Assessment of aerosol dry depositions and their impact on snow composition at an Arctic urban site

Dry depositions contribute to regulate the lifetime of the aerosol in the atmosphere and at the same time they are responsible for the surface flux of nutrients, reactive compounds and pollutants. In polar areas, in particular, atmospheric depositions represent an important source of uncertainty in assessing the lifetime of particulate matter and short-living climate forcers (including black carbon, cloud condensation nuclei and ice nuclei). Dry depositions are deemed to affect snow composition (in terms of reactive compounds, light-absorbing species and persistent pollutants), although its relative importance with respect to wet depositions remains undetermined. There is a paucity of observational data of size-segregated particle fluxes in polar areas, which remains a challenge for the development of reliable parameterizations, given the peculiarities of the turbulence in the polar boundary layer as it is affected by the low solar angles, the presence of a snowpack, the strong surface radiative cooling.

The Alaskan Layered Pollution and Chemical Analysis (ALPACA) experiment is the first, comprehensive air quality study at a urban Arctic location. In the frame of ALPACA, atmospheric transport and vertical distribution of anthropogenic aerosols were investigated by a suite of experimental and modelling approaches. At the same time, the characteristic of the atmospheric boundary layer and surface fluxes of energy and particles have been investigated, while the composition of surface snow was determined on a daily basis. ALPACA was conducted in Fairbanks (AK, US) in Jan – Feb 2022 and comprehensive boundary layer observations were carried out at the sub-urban “Farm” location, over a large, flat terrain with little local pollution sources. In the dark Arctic winter, minimum temperatures dropped as low as -35 °C during the first part of the campaign. As a result of the surface cooling, the temperature gradient reached 10 °C in the first 10 meters above the ground. However, surface-based inversions were systematically perturbed by changes in the surface radiative budget caused by the intermittent presence of clouds and by surface winds promoted the thermal gradients between the Fairbanks plain and the surrounding elevated terrains. Whenever the surface inversions shrank and sufficient amount of aerosol was present in the lower levels, a clear surface particle surface flux was observed by means of an eddy-covariance technique. Such fluxes were intensified during the first part of the campaign when anthropogenic pollution developed in the lower atmospheric layers. In the same period, inorganic and organic compounds in surface snow progressively accumulated in absence of precipitations. The assessment of size-segregated particle fluxes and the analysis of particulate matter composition enabled to quantitatively assess atmospheric dry depositions. Their contribution to the evolution of snow chemistry was species-dependent, but in general dry depositions were found to be a significant sources of pollutants in snow during ALPACA. The effect of meteorology, vertical aerosol distribution and aerosol mixing state on the fluxes on the snowpack are discussed.