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Contribution of Wildland-Fire Smoke to US PM₂.₅ and Its Influence on Recent Trends

O’Dell, Katelyn, Ford, Bonne, Fischer, Emily V., Pierce, Jeffrey R.
Environmental science & technology 2019 v.53 no.4 pp. 1797-1804
models, particulates, pollution, satellites, smoke, spring, summer, wildfires, Eastern United States, Western United States
Seasonal-mean concentrations of particulate matter with diameters smaller than 2.5 μm (PM₂.₅) have been decreasing across the United States (US) for several decades, with large reductions in spring and summer in the eastern US. In contrast, summertime-mean PM₂.₅ in the western US has not significantly decreased. Wildfires, a large source of summertime PM₂.₅ in the western US, have been increasing in frequency and burned area in recent decades. Increases in extreme PM₂.₅ events attributable to wildland fires have been observed in wildfire-prone regions, but it is unclear how these increases impact trends in seasonal-mean PM₂.₅. Using two distinct methods, (1) interpolated surface observations combined with satellite-based smoke plume estimates and (2) the GEOS-Chem chemical transport model (CTM), we identify recent trends (2006–2016) in summer smoke, nonsmoke, and total PM₂.₅ across the US. We observe significant decreases in nonsmoke influenced PM₂.₅ in the western US and find increases in summer-mean smoke PM₂.₅ in fire-prone regions, although these are not statistically significant due to large interannual variability in the abundance of smoke. These results indicate that without the influence of wildland fires, we would expect to have observed improvements in summer fine particle pollution in the western US but likely weaker improvements than those observed in the eastern US.