Condensation sinks for sulfuric acid are predicted using measured aerosol number distributions for five sites, Baring Head and Cape Grim (clean marine), Tarawa (equatorial clean marine), the East Antarctic Plateau and the free troposphere (ultra clean). The condensation sink determined at each site allowed an estimate of the minimum level of sulfur dioxide required for new sulfate particle formation at that site. The sulfur dioxide concentration required for new particle production is nearly two orders of magnitude larger than the observed average at the sites assessed, except the free troposphere. In the marine boundary layer any available sulfur dioxide is converted heterogeneously to sulfate via the condensation sink. This implies that a significant proportion of the sub-micron sulfate aerosol in the marine boundary layer is likely to have been entrained from the free troposphere, where a limited condensation sink and sufficiently high levels of sulfur species can support homogeneous nucleation.