The objective of the NATO Project OPAQUE, a measurement program on the Optical Atmospheric Quantities in Europe, is to develop a data base of atmospheric optical and IR properties in Europe. An important part of this program is the measurement of the distribution of optical properties of aerosols as a function of time, geography and weather. Six OPAQUE stations are presently operational. The concentration and size distribution of aerosol particles is being measured with PMS and Royco particle counters. To verify comparability of these measurements an intercomparison of all OPAQUE aerosol counters was conducted at the Air Force Geophysics Laboratory. Results emphasized the need for proper maintenance and calibration. Properly maintained, all instruments appear to agree within a factor of two. In a supporting effort AFGL has been conducting airborne measurements of aerosol distributions in several of the OPAQUE areas. Measurements of concentration and size distribution have been conducted at altitudes between 300 and 5000 meters. Vertical profiles show very frequently a well defined low level haze layer of 1.5 to 2 km thickness, and a significant change in size distribution with increasing altitude.