An evaluation of the indirect radiative forcing by aerosols requires knowledge about aerosol number densities, and more particularly the number of particles that can be activated in clouds. In this study we present a data set relating the total number (NTOT) and the number of particles with dry diameter >80 nm (N>80) to the aerosol volume and non-sea-salt (nss) SO4= mass (MSO4). The data refer to submicron aerosol and have been obtained in both clean and polluted conditions in the North Atlantic marine boundary layer (MBL). Over this whole range, the relationships of both NTOT and N>80 versus MSO4 are close to linear. Detailed aerosol dynamics modeling shows that dilution of the initial pollution aerosol by entrainment of free tropospheric (FT) aerosol is the major process determining these relationships. Entrainment further explains our observation that the contribution of nss-sulfate (i.e., (NH4)x(SO4)y) to the dry MBL aerosol mass decreases from over 85% near the continent to 45--70% in more remote and clean conditions, as smaller contributions of sulfate to the FT aerosol mass have been observed. Finally, the linear relationships between aerosol number and MSO4 suggest that the observed nonlinear relation between the number of cloud droplets and MSO4 must be mainly ascribed to nonlinearities in the cloud activation process. ¿ 1999 American Geophysical Union