12.2. Dry deposition of aerosol particles

12.2.1. Field measurements

Particle dry deposition measurements are usually performed by micrometeorological methods (gradient method, eddy covariance method) and throughfall method (Garland, 2001). Applied micrometeorological methods are similar as in case of gases (12.1.1). The througfall method is generally used in case of forest canopy for quantification of soil load (Draaijers et al., 1996).

12.2.2. Modelling the dry deposition of particles

Dry deposition of particles is highly depends on their characteristic size. Smaller particles deposited similarly as gases, while removal of larger ones is mainly governed be gravitational settling (sedimentation). Dry deposition processes for particles with a size between 10–1 and 1 µm are less efficient.

The dry deposition velocity after can be written as (see e.g. Sportisse, 2007):

,

(12.9)

where vg is the gravitational settling velocity (or terminal settling velocity), and the resistance terms Ra, and Rb are the same term as for gases (see above). As we assume that all deposited particles stick to the surface, the surface resistance (Rc) is zero for particles.

The settling velocity (vg) can be derived as:

,

(12.10)

where ρp is the density of the particle, dp is the particle diameter (assuming a spherical particle), g is the acceleration of gravity, C is a correction factor and η is the viscosity coefficient of air.