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Abstract We present measurements of volatile organic compounds (VOCs) and other trace gases taken in Salt Lake City, Utah in August and September 2022. As part of the Salt Lake regional Smoke, Ozone and Aerosol Study (SAMOZA), 35 VOCs were measured with two methods: a proton‐transfer‐reaction time‐of‐flight mass spectrometer (PTR‐ToF‐MS) and 2,4‐dinitrophenylhydrazine (DNPH) cartridges analyzed by high‐performance liquid chromatography (HPLC). Over two months, the total measured VOCs averaged 32 ± 24 ppb (mean ± standard deviation) with the hourly maximum at 141 ppb, and the total calculated OH reactivity averaged 3.7 ± 3.0 s⁻¹(maximum at 20.7 s⁻¹). Among them, methanol and ethanol were the most abundant VOCs, making up 42% of the ambient mixing ratio. Isoprene and monoterpenes contributed 25% of the OH reactivity from VOCs, while formaldehyde and acetaldehyde made up another 30%. The positive matrix factorization analysis showed 5 major sources of VOCs, with 32% of abundance being attributed to secondary production/biogenic sources, 44% from the combination of traffic and personal care products, 15% from industrial solvent use, and the rest from biomass burning (10%). Moderate smoke‐impacted days elevated various hazardous air pollutants (HAPs) on average by 45%–217% compared to smoke‐free days. The ratio of OH reactivity from NO_xto that from VOCs showed that ozone production was mostly VOC‐limited throughout the campaign, consistent with our modeling study. VOCs and NO_xboth showed increased OH reactivity due to smoke influence. NO_xfeatured increased reactivity on weekdays compared to weekends, an effect not shown for VOC reactivity during SAMOZA. 

Rapid production of formic acid in biomass burning smoke is not captured by the Master Chemical Mechanism (MCM) nor simplified GEOS-Chem chemistry, likely due to missing secondary chemical production. 

Abstract. The Arctic is a climatically sensitive region that has experienced warming at almost 3 times the global average rate in recent decades, leading to an increase in Arctic greenness and a greater abundance of plants that emit biogenic volatile organic compounds (BVOCs). These changes in atmospheric emissions are expected to significantly modify the overall oxidative chemistry of the region and lead to changes in VOC composition and abundance, with implications for atmospheric processes. Nonetheless, observations needed to constrain our current understanding of these issues in this critical environment are sparse. This work presents novel atmospheric in situ proton-transfer-reaction time-of-flight mass spectrometry (PTR-ToF-MS) measurements of VOCs at Toolik Field Station (TFS; 68∘38′ N, 149∘36' W), in the Alaskan Arctictundra during May–June 2019. We employ a custom nested grid version of theGEOS-Chem chemical transport model (CTM), driven with MEGANv2.1 (Model ofEmissions of Gases and Aerosols from Nature version 2.1) biogenic emissionsfor Alaska at 0.25∘ × 0.3125∘ resolution, to interpret the observations in terms of their constraints onBVOC emissions, total reactive organic carbon (ROC) composition, andcalculated OH reactivity (OHr) in this environment. We find total ambientmole fraction of 78 identified VOCs to be 6.3 ± 0.4 ppbv (10.8 ± 0.5 ppbC), with overwhelming (> 80 %) contributions are from short-chain oxygenated VOCs (OVOCs) including methanol, acetone and formaldehyde. Isoprene was the most abundant terpene identified. GEOS-Chem captures the observed isoprene (and its oxidation products), acetone and acetaldehyde abundances within the combined model and observation uncertainties (±25 %), but underestimates other OVOCs including methanol, formaldehyde, formic acid and acetic acid by a factor of 3 to 12. The negative model bias for methanol is attributed to underestimated biogenic methanol emissions for the Alaskan tundra in MEGANv2.1. Observed formaldehyde mole fractions increase exponentially with air temperature, likely reflecting its biogenic precursors and pointing to a systematic model underprediction of its secondary production. The median campaign-calculated OHr from VOCs measured at TFS was 0.7 s−1, roughly 5 % of the values typically reported in lower-latitude forested ecosystems. Ten species account for over 80 % of the calculated VOC OHr, with formaldehyde, isoprene and acetaldehyde together accounting for nearly half of the total. Simulated OHr based on median-modeled VOCs included in GEOS-Chem averages 0.5 s−1 and is dominated by isoprene (30 %) and monoterpenes (17 %). The data presented here serve as a critical evaluation of our knowledge of BVOCs and ROC budgets in high-latitude environments and represent a foundation for investigating and interpreting future warming-driven changes in VOC emissions in the Alaskan Arctic tundra. 

Abstract. The impact of biomass burning (BB) on the atmospheric burden of volatile organic compounds (VOCs) is highly uncertain. Here we apply the GEOS-Chemchemical transport model (CTM) to constrain BB emissions in the western USA at ∼ 25 km resolution. Across three BB emission inventorieswidely used in CTMs, the inventory–inventory comparison suggests that the totals of 14 modeled BB VOC emissions in the western USA agree with eachother within 30 %–40 %. However, emissions for individual VOCs can differ by a factor of 1–5, driven by the regionally averaged emissionratios (ERs, reflecting both assigned ERs for specific biome and vegetation classifications) across the three inventories. We further evaluate GEOS-Chemsimulations with aircraft observations made during WE-CAN (Western Wildfire Experiment for Cloud Chemistry, Aerosol Absorption and Nitrogen) andFIREX-AQ (Fire Influence on Regional to Global Environments and Air Quality) field campaigns. Despite being driven by different global BBinventories or applying various injection height assumptions, the model–observation comparison suggests that GEOS-Chem simulations underpredictobserved vertical profiles by a factor of 3–7. The model shows small to no bias for most species in low-/no-smoke conditions. We thus attribute thenegative model biases mostly to underestimated BB emissions in these inventories. Tripling BB emissions in the model reproduces observed verticalprofiles for primary compounds, i.e., CO, propane, benzene, and toluene. However, it shows no to less significant improvements for oxygenatedVOCs, particularly for formaldehyde, formic acid, acetic acid, and lumped ≥ C3 aldehydes, suggesting the model is missing secondarysources of these compounds in BB-impacted environments. The underestimation of primary BB emissions in inventories is likely attributable tounderpredicted amounts of effective dry matter burned, rather than errors in fire detection, injection height, or ERs, as constrained by aircraftand ground measurements. We cannot rule out potential sub-grid uncertainties (i.e., not being able to fully resolve fire plumes) in the nestedGEOS-Chem which could explain the negative model bias partially, though back-of-the-envelope calculation and evaluation using longer-term groundmeasurements help support the argument of the dry matter burned underestimation. The total ERs of the 14 BB VOCs implemented in GEOS-Chem onlyaccount for half of the total 161 measured VOCs (∼ 75 versus 150 ppb ppm−1). This reveals a significant amount of missing reactiveorganic carbon in widely used BB emission inventories. Considering both uncertainties in effective dry matter burned (× 3) and unmodeledVOCs (× 2), we infer that BB contributed to 10 % in 2019 and 45 % in 2018 (240 and 2040 Gg C) of the total VOC primaryemission flux in the western USA during these two fire seasons, compared to only 1 %–10 % in the standard GEOS-Chem.