A local optical closure experiment in Vienna

In a campaign in Vienna, the horizontal extinction over the central part of Vienna, the size dependent light absorption, and the mass size distributio n have been measured and a size resolved PIXE analysis has been performed. PIXE did not give a complete chemical analysis; therefore the chemical comp ounds and the amount of nitrogen and organic carbon had to be guessed. The extinction coefficient has been determined by Mic calculation using the mea sured size distribution, density and refractive index from the chemical com pounds. The closure was successful and a linear relationship between measur ed and calculated extinction coefficient was found, with the measured extin ction coefficient about 20% higher than the calculated one. The Angstrom ng strom exponent was also compared, the averages agreed but the measured valu es were more variable than the calculated ones. A sensitivity analysis show s that the accuracy of the determination of the optical properties of the a erosol is governed by the accuracies of all input parameters and the genera lly unknown state of mixing. This leads to uncertainties of the optical par ameters of at least +/- 20%. The accumulation mode makes a considerable con tribution to the extinction coefficient; therefore knowledge of the size di stribution, mass density and refractive index is critical. Lack of informat ion in the coarse mode or the nucleation mode is less critical. Usually ext inction 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 a nalyses or models. In that case larger uncertainties are to be expected. (C ) 2001 Elsevier Science Ltd. All rights reserved.