Processes determining the relationship between aerosol number and non-sea-salt sulfate mass concentrations in the clean and perturbed marine boundarylayer

An evaluation of the indirect radiative forcing by aerosols requires knowle dge about aerosol number densities, and more particularly the number of par ticles that can be activated in clouds. In this study we present a data set relating the total number (N-TOT) and the number of particles with dry dia meter > 80 nm (N->80) to the aerosol volume and non-sea-salt (nss) SO4 = ma ss (M-SO4). The data refer to submicron aerosol and have been obtained in b oth clean and polluted conditions in the North Atlantic marine boundary lay er (MBL). Over this whole range, the relationships of both N-TOT and N->80 versus M-SO4 are close to linear. Detailed aerosol dynamics modeling shows that dilution of the initial pollution aerosol by entrainment of free tropo spheric (FT) aerosol is the major process determining these relationships. Entrainment further explains our observation that the contribution of nss-s ulfate (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 obse rved. Finally, the linear relationships between aerosol number and M-SO4 su ggest that the observed nonlinear relation between the number of cloud drop lets and M-SO4 must be mainly ascribed to nonlinearities in the cloud activ ation process.