"Although footprint modelling has become an important and widely used tool in micrometeorology, the validation of existing models remains an outstanding problem. This study presents experimental approaches using natural tracer measurements to evaluate the performance of footprint models, which are intended to provide an inexpensive and practical alternative to footprint evaluation experiments releasing artificial trace gases. The approaches tested and discussed here are based on either eddy-covariance measurement complexes or small aperture scintillometers operated during field scale experiments in an area of well-defined heterogeneity. We tested two different footprint validation approaches based on natural tracer measurements: firstly, a comparison of measured flux differences and modelled land use differences for pairs of measurement positions and secondly, a correlation analysis between measured and modelled quantities using reference measurements. We tested these footprint validation approaches on two different footprint models, an analytical flux source area model and a forward Lagrangian stochastic trajectory model. For the test we utilised measurements from an intensive field scale measurement campaign from summer 2002 instead of performing a new experiment to derive the required data set. The results of this study clearly demonstrate that the two approaches tested here based on natural tracers provide a valuable tool for footprint validation purposes. The first approach highlighted qualitatively the agreement between flux differences and differences in the land use of the surrounding terrain determined by footprint calculations, while the second approach, the correlation analysis between measured and modelled quantities, also allowed the comparison of the two footprint models on a quantitative basis. However, due to experimental shortcomings of the employed data set, additional scatter compromised the accuracy of the results; therefore, no significant differences between both footprint models could be found in this study. (c) 2006 Elsevier B.V. All rights reserved."