In this study an attempt is made to investigate the aerosol spatial and size distributions at different heights over the Greater Athens Area (GAA), Greece, under sea breeze conditions and clear sky and to further discuss possible implications for aerosol characteristics. The data used are airborne measurements of aerosol collected during two flights that were performed within the context of the 1997 STAAARTE experimental campaign. The aerosol measurements cover particle diameters from 0.1 to 45.5 microm. The horizontal and vertical distribution revealed that higher concentrations exist within or just above the atmospheric boundary layer, while greater concentrations are observed over the sea compared to land at high altitudes. At all altitudes the number size distributions show dominant diameter ranges between 0.1 and 0.3 microm at all altitudes. The volume distributions are characterised by two modes, one in the accumulation and the other in the coarse particle regime. At lower altitudes, fresh combustion emissions more likely cause the predominance of the size range 0.1-0.3 microm while enhanced physical and chemical processes that favour the growth of smaller particles to larger sizes could also act. The relative humidity does not seem to affect the observed number size distributions at low altitudes, where relative humidity is below 70% while at 4000 m the distributions seem to change over the sea where the humidity increases.