[1] A simple dimensionless parameter, L, is shown to determine whether or not new particle formation can occur in the atmosphere on a given day. The criterion accounts for the probability that clusters, formed by nucleation, will coagulate with preexisting particles before they grow to a detectable size. Data acquired in an intensive atmospheric measurement campaign in Atlanta, Georgia, during August 2002 (ANARChE) were used to test the validity of this criterion. Measurements included aerosol size distributions down to 3 nm, properties and composition of freshly nucleated particles, and concentrations of gases including ammonia and sulfuric acid. Nucleation and subsequent growth of particles at this site were often dominated by sulfuric acid. New particle formation was observed when L was less than $1 but not when L was greater than $1. Furthermore, new particle formation was only observed when sulfuric acid concentrations exceeded 5 Â 10 6 cm À3 . The data suggest that there was a positive association between concentrations of particles produced by nucleation and ammonia, but this was not shown definitively. Ammonia mixing ratios during this study were mostly in the 1 to 10 ppbv range.

[1] Continuous measurements of hygroscopicity and volatility of atmospheric aerosol particles of 4-10 nm diameter were conducted with a nanometer tandem differential mobility analyzer (Nano TDMA) during the Aerosol Nucleation and Real-time Characterization Experiment (ANARChE), which took place in Atlanta in July and August 2002. In the Nano TDMA measurements, particles were exposed to either a high humidity ($90% RH) or an elevated temperature ($100°C) downstream of the first differential mobility analyzer (DMA) and were then resized by the second DMA to determine the change in size due to water uptake or evaporation. There were several days when nucleation occurred and high concentrations of sub-20 nm particles were observed, and during those events, particles of 4-10 nm diameter were very hygroscopic and nonvolatile. These observations, together with parallel Thermal Desorption Chemical Ionization Mass Spectrometer (TDCIMS) measurements of sub-20 nm particle composition, suggest that the particles were mostly composed of ammoniated sulfates. This finding strongly supports the hypothesis that the nucleation and subsequent growth of nanoparticles were driven by reactions involving sulfuric acid and ammonia during this study.