. . . . . . . . . . . . "We report hourly surface observations of PM2.5, CO, NOx, 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, PM2.5, 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, NOx 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 PM2.5 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 PM2.5, whereas non-cancer risks are dominated by formaldehyde, benzene, acrolein, and acetaldehyde. GEOS-Chem captured major smoke intrusions but underestimated CO, PM2.5, 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." . "Characterizing emissions, chemistry, and health impacts of aged wildfire smoke in a western US city" . . . "lixu.jin@umontana.edu" . "January 21 2026" . "2025" . . "Emily Regalado" . "2026-05-01T17:51:21.441Z"^^ . . . . . . . . . "RSA" . "MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAxzr6UBGMW6c8tegz0babaledWUEQ0PLDE4tp7Iinbe2DZtAtY5JUptKYuStWDZx+QER4808P8dejNWRnBDzgthYJm/AyNSXflHSJhz2+NC+h7RylOLxbwLEQocmyKKiYxa2gT85m6ajVL2M6TnfG67nnK+K2f7iCGL6wYXRITD1q+7+5SWqBdDXIV921W4IKWaD2GJk+NRBoOqQhbsrk8Tn5XsNd7DMYVHk47oMDGbeBnrOIoRPsbBgAcoCsxxhiB9yN6Lf8EUbnlXVEDzJuZk048L1BDZL+6nkA8btTQGP2ijUFWA7rTrod3LjUDQWLZS95njjl867dtmv/znYkzwIDAQAB" . "IMFhKVm+GQkpWrA7cnpcSc0sOaE/oZ7bGiSLlhpgiWkaVGUseL0EBSep2r74/WzTSFjOISJC0REuavoyJl4GSDOt7KxySL7y0iO3i4QJsaS+xxnEjV3CgmdupkiyIASoWOcTkEe0wGftLm6IqZgCbH/GPwlfdUBuI5GMC+cIs3R9/adH6PO+Nk6GZosyBktvqWwId03WcpzDQhmJifcYKfoeadVYLmbwvyiMnIvABt7BnTqYJa9XEGPONA/cgz4CNsHWZsPPg3caBTo8aOQVk27JSkzBBMkPlbe5TxnpLAWHsgirBXu0RSVFMCFbzj24YqayhDm2ICYo/oQ3NrRyHw==" . . .