Abstract
In situ measurements of dry aerosol optical properties were made during the North Atlantic Aerosol Characterization Experiment in the vicinity of the Canary Islands. Profiles of aerosol scattering in 3 wavelengths and absorption coefficients are presented for 2 cases where North African mineral dust layers were found above a marine boundary layer (MBL) influenced by varying pollution level. The scattering and absorption coefficients showed large differences in magnitude and variability; not only between the 2 cases of mineral dust (e.g., the mean scattering in the green wavelength was 9.8×10-6±1.6×10-6 m-1 and 2.1×10-5±7.3×10-6 m-1), but also within a given layer (e.g., from less than 1×10-5 to 3.1×10-5 m-1 during the second flight). This was also true for the polluted MBL where it is likely due to origin, age and history of the air mass as well as on the mixing state of the MBL. The scattering, backscattering and absorption coefficients have been used to calculate aerosol optical properties such as hemispheric backscatter fraction, single scattering albedo, aerosol optical depth and the Ångström exponent, which are also presented. The hemispheric backscatter fraction was found to be smaller in the mineral dust layer than in the MBL, as was the wavelength dependence on scattering. The single scattering albedo of dry aerosols was found to vary between 0.73±0.12 and 0.91±0.12 in the dust layer and showed large variability within and between the dust cases. This might be due to the mixing state of the dust layer, but could also be explained by the dust having different optical properties depending on local origin within the same major source region.