<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>

<p>Local air pollution sources in the Arctic lead to poor air quality in Arctic cities, particularly during the winter months. Fairbanks in central Alaska, is a prime example of such an Arctic city which suffers from acute wintertime pollution episodes. The topography of Fairbanks (situated in a basin), coupled with strong surface-based temperature inversions, contributes to stable meteorological conditions that hinder the dispersion of pollutants and surface temperatures reaching -40 °C. These harsh winter conditions result in enhanced domestic and power plant combustion emissions. Stable meteorological regimes are frequently interspersed with less stable episodes, resulting in vertical mixing between surface and elevated inversion layers. However, there are many uncertainties in our understanding about pollution sources and secondary aerosol formation under cold, dark winter conditions, where photochemistry is limited. These issues were addressed through the collection of comprehensive datasets on atmospheric composition and meteorology in Fairbanks, during the international ALPACA (Alaskan Layered Pollution and Chemical Analysis) field campaign in January and February 2022. Data were collected at the surface and vertical profiles were collected using a tethered balloon (EPFL Helikite).</p> <p>Here, we examine the relative contributions and distributions of power plant emissions, emitted above the surface, and surface emission sources to pollution levels in the Fairbanks region. The FLEXPART-Weather Research and Forecasting (WRF) Lagrangian particle dispersion model, driven by meteorological fields from WRF-Environmental Protection Agency (EPA, Alaska) simulations is deployed. Firstly, model runs are used to evaluate the transport and dispersion of emissions from power plants at several altitudes in and around Fairbanks. Surface-based and elevated temperature inversions, characteristic of the winter boundary layer in Fairbanks, are considered in a parameterisation of power plant plume injection heights, and temporal variations in these emissions is also taken into account. Secondly, the extent to which power plant emissions are contributing to surface pollution is investigated using power plant (point source) and sector-based surface EPA emissions at 1.3km resolution at hourly time resolution during the 2022 campaign period. Model results are evaluated against available vertical profile and ground-based observations from ALPACA 2022. Power plant plumes are simulated aloft at several ALPACA measurement sites, as validated by vertical profile observations. The simulations indicate that power plant emissions are mixed down towards the surface in some cases. These results also provide insights into relative source contributions from each power plant in Fairbanks within the vertical profile of the lower atmospheric boundary layer, which could be used as tool for source apportionment studies.</p>