Measurements of aerosol optical depth above 3570 m asl in the North Atlantic free troposphere: results from ACE-2

As part of the 2nd Aerosol Characterisation Experiment (ACE-2), conducted during summer 1997 in the North Atlantic region between the Canary Islands and Portugal, we measured aerosol optical depths (AOD) at a mid-tropospheric site, near the top of the volcanic mountain "El Teide" (28 degrees 16'N, 16 degrees 36'W, 3570 m asl). Our instrument was located at the highest altitude in a network of sunphotometers that extended down to sea level. Clear conditions dominated the ACE-2 period, and, although suggested by back-trajectories at 300 hPa, no evidence of anthropogenic pollution was found in our data. Three distinct dust episodes were observed. Vertical soundings and back trajectories suggested mineral dust from the Sahel region as a source. During these episodes, AOD increased an order of magnitude with respect to background conditions (from 0.017 up to 0.19 at lambda = 500 nm). A shift towards neutrality of the extinction spectral dependence (Angstrom exponent a down to 0.13), indicated that the coarse mode (particle diameter > 2 mu m) dominated the aerosol size distribution. For 6 days during the episodes of mineral dust, a monomodal size distribution between 2 and 20 mu m diameter was obtained from Mie based size distribution calculations. Estimates, at 500 nm, of the single scattering albedo omega(0) (0.87-0.96), and the aerosol asymmetry parameter g (0.72-0.73) suggest that the dust layer causes a net cooling forcing at the top of the atmosphere.