Aerosol properties were measured during two transects over the North Atlantic between Halifax (Nova Scotia, Canada) and the Moroccan Coast. Measurements of the chemical composition of total aerosol, of the black carbon concentration and of the number size distributions with particle diameter Dp in the range 16 nm >Dp>1 μm were made. The e-folding lifetime of the black carbon aerosol, coming from the northeast American continent and transported eastward over the ocean, was estimated to be 15 hours. the non-sea-salt (nss) fraction of the sulfate concentrations encountered during this campaign spans a 3 order of magnitude range (0.002 μg m-3 to 19 μg m-3) and shows a high correlation with black carbon. The measured bimodal aerosol size distributions were analysed in order to yield number concentrations of the nuclei and the accumulation mode (ACM), the latter being interpreted as cloud-processed particles and thus as cloud condensation nuclei (CCN). A strong positive correlation was found between ACM number concentration and nss-sulfate load over the whole concentration range, i.e., for clean to polluted air masses. Furthermore, our regression between nss-sulfate and ACM number concentration also agrees well with results from other investigators where CCN or cloud droplet concentrations were related to nss-sulfate at a variety of geophysical locations and degrees of pollution. The composite data set shows that the nss-sulfate-CCN relationship from baseline conditions to anthropogenically conditioned aerosol, happens via a smooth transition which can be described by a linear regression on a logarithmic scale. ¿ American Geophysical Union 1995.