Introduction

Any constituent of the atmosphere (thus including air pollutants) may be removed and transferred to the earth's surface by a variety of mechanisms. This transfer is known under the generic term of deposition. The choice of this term is not very appropriate in so far as it covers a variety of processes that may be considerably different from each other. To give a few examples, gaseous molecules such as ozone are taken up by vegetation by penetration into the plant tissues or are destroyed at the surface by reaction. Such a process is not really deposition, but by convention, the term is used. Particulate matter (atmospheric aerosol) is really deposited as a consequence of two contributing processes: gravitational settling and atmospheric turbulence, which brings the particles in contact with the surface in a quasi-random way. Removal of dissolved matter may occur by precipitation; in this case, the term fall would be more appropriate. However, the term deposition is used widely. By convention, these processes are subdivided in two categories. The expression dry deposition is used for any transfer process of gaseous or particulate matter from the atmosphere to the surface. Wet deposition corresponds to the transfer of dissolved matter to the surface. I will deal with these two categories successively. Table 23.1 summarizes the deposition processes, the main mechanisms by which they occur, and the substances involved.