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

Abstract. We report hourly surface observations of PM_2.5, CO, NO_x, O₃, 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. O₃ showed a non-monotonic response to wildfire smoke: MDA8 O₃ 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 noncancer hazard index (HI) would reach 3.0, meaning substantially elevated acute risks during high-smoke periods. 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.