Standardization of a resistance model for the calculation of nitrogen deposition in the updated German standard VDI 3782-5

The German standard VDI 3782-5 "Deposition Parameters" (German/English, www.vdi.de) provides deposition velocities and washout rates for various gaseous substances and particles. It is applied in local and mesoscale dispersion modelling, for example in the context of the German regulation on Air Quality Control (TA Luft). The current version of the standard dates from 2006. It is based on findings from a limited number of studies that led to the implementation of relatively simple descriptions and only rough estimates of atmospheric nitrogen deposition. The standard is currently undergoing a rigorous scientific revision by the authors on behalf of the VDI.

The updated standard will specify, among others, a model for the calculation of surface resistances, including compensation points for NH₃. The model is based on DEPAC (RIVM, Netherlands) and implemented in Java program (JDepac). JDepac allows parameter variations and time series calculations. Input parameters include date and time, geographical location, land use, meteorological data and, for NH₃, information on current and past loads. Default options are provided for missing input. Output quantities are, among others, resistances, deposition velocities, and deposition fluxes of NH₃, NO, NO₂, HNO₃, SO₂, O₃, Hg and particles.

JDepac is compared to various deposition measurements and results from mesoscale models. For NH₃, effects of the compensation point on the resulting deposition velocities are investigated. JDepac is used to calculate temporal averages of deposition velocities for different land use classes. In combination with dispersion calculations, effective deposition velocities are derived from the calculated deposition fluxes and concentrations. These simpler parameters are straightforward to apply in local dispersion modelling. JDepac itself allows more sophisticated calculations and can be coupled to dispersion and chemical transport models.

The updated standard VDI 3782-5 and its OpenSource tool JDepac are intended to serve as a state-of-the art, practical, and transparent reference for both local and mesoscale calculations of nitrogen deposition. In addition, the standard contains descriptions for the calculation of deposition velocities and washout rates of particles, the calculation of deposition probabilities for Lagrangian particle models, and the effects of drop displacement in wet deposition.

The updated standard is expected to serve as a useful tool for example in the decision process of facility planning and its licensing procedure conducted by local authorities, which is especially critical for the impact assessment on ecosystems under the EU Habitats Directive. In addition, the updated standard is expected to support the harmonization of air pollution modelling within the implementation of the new (2024) EU Ambient Air Quality Directive.