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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. 

We present emission measurements of volatile organic compounds (VOCs) for western U.S. wildland fires made on the NSF/NCAR C‐130 research aircraft during the Western Wildfire Experiment for Cloud Chemistry, Aerosol Absorption, and Nitrogen (WE‐CAN) field campaign in summer 2018. VOCs were measured with complementary instruments onboard the C‐130, including a proton‐transfer‐reaction time‐of‐flight mass spectrometer (PTR‐ToF‐MS) and two gas chromatography (GC)‐based methods. Agreement within combined instrument uncertainties (<60%) was observed for most co‐measured VOCs. GC‐based measurements speciated the isomeric contributions to selected PTR‐ToF‐MS ion masses and generally showed little fire‐to‐fire variation. We report emission ratios (ERs) and emission factors (EFs) for 161 VOCs measured in 31 near‐fire smoke plume transects of 24 specific individual fires sampled in the afternoon when burning conditions are typically most active. Modified combustion efficiency (MCE) ranged from 0.85 to 0.94. The measured campaign‐average total VOC EF was 26.1 ± 6.9 g kg⁻¹, approximately 67% of which is accounted for by oxygenated VOCs. The 10 most abundantly emitted species contributed more than half of the total measured VOC mass. We found that MCE alone explained nearly 70% of the observed variance for total measured VOC emissions (r² = 0.67) and >50% for 57 individual VOC EFs representing more than half the organic carbon mass. Finally, we found little fire‐to‐fire variability for the mass fraction contributions of individual species to the total measured VOC emissions, suggesting that a single speciation profile can describe VOC emissions for the wildfires in coniferous ecosystems sampled during WE‐CAN.

 

Reactive nitrogen (N_r) within smoke plumes plays important roles in the production of ozone, the formation of secondary aerosols, and deposition of fixed N to ecosystems. The Western Wildfire Experiment for Cloud Chemistry, Aerosol Absorption, and Nitrogen (WE‐CAN) field campaign sampled smoke from 23 wildfires throughout the western U.S. during summer 2018 using the NSF/NCAR C‐130 research aircraft. We empirically estimateN_rnormalized excess mixing ratios and emission factors from fires sampled within 80 min of estimated emission and explore variability in the dominant forms ofN_rbetween these fires. We find that reduced N compounds comprise a majority (39%–80%; median = 66%) of total measured reactive nitrogen (ΣN_r) emissions. The smoke plumes sampled during WE‐CAN feature rapid chemical transformations after emission. As a result, within minutes after emission total measured oxidized nitrogen (ΣNO_y) and measured totalΣNH_x(NH₃ + pNH₄) are more robustly correlated with modified combustion efficiency (MCE) than NO_xand NH₃by themselves. The ratio of ΣNH_x/ΣNO_ydisplays a negative relationship with MCE, consistent with previous studies. A positive relationship with total measuredΣN_rsuggests that both burn conditions and fuel N content/volatilization differences contribute to the observed variability in the distribution of reduced and oxidizedN_r. Additionally, we compare our in situ field estimates ofN_rEFs to previous lab and field studies. For similar fuel types, we findΣNH_xEFs are of the same magnitude or larger than lab‐based NH₃EF estimates, andΣNO_yEFs are smaller than lab NO_xEFs.

 

Wildfires are a major source of gas‐phase ammonia (NH₃) to the atmosphere. Quantifying the evolution and fate of this NH₃is important to understanding the formation of secondary aerosol in smoke and its accompanying effects on radiative balance and nitrogen deposition. Here, we use data from the Western Wildfire Experiment for Cloud Chemistry, Aerosol Absorption, and Nitrogen (WE‐CAN) to add new empirical constraints on the e‐folding loss timescale of NH₃and its relationship with particulate ammonium (pNH₄) within wildfire smoke plumes in the western U.S. during summer 2018. We show that the e‐folding loss timescale of NH₃with respect to particle‐phase partitioning ranges from ∼24 to ∼4000 min (median of 55 min). Within these same plumes, oxidation of nitrogen oxides is observed concurrent with increases in the fraction ofpNH₄in each plume sampled, suggesting that formation of ammonium nitrate (NH₄NO₃) is likely. We find wide variability in how close ourin situmeasurements of NH₄NO₃are to those expected in a dry thermodynamic equilibrium, and find that NH₄NO₃is most likely to form in fresh, dense smoke plumes injected at higher altitudes and colder temperatures. In chemically older smoke we observe correlations between both the fraction ofpNH₄and the fraction of particulate nitrate (pNO₃) in the aerosol with temperature, providing additional evidence of the presence of NH₄NO₃and the influence of injection height on gas‐particle partitioning of NH₃.