Ground-based sunphotometry measurements can be used to investigate atmospheric aerosol optical properties, such as the volume size distribution, an important parameter in the study of the effect of aerosol on atmospheric processes. Most inversion algorithms assume constant aerosol optical characteristics over the whole air column. In this work we present observational evidence of the limitations of this simplifying assumption in cases where the aerosol vertical
structure is highly inhomogeneous. During the field campaign VELETA 2002, carried out in Granada (Spain), a quite complete characterization of the atmospheric aerosol was obtained by simultaneously measuring the columnar aerosol characteristics, by means of CIMEL C318 sun-tracking photometers, the size-segregated near-surface aerosol mass concentration by a GRIMM 1108 dust monitor and the aerosol vertical profiles by a lidar system. During the last days of the campaign, a dust-rich air mass from the Sahara reached the site, producing a multilayered structure on the aerosol vertical profile. The ground level size distributions can be compared with the columnar ones using retrieved scale height values from a lidar extinction coefficient profiles, corresponding to the altitude where the integrated extinction is equal to 1-e-1 of the AOD. Comparisons of the column-integrated and the modified ground-level aerosol size distributions show a good agreement in the days previous to the arrival of the Saharan intrusion, when the aerosols are homogeneously distributed in a well-mixed boundary layer. But, when the vertical homogeneity is reduced due to elevated layers containing desert dust, the column properties clearly deviates from the surface properties. This indicates the importance of verifying the vertical distribution of aerosol in order to correctly relate column and ground-level optical properties.