| | | 2.5 Deposition pathways |
There are two pathways for the atmospheric deposition, wet and dry. The two processes are completely different in their nature, one depends on solubility and whether it rains or not, the other depends on a whole number of physical, chemical and biological properties of the surface. In this section the processes of wet and dry deposition will be described. How to quantify the deposition is the subject of Sections and .
Wet deposition is the process where gaseous and particulate components are scavenged (taken up) by the means of cloud, fog and rain droplets or snowflakes and subsequently transferred to the ground. Four different processes can be distinguished [411]:
In this text we will focus on the most common of these processes, namely the precipitation scavenging.
Three steps are necessary for wet deposition to take place. Firstly the pollutants need to come into contact with condensed water in the atmosphere, secondly the pollutants must be scavenged by the droplets and thirdly it then has to start raining before the condensed water evaporates back into water vapor, thereby releasing the pollutants back into the air.
Concerning the wet scavenging, two different processes can be distinguished: below-cloud scavenging and in-cloud scavenging. The first refers to the process where pollutants are taken up in falling raindrops. This is also denoted rainout. The second refers to the process where pollutants are taken up in droplets inside the cloud. These pollutants will only wet deposit if it starts raining from the cloud in which case the process is denoted washout.
Most of the pollutants in the air are deposited in the beginning of a rain event, i.e. the air is left more or less clean afterwards. The result is that wet deposition is a very episodic phenomenon where the day to day variation can cover several orders of magnitude.
Dry deposition is the process of transferring gaseous and particulate components to the ground in the absence of precipitation. Turbulence in the air is responsible for this process together with a number of other factors. Turbulence in air can best be described as a movement of the air in eddies and whirls which twine in and out of themselves. As the air in this way describes a circle-like motion, some of the pollutants in the air ends up sticking to the surface, and this process is denoted dry deposition. It depends on a number of factors, some of which include:
In general the deposition follows the turbulence level, i.e. the higher the level of turbulence, the more material is transported to the surface and subsequently deposited. The deposition also follows the roughness of the surface as a consequence of the greater production of turbulens over a rough surface. The deposition to a surface covered by vegetation is furthermore enhanced by the uptake of components through the stomatal openings, which again has a diurnal as well as a seasonal variation. The result is that the deposition is higher to surfaces covered by forest (particularly in the growth season) or buildings than to smooth surfaces like calm water surfaces or fields with low vegetation.
The vegetation itself influences the dry deposition through the process of capture of gases. This process depends on the ability of the plant to take up the pollutant in question. Uptake will e.g. not take place if the leaf pores (stomata) are closed as is the case during night and winter time. For gaseous compounds it is the chemical properties such as reactivity and solubility that are important, whereas for particulate materiale it is the physical properties such as hygroscopisity, size, density and shape that influence the dry deposition process. Finally some chemical species are insoluble in water and will therefore not deposit to water surfaces or wet (e.g. dew-covered) land surfaces.
In the most widely used formulation for dry deposition it is assumed that the dry deposition flux is directly proportional to the concentration C of the chemical component being deposited:
F=-vd Cwhere F is the vertical dry deposition flux, i.e. the amount of material depositing to a unit surface per unit time. The constant of proportionality between flux and concentration, i.e. -vd is denoted the deposition velocity and has the units of length per unit time. The advantages of this formulation of dry deposition is that all of the complexities of the process are represented in one single parameter, the deposition velocity. The disadvantage is that vd depends on many different physical and chemical properties and therefore can be quite difficult to quantify. An example of a vd parameterisation is given in Box .
In order to determine the deposition velocity vd several different descriptions have been constructed, the most popular being the resistance model for dry deposition (see e.g. [411]). In this description the deposition is represented by three steps: (1) aerodynamic transport down through the atmospheric surface layer to a very thin layer of stagnant air just adjacent to the surface; (2) molecular (for gases) or Brownian (for particles) transport across this thin stagnant layer of air called the quasi-laminar sublayer, to the surface it self; and (3) uptake at the surface. Each of these steps contributes to the value of the deposition velocity vd.
In order to actually calculate a value of vd it has proven useful to think of the three steps of the deposition process mentioned above as a series of resistances to deposition (the analogue is to electrical resistances in series and the calculation method when electrical resistances are combined). For dry deposition the resistances are determined:
When the values of the resistances have been determined the deposition velocity can be calculated as:
vd=(1)/(ra+rb+rc)
Several different mathematical expressions are proposed for the three different resistances, however many of these are quite complicated and the interested reader is referred to [411] for further investigations.
| | | 2.5 Deposition pathways |