/m 3 , wildfire emissions can explain 90% of total PM 2.5 , with smoke transported from Canada contributing 25–50% in northern states, according to model sensitivity simulations. Fire emission uncertainties pose challenges to accurately assessing the impacts of fire smoke on air quality, radiation, and climate.