Footprint Estimation over Rough Urban Surfaces (FERUS)

When measuring concentrations or turbulent fluxes of atmospheric pollutants (but also water vapor or even heat) it is most often of crucial interest which portion of the upwind surface area has contributed to what degree to this measurement. This upwind surface influence area is called footprint and depends on the measurement site (e.g., measurement height, nearby surface characteristics) and the atmospheric state (stability). Footprints (the area) and the footprint function, i.e. the relative strength of the contribution from any location within the footprint, are determined using methods of atmospheric pollutant dispersion modeling. Despite the apparent need for pollutant source appointment in urban areas, there is no footprint model available that specifically would take into account the dispersion characteristics in the urban atmosphere. The present project aims at changing this. Turbulence, and hence dispersion characteristics in the lowest tens to hundreds of meters over an urban surface are strongly influenced by the presence of buildings (i.e., solid roughness elements) and other structures, their spatial distribution, density and height. Knowledge on this turbulence structure near urban surfaces and from urban pollutant dispersion modeling will be used for the development of an urban footprint model. The model will be tested (validated) using urban tracer release experiments (there are not so many) and recent measurements on the roof of the university building of the applicant. A sensitivity analysis on the input parameters to the model will furthermore be performed in order to detect the most influential among them. The urban footprint model will finally be generalized to other rough surfaces, i.e. other surfaces covered with tall roughness elements such as trees (forested surfaces).

In urban areas there is, on the on hand, the problem that we have many different pollutant sources, and consequently often high pollutant concentrations. On the other hand, pollutant dispersion models have to make a number of assumptions which are not fulfilled - particularly in urban environments. This leads to the situation that we need footprint modeling (source attribution for a measurement location - for example if there are exceptionally high concentrations we need to know 'where they come from') in an environment most, where the tools are least appropriate. This project therefore aims at designing an urban footprint model, which uses alternative approaches in case of the above-mentioned assumptions and is thoroughly tested based on available data from full-scale and (physical) modeling results.