The formation of new atmospheric particles, roughly 1--2 nm in diameter, has drawn a lot of attention recently because of (1) their possibly negative effects on human health and (2) their potential to grow into cloud condensation nuclei, thus affecting cloud formation as well as the global radiation budget. At these small sizes, the Kelvin effect prevents condensation of any compound in sufficient abundance for producing substantial growth. But heterogeneous reactions may play a critical role in helping these nm-sized nuclei cross the huge Kelvin-effect barrier and facilitating further condensation of organic vapors. Its implications in the marine boundary layer and engine exhausts are discussed.