Dry Deposition of Particles to Water Surfaces

Transport of particles to water surfaces by dry deposition is more complicated than the transport to terrestrial surfaces. This complication is brought about by the fact that water is a smooth surface with intermittent wave action. The waves create bursting bubbles and spray formation which make the modeling of dry deposition to water a very difficult task. The situation is complicated by high humidity near the water surface, the temperature difference between air and water, and movement of water surface. Wind speeds tend to be higher over water than over nearby land.

In spite of the above-mentioned complications, a number of approaches have been made to improve our understanding of dry deposition of particles over the aquatic surfaces over the last two to three decades, with the latest studies within the EU ELOISE projects.

First models simulating the dry deposition to water surfaces were two-layer models including particle transport by eddy diffusion and sedimentation by gravity in the first layer and transport through the boundary layer by Brownian diffusion, inertial impaction, and other mechanisms. The effect of hygroscopic growth due to high humidity in the boundary layer was also incorporated into the models. However, these models assumed a perfect sink surface.

Further development of the dry deposition models to the water surfaces focused on accounting for the breaking of waves by considering the deposition processes to smooth and breaking areas of the water surface, separately.

Major improvement of our understanding of processes involved in dry deposition of fine particles to marine systems have been made through the EUROTRAC projects on Air-Sea Exchange (ASE) and CAPMAN (Coastal Air Pollution Meteorology and Air Sea Nutrient Exchange). An important aspect of the ASE studies has been on the establishment of the magnitude and importance of atmospheric deposition of various nutrients and contaminants to the European coastal waters. Dry deposition of particles has been identified as one of the main pathways of nitrogen to the coastal and sea waters which gives rise to the eutrophication events in various coastal areas around the globe. According to studies within the ELOISE (http://www.eloisegroup.org) projects, as much as 30% of nitrogen input to the European seas comes through the atmospheric deposition, although the main part of it occurs through wet deposition.

Major modeling activity of atmospheric deposition of fine particles containing nutrients and contaminants to the water surfaces, including dry deposition, has been carried out within the international conventions aiming at the protection of the European seas, including the Oslo and Paris Commissions (OSPARCOM) program on reduction of land-based pollutants transported to the North Sea and the North-East Atlantic

(http://www.ospar.org), and the regional programs for the Baltic Sea (Helsinki Commission (HELCOM), (http://www.helcom.fi) and the Mediterranean Sea (Barcelona Convention). Major international activity to assess source-receptor relationships for nutrients and contaminants transported with air masses and undergoing the atmospheric deposition to water and terrestrial surfaces has been carried out within the UN Economic Commission for Europe (UNECE) Convention on LongRange Transboundary Air Pollution Transmission (LRTAP) (http://www.unece.org).

The above-mentioned model activities are supported by long-term and short-term measurement programs. The results of these measurements confirm the importance of dry deposition of particles as one of the main pathway of nutrients and contaminants to the water surfaces on regional and even global scale (e.g., the work within the IGBP Land-Ocean Interactions in the Coastal Zone (LOICZ) Project, http://www.loicz.org).

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