Characterization of size-segregated particles turbulent fluxes in an Arctic city (Fairbanks, Alaska)

Wintertime air pollution affects air quality of Arctic and sub-Arctic urban areas, because of the coupling between strong local emissions for residential heating and energy production and poor atmospheric dispersion associated with a stratified planetary boundary layer. Aerosols represent priority pollutants in such environments, and their behaviour in the Arctic wintertime boundary layer not only impacts air quality but also determines deposition on snow or ice surfaces, leading to chemical and physical modifications in the snowpack. The interactions between boundary layer meteorology and air pollution were the focus of the international ALPACA (Alaskan Layered Pollution and Chemical Analysis) field campaign held in January and February 2022 in Fairbanks (AK, USA). The aim of the present work is to analyse the fluxes of atmospheric particles in at a urban background site in Fairbanks, based on continuous observations of aerosol concentration, size distributions, and size-segregated deposition velocities. The EC system was installed at the suburban site of UAF (University of Alaska Farm), located nearby the foothills bordering the city basin. The main micrometeorological parameters and fluxes (wind field, friction velocity, turbulent kinetic energy, and sensible heat flux) were characterized in terms of boundary layer conditions (occurrence of thermal inversions, dynamic stratifications, vertical wind shear, slope currents, coherent turbulence structures). The aerosol eddy covariance system was based on a condensation particle counter (CPC) - able to measure particles down to 5 nm in diameter - and an Optical Particle Counter Optical Particle Counter (OPC) for evaluating particle fluxes in the accumulation mode (0.25 < dp < 0.8 μm) and quasi-coarse mode (0.8 < dp < 3 μm). The median number concentration was 13 E+3 cm⁻³, 76 cm⁻³ and 0.3 cm⁻³ for ultrafine, accumulation and quasi-coarse particles mode, with higher concentrations found at low wind speeds. The particle fluxes showed a net emission pattern for the ultrafine, accumulation and quasi-coarse dimensional mode, especially in daytime, with average values of 203, 0.3, and 0.02 cm^-2 s^-1 respectively. Deposition periods were observed most frequently for air masses from the city located to the east, while local emission sources due to traffic lead emission fluxes, especially in the accumulation mode. We discuss the particle flux measurements in the context of parallel aerosol and gaseous pollutants determined by fixed and mobile platforms (a tethered balloon and a car) as well as of determinations of depositions in the snow pack across the Fairbanks area.