Wildfires are the second largest source of volatile organic compounds (VOCs) in the western US (Hoesly et al., 2018; Jin et al., 2023). Some VOCs are designated hazardous air pollutants (HAPs) due to their carcinogenicity and potential for acute or chronic health effects (Naeher et al., 2007; Reid et al., 2016). Lengthening and intensified fire seasons in recent decades have posed direct air quality risks to rural populations and the inhabitants of ∼ 50 million US homes that already lie within the fire-prone wildland–urban interface (WUI), a figure expected to grow by one million every three years (Burke et al., 2021). Moreover, recent years have shown that western US and Canadian biomass burning (BB) smoke frequently impacts cities in western North America and can reach population centers thousands of kilometers downwind on the East Coast (Yu et al., 2024). Despite the growing threat of more frequent and intense wildfires, the health effects of HAPs emitted from wildfires are not well documented, and residents often lack chemically resolved data on the pollutants to which they are exposed, because of limited ground-level measurements. Here, we combine hourly ground-based observations of VOCs and criteria pollutants with the GEOS-Chem chemical transport model (CTM) to characterize the composition, chemical evolution, and health implications of aged wildfire smoke.

Past observational efforts have provided valuable yet incomplete insights into the health risks associated with wildfire smoke exposure (Gould et al., 2024). For example, some studies have targeted near-source fireline PM (Navarro et al., 2021), CO (Semmens et al., 2021), or high-altitude plumes sampled by aircraft (O'Dell et al., 2020), and others have examined speciated VOCs and/or aerosol composition (Akagi et al., 2014; Fiddler et al., 2024; Joo et al., 2024; Liang et al., 2022). Speciated, near-surface data where the majority of human exposure to fresh and aged smoke actually occurs are still seriously under sampled. The lack of sufficient measurements also limits the evaluation of CTMs, which are widely used for smoke forecasting and health-risk assessments. Recent aircraft observations show that GEOS-Chem (a commonly used CTM) underestimates BB CO and key HAPs such as formaldehyde by a factor of three or more (Jin et al., 2023). However, analogous ground-based validation remains absent. Because near-surface chemistry differs fundamentally from lofted plumes (e.g., ambient temperature, nighttime oxidants, boundary layer mixing) (Decker et al., 2019; Pagonis et al., 2023), ground observations are urgently needed to improve model validation and exposure assessments.

The western US fire season of 2020 produced some of the most extreme smoke levels on record in OR, WA, and CA (Albores et al., 2023; Reilly et al., 2022), with September standing out as the peak month (Abatzoglou et al., 2021; Mass et al., 2021). More than 1.6 million ha burned in California alone, releasing an estimated ∼ 127 TgCO2-equivalent, nearly twice the state's cumulative 2003–2019 greenhouse-gas reductions (Jerrett et al., 2022). These BB events provide an ideal natural laboratory for quantifying the composition of aged smoke, and testing CTM capabilities for representing regional smoke and assessing the continental-scale impacts of wildfires.

In this study, we present hourly ground-based measurements of 75 VOCs (including 15 HAPs), PM2.5, CO, NOx, and O3 collected in aged (mostly > 3 d) regional wildfire smoke during September 2020 in Missoula, Montana – a representative northwestern US city frequently impacted by regional wildfire smoke. Leveraging this comprehensive dataset, we (i) characterize the temporal evolution of VOCs and criteria pollutants (Sect. 4), (ii) quantify species-specific BB enhancements (Sect. 5), (iii) investigate O3–PM2.5 relationships (Sect. 6), (iv) assess public health risks using regulatory exposure metrics for PM2.5 and HAPs (Sect. 7), and (v) evaluate GEOS-Chem simulations against observations to diagnose model biases in BB emissions and chemistry, and their implications for health-risk estimates (Sect. 8).

Wildfire smoke at ground level is often detected using particle-based diagnostics, for example: (i) satellite-derived aerosol products such as the overhead information from the NOAA Hazard Mapping System (HMS) (Jaffe et al., 2022; O'Dell et al., 2021), (ii) column-integrated aerosol optical depth from AERONET, (iii) sustained elevations in surface PM2.5 relative to the general urban background (Selimovic et al., 2019, 2020), and (iv) combinations of these metrics (Kaulfus et al., 2017; McClure and Jaffe, 2018). Many studies also incorporate gas-phase tracers – initially CO, hydrogen cyanide (HCN), acetonitrile (ACN), and more recently, furan and maleic anhydride (MA) (Coggon et al., 2019; de Gouw et al., 2003, 2006; Li et al., 2000). Among these VOCs, furan is a sensitive indicator of fresh BB smoke due to its short lifetime and smaller emission amount from anthropogenic sources. In addition, MA is an oxidation product of furan and its derivatives, with a longer lifetime; thus, it has been proposed as a tracer of aged smoke (Coggon et al., 2019).

Here, we evaluate some current BB-impacted identification metrics in the context of our MSO case study and examine whether our VOC measurements provide additional value in identifying BB events in the urban setting. We first derive the hourly median diurnal urban background for BB tracers and other species using HMS “no-fire” days for September 2020. Any enhancement (ΔX) > 1.5 times the background is considered BB-impacted, and a provisional smoke day is defined when at least six such hours occur. We also tested the PM2.5/CO criterion (> 30 µgm-3ppm-1) proposed for urban smoke identification (Jaffe et al., 2022). However, at MSO this threshold classified nearly all days as smoke-impacted, consistent with the limitation that PM2.5/CO becomes non-discriminating when background PM2.5 and CO are low (Jaffe et al., 2022). We therefore do not use ΔPM2.5/ΔCO as a standalone classifier for this site.

Figure 2 summarizes the various daily BB classifications for September 2020 in Missoula. HMS alone identifies 20 smoke days (the greatest number among the diagnostics), but it can flag a day as smoke-impacted even when surface concentrations remain low if smoke is primarily aloft. For example, HMS triggers on 2 and 30 September (daily average PM2.5 is 8.5 and 5.5 µgm-3, respectively), which likely reflect elevated plumes that were not mixed down to the surface (Liu et al., 2024). By comparison, the ΔPM2.5 criterion identifies 16 d. Its diagnostic power depends strongly on aerosol loading at the surface.

When daily mean PM2.5 exceeded 35 µgm-3 (n = 7; 13–19 September), the ΔPM2.5 approach aligned with HMS and with all gas-phase tracers considered here (i.e., CO, ACN, furan, and MA), indicating concentrated smoke that coupled large aerosol mass with gaseous enhancements. A k-means clustering analysis (k = 3: smoke, no-smoke, uncertain) applied to pairwise combinations of CO, PM2.5, and each tracer corroborated this classification, reinforcing that at high aerosol loadings, routine ground-level PM2.5 alone can reliably identify smoke during summer in Missoula. We therefore classify these seven days as a high-confidence BB period.

At intermediate daily mean PM2.5 between 20 and 35 µgm-3 (n = 5; 5–6, 11–12, and 23 September), ΔPM2.5 still overlaps at least two gas-phase tracers but not all, signaling moderately aged or dispersed smoke and therefore medium diagnostic confidence. When the daily-mean PM2.5 falls below 20 µgm-3 (n = 4; 7, 21–22, and 24), PM2.5 overlaps with at most one gas tracer, and HMS does not indicate smoke on 24 September, so these low-load cases warrant only low-to-medium confidence. One additional case (21 September) meets the PM2.5 threshold but lacks corroboration from the gas-phase tracers, suggesting a potential false positive, although HMS indicates overhead smoke. Across September (a total of 30 d), the remaining 14 d do not meet the ΔPM2.5 criterion and are classified as non-smoke by this approach.

Gas-phase tracers extend the detectability of wildfire smoke beyond what is captured by HMS and PM2.5 alone, particularly during precipitation events. For example, on 20 September, CO and furan enhancements were observed in the absence of corroborating HMS smoke or elevated PM2.5. A rain event from the preceding day into the afternoon of 20 September efficiently scavenged aerosol mass, resulting in a sharp decrease of ∼ 30 µgm-3 in PM2.5, whereas gas-phase species were less affected and their enhancements persisted in the boundary layer.

Because no single tracer is definitive, we adopt a balanced multi-indicator rule: an hour is classified as smoke-impacted when at least three independent smoke indicators support smoke influence, based on HMS smoke detection and/or BB-associated species exceeding 1.5 × their background values for at least 6 h on that day. Applied to September 2020, this criterion identified 14 smoke days (5–7, 11–19, and 22–23 September). For subsequent sections, we describe the three smoke events, and we focus on the high-confidence BB period of 13–19 September to analyze its enhancement ratios, thereby minimizing uncertainties from marginal or non-BB sources.

Wildfires across the Pacific Northwest and northern California were exceptionally active in 2020 (Albores et al., 2023; Higuera and Abatzoglou, 2021; Neyra-Nazarrett et al., 2025; Reilly et al., 2022), and September delivered strong smoke signatures in Missoula. Applying the three-tracer classification described in Sect. 3, we identified 16 smoke-impacted days across confidence levels.

Figures 3 and S1 in the Supplement show three distinct multi-day smoke episodes affecting Missoula identified using the criteria described in Sect. 3, including 5–7 September (Event 1), 11–20 September (Event 2), and 22–24 September (Event 3). Because each episode involved multiple overlapping plumes from different fires with varying transport times, we refer to them as “events” rather than discrete “plumes”. Analysis of GOES-17 and VIIRS satellite imagery, and synoptic meteorological conditions indicates that these events were predominantly driven by southwesterly transport of wildfire smoke from Oregon, Idaho, and California, with likely influence from local smaller fires.

Event 1 (5–7 September) resulted from the California Creek Fire (United States Forest Service, 2020), which started on 4 September mixing with a localized, narrow plume moving east-northeast over the Bitterroot Mountains toward Missoula, as captured by GOES-17 imagery. Correspondingly, hourly PM2.5 increased rapidly from < 10 to 30 µgm-3 by 06:00 Mountain Daylight Time (MDT, UTC-6) on 6 September and CO from 100 to ∼ 300 ppb by 09:00 MDT the same day, with brief spikes (≤ 50 ppt) of the fresh-smoke tracer furan at 01:00 MDT. Pollutant concentrations returned to the urban background within 24 h after a post-frontal north-westerly wind flushed the valley.

Event 2 (11–20 September) was the longest and strongest impact on Missoula from the Oregon–California “megafire” complex that ignited in August and erupted on 7 September (Abatzoglou et al., 2021). Intense smoke on 8 September was advected westward offshore, where it became entrained into a cut-off low over the NE Pacific (around 42° N, 135° W), and subsequently advected eastward inland, mixing with smoke freshly emitted by the megafire complex and fires in Idaho. By 11 September, dense smoke formed a clearly visible “smoke river” that stretched across Washington, Oregon, and Idaho into western Montana. Back-trajectory analyses indicate that air parcels reaching Missoula had aged for at least two days. Consistently, smoke-impacted hours during this event exhibited pollutant enhancements of four- to fifteen-fold for CO, PM2.5, acetonitrile, and total measured VOCs (TVOCs), reaching peak values of 800 ppb CO, 120 µgm-3 PM2.5, 2 ppb acetonitrile, and 85 ppb TVOCs in mid-September. Strong correlation (r2=0.9; Fig. S2) among PM2.5 measured at the Boyd Park and other tracers measured at the UM campus indicates spatially coherent smoke influence across Missoula at the surface. Brief furan spikes (≤ 0.7 ppb) on 12–13 and 18–19 September suggest additional influence from local fires. Overall, this event accounts for ∼ 80 % of smoke hours in this month and is the focus of the enhancement ratio analysis (Sect. 5).

Event 3 (22–24 September) was a weaker resurgence of smoke from the same Oregon–California fires in Event 2. A preceding rain event on 20 September coincided with a rapid decrease in the 24 h mean PM2.5 from 40 to 10 µgm-3, whereas most gas-phase species decreased less, with CO declining from 500 to 350 ppb and total VOCs from 50 to 30 ppb. This pattern suggests preferential particle removal via precipitation scavenging and may provide a physical explanation for “smoke-present but low-PM2.5” days. It also highlights that smoke identification based on column indicators (e.g., HMS) may not always reflect surface PM exposure when precipitation occurs. Subsequent south-westerly flow reintroduced the residual, aged smoke into the region. As a result, 24 h PM2.5 concentrations reached 40 µgm-3 and hourly CO peaked at 350 ppb before a front cleared the valley, which was especially evident for PM2.5 but still leaving fire influences for trace gases.

Figure 4 compares pollutant concentrations during smoke-impacted and background periods in September 2020. On average, MDA8 ozone increased by 3 ppb (7 %) during smoke episodes, while NOx showed no visible change. In contrast, CO, PM2.5, and TVOCs were significantly elevated, with hourly concentrations increasing by a factor of 3–8 compared to urban background. Average hourly CO concentrations increased from 160 ± 80 ppb (mean ± SD of hourly data) to 420 ± 205 ppb on smoke days. PM2.5 showed even larger enhancements, rising from 6.1 ± 4.5 to 43 ± 34 µgm-3. The background CO and PM2.5 concentrations agree with typical western US urban levels (150–200 ppb and 5–10 µgm-3) (Cope et al., 2024; Lill et al., 2022; Lopez-Coto et al., 2020; Pfister et al., 2011).

Measured hourly TVOCs tripled from 23 ± 15 to 55 ± 24 ppb, with individual species enhancements summarized in Table S2 in the Supplement. Ten VOCs increased by factors of ∼ 4–7, five of which were furanoids or their derivatives. These species included furfural, methylfurfural, 2-furanmethanol, 2-furanone, 5-hydroxy-2-furfural/2-furoic acid, maleic anhydride, acrylonitrile, methyl benzoic acid, methyl methacrylate, and anisole. Forty-eight VOCs increased by a factor of 2–4, while 16 species increased by a factor of 1–2. Fifteen of the 75 measured VOCs are on the EPA's list of HAPs, and they collectively represented 51 % of TVOCs by average molar mixing ratio. During smoke events, total HAPs (THAPs) increased threefold relative to background periods (28 ± 13 ppb vs. 11 ± 7 ppb), with individual species exhibiting enhancements ranging from 40 % (dichlorobenzene) to nearly fivefold (acrylonitrile and maleic anhydride). Figure S3 shows that OH reactivity (OHR) from CO and TVOCs approximately doubled during smoke-impacted days compared to background (13.4 vs. 6.5 s-1). During background periods, isoprene was the largest individual contributor to OHR (0.9 s-1; 13.6 % of total OHR), followed by monoterpenes (10.4 %), formaldehyde (9.0 %), CO (8.8 %), and other individual VOCs. During smoke-impacted days, CO became the largest combustion-related contributor (1.4 s-1; 10.7 %), and the relative contribution of monoterpenes fell to 5.9 %, consistent with previous urban smoke OHR analyses in Boise, ID (Permar et al., 2023). Other VOC groups show similar contributions to OHR in both periods. Together, these enhancements underscore the strong influence of regional wildfire smoke on western US air quality and motivate continued monitoring of smoke exposure in communities such as Missoula.

We further apply a photochemical-age framework to determine whether VOC enhancement ratios (EnRs) measured in aged smoke reaching Missoula still preserve interpretable source information after multi-day transport and mixing (Fig. 5). Originally developed for urban plumes, this framework provides a useful diagnostic for separating direct emissions, chemical removal and formation (de Gouw et al., 2017). Here, photochemical age is an oxidation-based metric inferred from the VOC ratio clock and should not be interpreted as the exact physical time since emission. Negative values occur when the observed toluene-to-benzene enhancement ratio exceeds the assumed initial ratio and indicate that the clock assumptions are not fully satisfied for those observations, and large uncertainty exists (Sect. S1 in the Supplement). For primary VOCs, linear fits of ln(EnR) versus photochemical age yield back-extrapolated time-zero emission ratios (ERs) and effective OH rate constants (kOH) under the assumption of plume-integrated aging. For oxygenated VOCs (OVOCs), the same relationships are interpreted qualitatively because secondary formation can offset chemical loss. We provide methodological details in Sect. S1.

For relatively long-lived primary aromatics, the framework-inferred ERs are physically reasonable, indicating that aged smoke arriving in Missoula still retains measurable source signatures for these species, though the high general urban background, the initial fire emission ratio, and the potential mixing of different sources during transport introduce uncertainties that are hard to resolve. Nevertheless, if we infer back-extrapolated time-zero ERs, this approach estimates 2.05 ± 0.02 ppbppm-1 for benzene and 1.42 ± 0.02 ppbppm-1 for toluene relative to CO. These values agree with near-source ranges reported from the WE-CAN and FIREX-AQ campaigns within ∼ 20 % (benzene: 1.8–2.3 ppbppm-1; toluene: 1.2–1.5 ppbppm-1) (Gkatzelis et al., 2024; Permar et al., 2021), and are the same as what we inputted for the photochemical age calculation (benzene/toluene fire emission ratio of 0.7 ppbppb-1; Sect. S1; Eq. S1). This agreement indicates that the framework can recover plausible source-like ERs for relatively unreactive primary species even after multi-day transport. However, inferred ERs for C8–C10 aromatics are higher than published near-source wildfire values by roughly a factor of 2–3, suggesting that these species were substantially influenced by non-fire sources during transport. Urban mixing is a likely contributor, consistent with city-based measurements showing elevated aromatics relative to CO during wildfire-smoke periods (Cope et al., 2024).

Beyond ER derivation, the ln(EnR)-age framework also provides insight into the net photochemical evolution of VOCs and is not sensitive to an accurate time-zero estimate, unlike ERs. For primary VOCs, EnRs decreased significantly with photochemical age (p≪ 0.01), and the relative decay rates broadly tracked their OH rate constants (kOH). For example, benzene exhibited only modest decay over 1 week of photochemical aging, whereas toluene and C8 aromatics decayed more rapidly, consistent with their higher reactivity. The inferred kOH values (Sect. S1; Eq. S7) were (9.8 ± 1.6) × 10⁻¹³ and (5.6 ± 0.2) × 10⁻¹² cm3molec.-1s-1 for benzene and toluene, respectively. These values agree with the literature within ∼ 10 %–20 % and, again, agree with our photochemical age estimates. For C8 aromatics, the inferred kOH (7.9 × 10⁻¹² cm3molec.-1s-1) lies between those of ethylbenzene and the xylene isomers, as expected for a lumped mixture.

The framework becomes less robust for more reactive hydrocarbons and OVOCs. Effective kOH values inferred for C9–C10 aromatics and small alkenes are lower than literature values by factors of 3 or more, indicating that simple plume-integrated first-order loss does not adequately describe these compounds in aged smoke. A likely reason is that the most reactive species were already preferentially removed during the earliest, OH-rich stage of plume evolution, before the smoke reached Missoula. Additional uncertainty likely arises from dilution, urban mixing, background correction, and species lumping. OVOCs such as methanol, formaldehyde, and acetaldehyde, exhibited weaker and often statistically insignificant ln(EnR)-age trends (p ≈ 0.02–0.21), and in several cases the relationships flattened with age. Such behavior is consistent with secondary production partially offsetting chemical loss, while additional contributions from non-fire sources such as biogenic emissions may further obscure the relationships. Together, these results show that the ln(EnR)-age framework remains informative for relatively long-lived primary VOCs, but becomes progressively less diagnostic for reactive hydrocarbons and especially OVOCs in regionally aged smoke.

To complement the age-framework analysis, we also calculated event-integrated EnRs of PM2.5 and 27 VOCs relative to CO as bulk descriptors of the aged-smoke event. These values provide an observational reference for VOC-to-CO relationships in regionally aged smoke and are also used for model evaluation in Sect. 8. Species-level values and literature comparisons are provided in Table S3 and Fig. S4.

Figure 6 shows the relationship between maximum daily 8 h average (MDA8) O3 and daytime mean PM2.5 at Missoula during September 2020. During the strongest smoke episodes, observed MDA8 O3 was suppressed by up to ∼ 15 ppb (e.g., 6–7 September; Fig. 6A). These decreases coincide with ∼ 50 % reductions in downwelling shortwave radiation (SW) (Fig. S5A), a first-order proxy for actinic flux and photolysis J(NO2) and J(O1D), consistent with reduced radical production and in situ O3 formation. Consistent with Fig. 4, MDA8 O3 was only ∼ 3 ppb (7 %) higher during smoke-impacted periods than during the September 2020 background, while NOx showed no discernible enhancement. In addition, heavy smoke was associated with lower observed daytime maximum temperature and a shallower planetary boundary layer height (PBLH) (Fig. S5B–C), which may further reduce surface ozone by slowing reaction rates and weakening entrainment of O3-rich air aloft.

Across the limited number of BB days, the relationship between MDA8 O3 and daytime mean PM2.5 is non-monotonic (Fig. 6B). A locally weighted scatterplot smoothing (LOWESS) curve suggests that MDA8 O3 increases at low PM2.5 (< ∼ 20 µgm-3), but decreases at moderate PM2.5 (∼ 20–60 µgm-3). The non-monotonic relationship may reflect limited local O3 production in the absence of a concurrent NOx enhancement, together with reduced photochemistry at higher aerosol loadings.

The non-monotonic behavior is not unique to Missoula but is also evident across other northwestern sites during the same September 2020 period. At Cheeka Peak (WA) and Eugene (OR), both of which were near BB sources during that month, daily PM2.5 reached up to 250 and 450 µgm-3, respectively. O3 similarly increased with PM2.5 under low smoke but leveled off or declined at higher aerosol loadings (Fig. S6). Similar nonlinear O3–PM2.5 relationships have been reported in multi-year, multi-site analyses across the western US, with O3 increasing with PM2.5 under light-to-moderate smoke and decreasing under heavier smoke (Buysse et al., 2019; McClure and Jaffe, 2018).

Figure 7 summarizes the upper-limit chronic inhalation risks from wildfire smoke, assuming that a 2020-style wildfire season were to recur annually over a 70-year lifetime. We do not seek to estimate the total public-health burden of wildfire smoke, but instead use several well-defined screening metrics for PM2.5 and HAPs with available toxicity values as a transparent basis for comparison. Methodological details are described in Sect. S2. Over lifetime exposure (70 years), the total excess cancer risk attributable to wildfire smoke is 100 cases per million people. Roughly 90 % of this smoke-driven risk is attributed to PM2.5, with the remaining 10 % arising from HAPs combined. Among HAPs, the major contributors are formaldehyde (42 % of HAP cancer risk), benzene (34 %), acetaldehyde (8 %), acrylonitrile (9 %), and naphthalene (6 %). These relative contributions agree with recent aircraft observations of lofted plumes and confirm that PM2.5 dominates cancer risk both aloft and at ground level (Pye et al., 2024).

For context, the climatological annual-mean of non-smoke background PM2.5 is ∼ 6 µgm-3 in Missoula. If the same BB-impacted days had instead experienced typical non-smoke background concentrations, the corresponding PM2.5-attributable excess cancer risk would be ∼ 15 cases per million people. Thus, the 2020-style wildfire season increases the PM2.5-attributable cancer risk by ∼ 7× relative to the PM-related baseline. Even using a 10-fold lower PM2.5 unit-risk estimate (4.8 × 10⁻⁵ µg-1m3), PM2.5 still accounts for ∼ 50 % of the total cancer risk, with the total risk remaining ∼ 20 cases per million people (i.e., ∼ 30 % higher than the baseline). Thus, even when the community's exposure to wildfire smoke is annualized and a lower-bound PM2.5 unit-risk estimate is used, aged wildfire smoke at the level experienced in September 2020 in Missoula still shows a quantifiable carcinogenic burden.

Non-cancer chronic risk is expressed as a hazard index (HI), with the dominant effects in this study associated with respiratory and cardiopulmonary systems. The full calculation framework is provided in Sect. S2. The calculated HI = 3 indicates appreciable potential for adverse effects. HAPs account for ∼ 90 % of this index, driven by acrolein (60 % of HAP non-cancer risk), formaldehyde (24 %), acetaldehyde (3 %), and all other species (< 1 % each). PM2.5 contributes to the remaining ∼ 10 %. The result contrasts with aircraft-based assessments, in which PM2.5 dominated non-cancer risk (O'Dell et al., 2020; Pye et al., 2024). The lower relative contribution of PM2.5 observed here likely reflects the evaporation of semi-volatile particulate mass as plumes descend from aloft to the surface (Pagonis et al., 2023; Selimovic et al., 2019, 2020). If PM2.5 were assumed 10 times more toxic, it would instead contribute ∼ 70 % of the HI.

No acute reference exposure levels (RELs) are available for PM2.5, so the acute HI analysis was restricted to HAPs. During September 2020, 719 of 720 one-hour windows exhibited HI < 1, implying negligible acute non-cancer concern. A single exceedance (19 September, HI = 1.05) was apportioned to acrolein (52 %), formaldehyde (32 %), benzene (12 %), and other HAPs (< 5 %). Expanding to eight-hour windows (Fig. S7), 49 % of periods remained below the HI = 1 threshold, whereas 51 % exceeded it, aligned with smoke incursions. Of the exceedances, 30 % fell within HI = 1–2 and 21 % reached up to HI = 5. The 8 h hazards were driven by formaldehyde (38 %), acrolein (31 %), and benzene (29 %), with all other HAPs contributing < 2 %.

The recurrence of the same three drivers across acute and chronic metrics highlights two practical needs. First, smoke-impacted communities would benefit from continuous, high-time-resolution monitoring of formaldehyde, acrolein, and benzene to track rapidly evolving exposures and guide advisories. Second, because particle-only strategies do not address these gases, indoor interventions should pair particle filtration (HEPA or MERV 13+ for PM2.5) with sorbent media (e.g., activated carbon) when using indoor air cleaners during smoke events (Maximoff et al., 2022; May et al., 2021).

Missoula, Montana (46.9° N, 114.0° W), experienced persistent and chemically complex wildfire smoke during September 2020, reflecting the widespread influence of wildfires across California and the Pacific Northwest. We report hourly surface observations of PM2.5, CO, NOx, O3, and 75 speciated VOCs in Missoula during the wildfire-smoke-impacted month of September 2020. Of the 75 measured VOCs, 15 were classified as HAPs by the EPA. Leveraging comprehensive measurements, we quantified EnRs of PM2.5 and VOCs relative to CO in smoke aged for several days, and characterized the temporal evolution of criteria pollutants and VOCs under smoke-impacted and no-/low-smoke conditions. We performed nested GEOS-Chem simulations and compared them with observations to constrain BB emissions and photochemistry in the model. Finally, we assessed public health risks based on regulatory exposure metrics for PM2.5 and HAPs.

We find that combining gas-phase tracers with conventional particle-based diagnostics improves the identification of surface-level wildfire smoke in urban environments. While traditional aerosol-based smoke diagnostics (HMS, AERONET, and PM2.5) remain useful for detecting dense, optically thick plumes, they often misclassify events when plumes are aloft, optically thin, or when rainfall removes aerosols. In contrast, VOC tracers, especially furan and maleic anhydride (MA), provide complementary value: furan identifies fresh smoke, while MA captures more aged plumes. We thus develop a balanced multi-indicator rule that requires concurrent enhancements in at least three independent tracers, thereby capturing smoke episodes with higher confidence. Three major smoke events driven by long-range transport resulted in multi-day elevations of CO and total VOC abundances by factors of 2–3, while ozone increased only modestly and NOx showed little to no change. In addition, PM2.5 and total HAPs (which comprised approximately half of the measured TVOCs by molar fraction) increased by factors of ∼ 3–8 during smoke periods. Concurrently, the total measured OH reactivity approximately doubled (13.4 vs. 6.5 s-1), shifting the relative OHR contributions from biogenic tracers in background conditions toward CO and BB VOCs during smoke. These results highlight the strong influence of regional wildfire smoke on air quality in the western US.

Photochemical-clock analysis constrains ERs for a wide range of primary VOCs in aged BB smoke, yielding values consistent with those reported for western US wildfires. The observed decay patterns of primary VOCs broadly follow their known OH rate constants (kOH). Some underestimation of kOH (by factors of ≈ 3) for reactive VOCs likely arises from rapid depletion, dilution, or background contamination. OVOCs such as methanol, formaldehyde, and acetaldehyde also decrease with age, but their shallow slopes indicate substantial secondary production offsetting primary decay.

Intense smoke episodes in Missoula suppressed surface ozone, with MDA8 O3 declining by up to ∼ 15 ppb (6–7 September), coincident with 50 % reductions in solar radiation and lower planetary boundary layer heights (PBLH), confirming that reduced photolysis and weaker mixing limit in situ ozone formation. Across smoke-impacted days, the MDA8 O3–PM2.5 relationship was non-monotonic: ozone increased under light smoke (PM2.5 < ∼ 20 µgm-3) but tended to flatten or decrease at higher smoke loadings (≳ 20 µgm-3).

Chronic and acute health assessments indicate that prolonged exposure to 2020-level wildfire smoke in Missoula poses quantifiable health risks. If a season of similar intensity were to recur annually, the smoke-attributable excess lifetime cancer risk would be ∼ 100 cases per million people (≈ 7× the non-smoke baseline of ∼ 15 per million for the same days), with PM2.5 accounting for ∼ 90 % of the cancer burden. The chronic non-cancer hazard index (HI) is ∼ 3, with ∼ 90 % attributable to HAPs (driven primarily by acrolein and formaldehyde) and the remaining ∼ 10 % to PM2.5. For acute non-cancer risk, 1 h HI exceedances are rare, but 8 h averaging reveals frequent smoke-aligned exceedances (∼ 51 % of periods with HI > 1). Across both chronic and acute metrics, the repeated importance of the same few HAP drivers (notably acrolein, formaldehyde, and benzene) indicates that mitigating wildfire-related health impacts should extend beyond particle-only strategies to include gas-phase toxics.

The GEOS-Chem simulation reproduces the timing and transport of major smoke plumes (R2 = 0.8–0.9) but underestimates the magnitude of smoke enhancements, with model low biases of ∼ 30 %–40 % for CO and PM2.5 and ∼ 60 %–90 % for most VOCs. The model overestimates OH exposure by roughly 100 %, leading to excessive chemical loss and artificially low EnRs for reactive VOCs. Persistent low EnRs for oxygenated VOCs further suggest missing secondary production during multi-day aging, whereas the +33 % bias in PM2.5 EnR points to overly efficient secondary aerosol formation and/or retention in the GEOS-Chem simplified “simple SOA” treatment. Tripling BB emissions (3× BB) largely improves the modeled primary compounds (e.g., benzene from -70 % to -10 %) and reduces the model bias of OH exposure to ∼ 30 % within smoke-impacted periods, but the modeled PM2.5 is worse (from -30 % to +80 %), demonstrating that uniform emission scaling cannot reconcile errors in both gases and aerosol. Together, these results highlight that reducing model biases in smoky environments requires not only better fire emission magnitudes but also improved representation of missing OH reactivity (e.g., furanoids), secondary OVOC formation, and smoke SOA parameterizations.

GEOS-Chem exhibits a persistent positive bias in September MDA8 O3 in Missoula (+15 ppb on average) and fails to reproduce observed day-to-day variability, regardless of the BB emissions scenario (base, 3 × BB, or noBB). This is in part due to the modeled O3 being too high for general urban background, and the model bias further amplifies on smoke days, reaching +70 ppb and increasing with observed daytime PM2.5 (r ≈ 0.7). More importantly, the model fails to reproduce the observed non-monotonic O3–PM2.5 response, where observations flatten or decline beyond ∼ 30–40 µgm-3. Instead, GEOS-Chem continues to increase O3 with PM2.5. The same monotonic behavior is also present in the independent CMAQ-based AIRPACT forecast system across Missoula and other western US sites, pointing to a shared CTM limitation in representing nonlinear ozone responses under intense BB smoke. These model biases are likely related to overestimated radiation, warm temperature biases, PBLH/entrainment errors, and potentially too-weak heterogeneous NO3/N2O5 loss in CTMs, although isolating the dominant drivers will require additional observational constraints.

GEOS-Chem underpredicts the smoke-attributable lifetime excess cancer risk by 40 % (63 vs. 100 cases per million), due to the underestimated PM2.5. The model's capability to predict non-cancer HI is even worse (∼ 0.3 in GEOS-Chem vs. 3 in observations). The model also underrepresents acute hazards, predicting limited 8 h exceedances (15 h with 8 h HI > 1; max 8 h HI = 1.2), far fewer and weaker than observed (476 h; max 8 h HI = 6). Overall, these results indicate that GEOS-Chem can provide a defensible screening-level estimate of PM2.5-dominated cancer burden, whereas HI estimates remain unreliable without improved BB VOC speciation, secondary production pathways, and inclusion of high-potency toxics such as acrolein.