is a subarctic city with fine particle (PM 2.5 ) concentrations that exceed air quality regulations in winter due to weak dispersion caused by strong atmospheric inversions, local emissions, and the unique chemistry occurring under the cold and dark conditions. Here, we report on observations from the winters of 2020 and 2021, motivated by our pilot study that showed exceptionally high concentrations of fine particle hydroxymethanesulfonate (HMS) or related sulfur-(IV) species (e.g., sulfite and bisulfite). We deployed online particle-into-liquid sampler−ion chromatography (PILS-IC) in conjunction with a suite of instruments to determine HMS precursors (HCHO, SO 2 ) and aerosol composition in general, with the goal to characterize the sources and sinks of HMS in wintertime Fairbanks. PM 2.5 HMS comprised a significant fraction of PM 2.5 sulfur (26−41%) and overall PM 2.5 mass concentration of 2.8−6.8% during pollution episodes, substantially higher than what has been observed in other regions, likely due to the exceptionally low temperatures. HMS peaked in January, with lower concentrations in December and February, resulting from changes in precursors and meteorological conditions. Strong correlations with inorganic sulfate and organic mass during pollution events suggest that HMS is linked to processes responsible for poor air quality episodes. These findings demonstrate unique aspects of air pollution formation in cold and humid atmospheres.

In Fairbanks, Alaska, (64.8°N) surface-based inversions (SBIs), where temperature increases with altitude above ground-level, are frequent during winter, occurring historically over 50% of the time from November to February/ March (Bourne et al., 2010;Fochesatto & Mayfield, 2013;Tran & Mölders, 2011). These SBIs are most likely to occur in the dark winter when skies are clear as heating by incoming radiation is absent, but thermal radiation is lost continuously from the ground, leading to radiative cooling at the surface (Wendler & Jayaweera, 1972). When low-level clouds are present, they increase downwelling thermal radiation toward the surface, causing a warming effect. Therefore, clouds weaken SBIs, an effect commonly observed in Fairbanks (Bourne, 2008;Kankanala, 2007;Wendler & Jayaweera, 1972). Sunlight has potential to end an SBI, but daylight hours are short and surface heating from solar radiation is weak due to low light intensity and the high shortwave reflectivity of snow on the ground, so it is common to see SBIs persist during winter daytime in Fairbanks (Bourne, 2008;Kankanala, 2007;Tran & Mölders, 2011).Wintertime SBIs in Fairbanks have a complex, multilayered structure. Layers of air with larger positive temperature gradients are observed from the surface to around 100 m and weaker positive gradients are observed above 100 m. In an analysis of Fairbanks radiosonde data, Fochesatto and Mayfield (2013) defined the top of the

Fairbanks-North Star Borough (FNSB), Alaska perennially experiences some of the worst wintertime air quality in the United States. FNSB was designated as a "serious" nonattainment area by the U.S. Environmental...

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<p>The ALPACA (Alaskan Layered Pollution and Chemical Analysis) field campaign (January-February 2022) aimed to collect new data to document Arctic wintertime air pollution. State of the art instrumentation was deployed in Fairbanks, Alaska to characterise inorganic/organic aerosols, vertical layering and mixing of aerosols and precursors, and meteorology at sites influenced by local anthropogenic emissions and background Arctic Haze.</p><p>Vertical profiles of the boundary layer composition were collected from an instrumented tethered ballon (helikite) deployed at the UAF-Farm site in West Fairbanks. The Helikite payload included instruments dedicated to the characterisation of particles (concentration, composition, size distribution) and to measurement of trace gases with dedicated analysers for O3, CO and CO2 and a MICROMEGAS instrument. MICROMEGAS is a light-weight package based on low-cost <em>Alphasense</em> electrochemical sensors for trace gases (CO/O3/NO/NO2). This instrument was also deployed on the ground close to reference-grade trace gas analysers at the CTC measurement site in downtown Fairbanks, and onboard a vehicle for 2D-mapping of pollution within and around Fairbanks.</p><p>Low-cost electrochemical sensors are sensitive to temperature and humidity and require careful calibration and validation. We first introduce the calibration method based on multi-linear regression with the collocated CTC reference measurements. The performance (biases, correlation coefficients, RMSDs) of the calibrated data are then evaluated against CTC observations not used for the calibration. Cases of vertical helikite profiles with polluted layers related to specific dynamical conditions (temperature inversions, wind regimes…) are investigated. Tracer-tracer relationships (CO, NO, NO2 versus CO2 ; NOx versus Ox) together with meteorological observations are used to examine air mass origins (domestic combustion, vehicles, power plants), as well as dilution and chemical transformation of the sampled pollution plumes.</p>