Dry deposition of particles to building surfaces and soiling

A separated turbulent flow around a cube with slightly rough surfaces of 3=3 cm area was produced to simulate particle deposition on edges or other small scale structures of building walls. Polydisperse soot particles with a mass mean diameter of 0.8 mm produced by atomising a suspension of India ink and monodisperse fluorescent 0.6 mm latex spheres were used. The deposition velocity of the soot particles was determined by measuring the change in reflectance of the surrogate surfaces and the light absorption coefficient of the aerosol. The deposited fluorescent spheres were counted by means of a fluorescence microscope. Deposition velocities of the fluorescent spheres used in outdoor experiments were equal to deposition velocities of the soot particles used in a small wind tunnel. The density of the deposit was different on each side of the cube and also inhomogeneously distributed on each individual surface. The highest deposition was always found on the edges of the cube. This peculiar deposition pattern could be explained by the characteristics of the turbulent flow field around the surfaces which was measured by laser Doppler anemometry. Modelled soiling constants calculated with the help of the measured deposition velocities were up to a factor of 10 higher than values obtained for a flat plate in a simple boundary layer flow.