During 2006, a study is undertaken at Uccle to examine the relationship between the vertical column-integrated Aerosol Optical Depth (AOD), the boundary layer aerosol characteristics, and the me teorological parameters, including air mass origin and mixing height. Continuous measurements of the AOD are being performed at 5 wavelengths (from 306 to 320 nm) with a Brewer ozone spectrophotometer. Next to this instrument, four filter samplers (two with PM2.5 inldet and two with PM10 inldet) were set up to obtain aerosol samples for gravimetric determination of the particulate mass (PM) and for detailed chemical aerosol characterisation. The samples are collected during the daytime onldy and on days with no or few clouds when 50% or more valid AOD data are expected. Here, the data obtained during the first 3 months of 2006 are presented; during this period were 35 days with aerosol samplings. The boundary layer mixing height, measured at noon, during the aerosol collection days, varied from 100 to 1750m. The daily mean AOD (at 320 nm) and the PM2.5 mass levels for the 3-month period ranged from 0.16 to 0.67 and from 5 to 70μ g/m 3, respectively. The PM2.5/PM10 mass ratio was, on average, 0.75. Secondary inorganic aerosols (sum of ammonium,nitrate, and sulphate) accounted for more than 50% of the gravimetric PM in PM2.5 (around 50% in PM10). Organic aerosol (estimated as 1.4 times organic carbon) and elemental carbon represented 32% and 4%, respectively, in PM2.5 (28% and 3%, respectively, in PM10). Crustal matter accounted for 3% in PM2.5 (10% in PM10). Preliminary examinations of the relationship between the AOD and the PM2.5 mass and mixing height, thereby using a mass extinction efficiency of 5 m 2/g, indicate that the boundary layer aerosols contribute, on average, for about 20% to the vertical column-integrated AOD.