Saharan dust transported and deposited in Finland on 23 February 2021

The Sahara Desert is the largest source of dust worldwide. Finland, north of 60 ^oN, is annually affected by long-range transported Saharan dust, which is most often observed as red sunrises and sunsets. Observations on dust deposition on ground are rare. On 23 February 2021, Saharan dust was transported and deposited in the southern part of Finland, reaching up a long way inland. At the time, the ground was covered with snow, and therefore the dust deposition was more easily detectable. The deposition was accompanied by freezing rain in the most southern part of the country, and snowfall further north.

Samples of dust in snow were collected by citizens and forwarded to the Finnish Meteorological Institute (FMI) following our researchers’ guidelines advertised in social media. Most samples were a solid residue from 2 dl of superficial snow, that had been either melted and filtered using coffee filters, evaporated on an aluminum foil, or decanted with the help of containers. In addition, fresh samples were collected for reference and were stored in a freezer for further analysis. Samples were received from over 500 locations and each of these contained one or more filtered, evaporated, or decanted dust samples. Dust was observed as far north as Vaasa and Kuopio (~63 ^oN).

The event was forecasted by the operational SILAM global atmospheric-composition suite of FMI (http://silam.fmi.fi) five days in advance. The suite is driven by the meteorology from the Integrated Forecasting System (IFS) model of the European Centre for Medium-Range Weather Forecast (ECMWF). According to the model results, the near-surface concentrations of desert dust in Finland on 23.02.2021 were negligible, while the total column reached 100-200 µg/m², and optical column thickness in some places was up to 0.2, which is enough to be visible. The scavenging of dust from aloft layers resulted in substantial contamination of snow. Light microscopy results indicate the presence of quartz particles in the range 5-15 µm compatible with desert dust. Processed samples from the Askola region (~60 °N), about 20 km north from the southern coastline, show depositions of ~1100 mg/m². Dust deposition amounts may vary greatly depending on the location and precipitation amounts. Our work also includes ice nucleation experiments, determination of particle size distributions, investigations on organic compounds, microplastics and microorganisms. The citizen science nature of the project will be used to promote and disseminate FMI’s research on aerosols through a specific outreach programme. Our study aims at producing information on latitudinal Saharan dust transport, as well as on deposition particle shapes, size distributions and ice nucleation ability of the particles detected in Finland, through the analysis of the collected samples.