Accurate evaluations of the optical thicknesses produced by tropospheric aerosol particles at visible and near-infra-red wave-lengths can be obtained from Sun photometer measurements taken at high mountain stations, provided that the attenuation due to other atmospheric constituents is calculated with high precision. For this purpose, particulate extinction models based on fourteen Junge-type size distribution curves of tropospheric particles were determined to represent the average spectral features of aerosol particle populations in the upper troposphere. Moreover, procedures suitable for determining the optical thicknesses produced by Rayleigh scattering, water vapour, ozone and nitrogen dioxide are herein proposed. In particular, use is made of a method which determines the atmospheric content of nitrogen dioxide by separation of gaseous absorption from particulate extinction. Sun-photometric measurements have been examined by following the above procedures and by using the particulate extinction models to obtain realistic estimates of both the total particle number and particulate mass loading in the atmospheric column.