7. Deposition

For some pollutants, what matters is not only the concentration in the air but also the amount which settles on the ground. For example, dust particles from a quarry may cause a nuisance down wind. Sulphuric acid which is created from power station emissions causes acidification of soil and can be harmful to plants and animals (e.g. destruction of forests and fish in lakes by acid rain). Methods for calculating the rate of deposition are needed. We will not go into details here. However, some important factors are: (i) Settling rate of particles. This is strongly dependent on the particle size and hence a knowledge of particle size distributions is important. (ii) Dry deposition by diffusion. The flux of a pollutant with concentration $C$ kgm$^{-3}$ through the ground is

F=-\varepsilon _z{\partial C\over \partial z}.\quad\hbox{kgm$^{-2}$s$^{-1}$.}

Note that in the image model for plumes reflecting from the ground, $\partial C/\partial z=0$ at the ground (by symmetry) and hence the deposition rate is zero. (iii) Wet deposition. Important processes are washout (collection of pollutant by rain drops as they fall) and rainout (collection of pollutants by cloud droplets which subsequently form rain drops). Knowledge of precipitation rates is clearly essential.

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