Optical properties of urban aerosols in the region Bratislava–Vienna I. Methods and tests

Aerosol optical data obtained by means of ground-based methods are applied to determine microphysical properties of aerosols in the atmosphere of Vienna-city. The measured aerosol extinction coefficient σσA serves as a source of information on the ambient aerosols. A large database of extinction efficiency factors for a set of irregularly shaped as well as the spherical particles of various sizes is pre-calculated and employed in the inversion procedure. The assumed particle models differ in chemical composition and are representative for most typical aerosol systems in the urban atmospheres. All database records are taken into a regularization scheme to solve the inverse problem for aerosol size distribution using measured extinction data. In addition, subsidiary data on spectral sky radiance are successfully incorporated into the mathematical model to retrieve the information on aerosol effective refractive index in the visible.As for Vienna, the aerosol extinction is a decreasing function of wavelength in visible spectrum—it indicates the predominance of sub-micrometer-sized particles in the atmosphere. The surface distribution function s(r)=dS/dr of aerosol particles customarily peaks at radii r≈0.2–0.3 μm, while the volume distribution function v(r)=dV/dr∼rs(r) has a mode at radii about 0.3–0.4 μm. Analysing size distributions dV/d log(r) for irregularly shaped particles it is shown that the daily profile of this function is smoothly evolving and almost typically accounts for a first mode at radii between 0.8 and 0.9 μm.