Characterizing emissions, chemistry, and health impacts of aged wildfire smoke in a western US city

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We report hourly surface observations of PM 2.5 , CO, NO x , O 3 , and 75 speciated VOCs in Missoula, Montana, during a strong smoke event in 2020. This study tests our current understanding of wildfire emissions, chemistry, and health effects as implemented in the GEOS-Chem chemical transport model. Three-or-more-day-old smoke transported from California and the Pacific Northwest increased CO, PM 2.5 , and total measured VOCs by factors of 2–8, with hourly maxima of 800 ppb, 120 µg m −3 , and 85 ppb, respectively. In contrast, NO x levels were not elevated compared to the urban background. O 3 showed a non-monotonic response to wildfire smoke: MDA8 O 3 increased under light smoke but flattened or declined when PM 2.5 exceeded ∼ 30–40 µg m −3 , a feature that GEOS-Chem failed to reproduce. A 2020-style wildfire season recurring annually would yield an excess lifetime cancer risk of 100-in-1 million or approximately 7 times the non-smoke baseline. The chronic non-cancer hazard index (HI) would reach 3.0, indicating appreciable potential for chronic non-cancer effects. About 90 % of cancer risks are from PM 2.5 whereas non-cancer risks are dominated by formaldehyde, benzene, acrolein, and acetaldehyde. GEOS-Chem captured major smoke intrusions but underestimated CO, PM 2.5 , and VOCs by 30 %–90 %. These model biases propagate to health metrics, with GEOS-Chem underestimating smoke-attributable cancer risk by ∼ 40 % and chronic HI by ∼ 10 times. We attribute the model errors to underpredicted fire emissions and unrepresented VOC chemistry, which together led to an overestimation of OH and insufficient secondary production.

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