Precipitation cleanses the air by capturing pollutants and depositing them onto the surface. The efficiency of this process can be expressed by the fractional depletion rate of pollutant concentrations in the air. This rate is generally characterized by the so-called scavenging coefficient. During the parameterization of wet deposition (or wet scavenging) in-cloud scavenging (rainout) and below-cloud scavenging (washout) are usually distinguished. However, for a correct estimation of wet deposition, explicit information about cloud and precipitation water at all model levels would be required. Instead of these complex parameterization schemes (see e.g. Sportisse, 2007), the wet deposition is often estimated by a simple method in chemical transport models. The species dependent scavenging coefficient (Λ in s–1) can take the simple form:
where P is the rate of rainfall (in mm h–1) and A and B are constants for a specific gas or aerosol particles. In some models A and B vary also in the function of scavenging type (rainout, washout), or in case of aerosol particles in the function of particle radius (Baklanov and Sørensen, 2001).
As the rain intensity is usually a very uncertain parameter, a very simple rain-out type method was proposed by Pudykievitz (1989) is based on a statistical parameterization using relative humidity data. In this approach, the wet removal is described by the following relation:
where RH, RH0 and RHs are the actual, the threshold and the saturated values of relative humidity, respectively and Λ is the scavenging coefficient. A suggested value for RH0 is 80%, which means, that wet deposition is only considered in the model, if the relative humidity is larger than 80%.
Wet deposition is the major sink process of aerosol particles (see also Chapter 9.). Particles are incorporated into cloud droplets and precipitation elements by several mechanisms. Some paper separates these processes into two main groups, which are the nucleation scavenging and impactation scavenging (e.g. Tost et al., 2006). In the initial phase of hydrometeor formation the nucleation scavenging of aerosol particles serve as cloud condensation nuclei (CCN) or ice nuclei (IN). In contrast to these mechanisms, impaction scavenging is a process, when aerosol particles collide with and stick to existing cloud droplets, or precipitation elements (e.g. raindrops, snow crystals).
The scavenging of atmospheric gases contains several processes. After their diffusion to the surface of hydrometeors, the gas molecules become dissolved in liquid drops or in a quasi-liquid layer at the surface of ice crystals. In the final step of scavenging mechanism, gases diffuse inside the hydrometeors. Detailed models of gas scavenging can be found for example in Sportisse and du Bois (2002) and Sportisse and Djouad (2003).
One of the main objectives of EMEP (European Monitoring and Evaluation Programme) is to provide information about deposition of different air pollutants, like acidifying pollutants. heavy metals (HM) or persistence organic compounds (POPs) (www.emep.int). Results of the European scale harmonized monitoring network and model simulations show large reductions in deposition of sulphur species during the last decades. However, orographic effects can lead to form local maxima in wet deposition. Due to high annual precipitation amounts, the wet deposition is typically high in southern Norway and the region around the Alps. Wet deposition of nitrogen ranges from less than 1 kg N ha−1 yr−1 to more than 20 kg N ha−1 yr−1. Deposition of oxidized nitrogen is generally somewhat higher than reduced nitrogen in Scandinavia and the Mediterranean, except for a few sites influenced by nearby agriculture. However, in the Benelux area and in Ireland, the contribution of ammonium deposition exceeds that of nitrate, reflecting regional agricultural sources of ammonia (Tørseth et al., 2012)
The wet deposition of calcium in Europe is significantly influenced by Saharan dust. Wet deposition rates exceeding 10 kg Ca ha−1 yr−1 are observed at sites in Spain, Portugal, Italy, Serbia and Croatia. Sites with high precipitation amounts located close to the sea also experience high rates of wet deposition due to sea salt calcium. (Hjellbrekke and Fjæraa, 2011).
Change of heavy metal deposition varied over the European countries. However, both modelling results and observations showed that wet deposition fluxes of both lead, cadmium and mercury are decreased between 1990 and 2010 (EMEP, 2012).
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