Characterization of size-segregated particles turbulent flux and deposition velocity by eddy correlation method at an Arctic site

Abstract. Estimating aerosol depositions on snow and ice surfaces and assessing the aerosol lifecycle in the Arctic region is challenged by the scarce available measurement data for particle surface fluxes. This work aims at assessing the deposition velocity of atmospheric particles at an Arctic site (Ny-Ålesund, Svalbard Islands) over snow, during the melting season and over dry tundra. The measurements were performed using the eddy‐covariance method from March to August 2021. The measurement system was based on a condensation particle counter (CPC) for ultrafine particle (UFP, < 0.25 μm) fluxes and an Optical Particle Counter (OPC) for evaluating particle size fluxes in the accumulation mode (ACC, 0.25 < dp < 0.7 μm) and quasi-coarse mode (CRS, 0.8 < dp < 3 μm). Turbulent fluxes in the ultrafine particle size range resulted prevalently downward especially in summertime. By contrast, particles fluxes in the accumulation and quasi-coarse mode were more frequently positive especially during the colder months, pointing to surface sources of particles from e.g., sea spray, snow sublimation or local pollution. The overall median deposition velocity (V_d⁺) values were 0.90, 0.62 and 4.42 mm s^-1, for UFP, ACC and CRS, respectively. Deposition velocities were smaller, on average, over the snowpack with median values of 0.73, 0.42 and 3.50 mm s^-1. The observed velocities differ of less than 50% respect to previous literature in analogous environments (i.e. ice/snow) in the particles in the size range 0.01–1 μm. At the same time, an agreement with the results of predictive models was found for only a few parameterizations, especially with Slinn (1982), while large biases were found with other models especially in the range 0.3 – 10 µm of particle diameters. Our observations show better fit with the models predicting a minimum deposition velocities for small accumulation mode particle sizes (0.1 – 0.3 μm) rather than for larger ones (~ 1 µm), which could result from an efficient interception of particles over snow surfaces which are rougher than the idealized ones. Finally, a polynomial fit parameterization was proposed in this study to describe the deposition velocity observations which properly represents their size-dependence and magnitude.