Growth rates of freshly nucleated atmospheric particles in Atlanta

During the Atlanta ANARChE Study of July and August 2002, atmospheric aerosol size distributions from 3 to 2000 nm were measured continuously with 5-min resolution. Sulfuric acid vapor concentrations were also measured. During regional nucleation events these data showed the presence of a nucleation mode that grew at rates ranging from 3 to 20 nm h^-1. In this paper we compare these measured modal growth rates with calculated rates that account for sulfuric acid condensation, intramodal coagulation of nucleation mode particles, and extramodal coagulation of nucleation mode particles with preexisting particles. Data collected during six time intervals were amenable to analysis. Calculated and measured growth rates were in reasonable agreement for the four events that involved growth below 40 nm (ratios of measured to calculated growth rates = 1.0, 2.1, 0.68, 0.60). Two of the three afternoon events involved growth above 40 nm, and in these cases, measured rates substantially exceeded calculated rates by factors of four to five, suggesting that our model did not account for all growth processes. We also compared observed rates of change in nucleation mode number concentration with calculated coagulation rates during these six time intervals. During the sub-40 nm growth events, particle concentrations changed at rates that were significantly below calculated coagulation rates. In two of these cases, particle concentrations increased during the growth period, suggesting that a source of particles was present. Measured size distributions suggest that particle production by nucleation continued during these events and contributed to this discrepancy. Concentrations during the super-40 nm events decreased at rates that exceeded calculated coagulation rates.