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 measurement data available 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 were prevalently downward, especially in summertime. In contrast, particle fluxes in the accumulation and quasi-coarse mode were more frequently positive, especially during the colder months, pointing to surface sources of particles from, for example, sea spray, snow sublimation, or local pollution. The overall median deposition velocity (Vd+) 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 by less than 50 % with respect to the previous literature in analogous environments (i.e. ice/snow) for 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, in particular 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 a better fit with the models predicting a minimum deposition velocity for small-accumulation-mode particle sizes (0.1–0.3 µm) rather than for larger ones (about 1 µm), which could result from an efficient interception of particles over snow surfaces which are rougher and stickier than the idealized ones. Finally, a polynomial fit was investigated (for the ACC-CRS size range) to describe the deposition velocity observations which properly represents their size dependence and magnitude. Even if this numerical fit is driven purely by the data and not by the underlying chemical–physical processes, it could be very useful for future model parameterizations.