Size distributions of urban Atlanta, Georgia, aerosols (0.003-2 microm) were measured from August 1, 1998 through August 30, 2000 as part of the Aerosol Research Inhalation Epidemiology Study (ARIES). Size distributions were measured five times per hour, and approximately 50,000 size distributions were measured during the 25-month study. This paper focuses on salient features of the sub-100-nm data. We examine concentrations of particles in six equally spaced logarithmic intervals and show that particles of different sizes have distinctly different behaviors. For particles between 10 and 100 nm, average concentrations tended to be highest during winter, during rush hour, and on week days. Concentrations of particles in the 3-10-nm range were elevated in the summer due to photochemically driven nucleation, and also during winter. We hypothesize that the elevated wintertime concentrations of these particles were associated with nucleation that occurs as vehicular emissions mix with the cool ambient air. In any given size range, distributions of concentrations tend to be lognormal, but significant deviations from lognormality were occasionally observed. For particles in the 3.2-5.6-nm diameter range, deviations were apparent in the summer when very high concentrations (up to 10(6) cm(-3)) were produced by photochemically driven nucleation. During 2 months of the study, deviations from lognormality for particles in the 32-56-nm diameter range occurred when anomalously high concentrations of 40-nm particles were observed.