scope:

Scope and objective

The guideline contains theoretical determination methods and values, which have been derived from measurements, for the deposition velocity and the washout rate of aerosols and trace gases. These parameters, briefly termed deposition parameters in the following, are required for calculating the dry and wet deposition flux to the ground or to near-ground boundary surfaces by dispersion models, such as Gaussian plume models (VDI 3782 Part 1 [T1]), Gaussian puff models (VDI 3945 Part 1 [T3]) and particle models (VDI 3945 Part 3 [T4]). They may also be used for estimating, on the basis of known trace substance concentrations in the air, the flux to the ground by dry and wet deposition. For modelling transport over long distances, as can be done using the particle model described in VDI 3945 Part 3 [T4], the washout of trace substances in clouds shall be additionally considered in the determination of wet deposition, to be able to close the mass balance.

Theoretical approaches are presented for the deposition parameters. An analytical calculation rule is described for aerosols, and numerical values for different surface conditions are given for selected trace gases. The information is based on a literature study; in Annex D, the evaluated literature and the values quoted therein are listed.

Due to the complexity of the physical processes contributing to deposition, some of the values given in literature have a wide range. Besides, not all of the numerous studies on this subject could be considered despite an extensive review of the literature. The recommendations of this guideline have to be seen against this background.