Atmospheric biogenic volatile organic compounds in the Alaskan Arctic tundra: constraints from measurements at Toolik Field Station
Authors: Selimovic Vanessa , Ketcherside Damien , Chaliyakunnel Sreelekha , Wielgasz Catie , Permar Wade , Angot Hélène , Millet Dylan B. , Fried Alan , Helmig Detlev , Hu Lu 
Published: June 2022

Abstract. The Arctic is a climatically sensitive region that has experienced warming at almost three 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, 68º38’ N, 149º36’ W), in the Alaskan Arctic tundra during May–June 2019. We employ a custom nested grid version of the GEOS-Chem chemical transport model, driven with MEGANv2.1 (Model of Emissions of Gases and Aerosols from Nature version 2.1) biogenic emissions for Alaska at 0.25 º × 0.3125º resolution, to interpret the observations in terms of their constraints on BVOC emissions, total reactive organic carbon (ROC) composition, and calculated OH reactivity (OHr) in this environment. We find total ambient mole 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 higher carbon containing biogenic emission 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 oxygenated VOCs 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 Alaska 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 Toolik 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 modelled 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.