Size distributions of 3-10 nm atmospheric particles: implications for nucleation mechanisms

The formation of new atmospheric particles by gas-to-particle conversion le ads to enhanced concentrations of nanoparticles. We have studied the format ion and growth of new particles in urban Atlanta and in the remote atmosphe re in locations ranging from the North Pole to Mauna Loa, Tasmania and the South Pole. Key to this work was our development of new measurement techniq ues for freshly formed nucleation mode particles between 3 and 10 nm. In th is paper we show that measured aerosol size distributions in the 3-10 nm di ameter range often increase with decreasing size down to our minimum detect able size of 3 nm, presumably because nucleation was occurring during the m easurement. Furthermore, we show that the Atlanta nucleation mode size dist ributions are consistent with a collision-controlled nucleation process in which accommodation coefficients for all collisions between condensing mole cules and molecular clusters and between molecular clusters are assumed to be equal to one, and in which evaporation from molecular clusters is neglec ted, as would be expected for a highly supersaturated vapour.