In a campaign in Vienna, the horizontal extinction over the central part of Vienna, the size dependent light absorption, and the mass size distribution have been measured and a size resolved PIXE analysis has been performed. PIXE did not give a complete chemical analysis; therefore the chemical compounds and the amount of nitrogen and organic carbon had to be guessed. The extinction coefficient has been determined by Mic calculation using the measured size distribution, density and refractive index from the chemical compounds. The closure was successful and a linear relationship between measured and calculated extinction coefficient was found, with the measured extinction coefficient about 20% higher than the calculated one. The Angstrom ngstrom exponent was also compared, the averages agreed but the measured values were more variable than the calculated ones. A sensitivity analysis shows that the accuracy of the determination of the optical properties of the aerosol is governed by the accuracies of all input parameters and the generally unknown state of mixing. This leads to uncertainties of the optical parameters of at least +/- 20%. The accumulation mode makes a considerable contribution to the extinction coefficient; therefore knowledge of the size distribution, mass density and refractive index is critical. Lack of information in the coarse mode or the nucleation mode is less critical. Usually extinction coefficients are not measured on a routine basis, instead they are calculated using available aerosol data such as size distributions obtained from models, and densities and refractive indices obtained from chemical analyses or models. In that case larger uncertainties are to be expected.