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As part of EPRI's Aerosol Research Inhalation Epidemiology Study (ARIES), measurements of aerosol size distributions in the 3 nm to 2 μm diameter range were carried out over a 24 month period beginning August 1998 in Atlanta, GA. An automated fine particle sampling system was developed for this study. The sampling system utilizes a nano-scanning mobility particle sizer (3 nm < D[sub p] < 50 nm), a standard scanning mobility particle sizer (20 nm < D[sub p] < 0.25 μm), and a laser particle counter (0.1 μm < D < 2 μm). Five complete size distributions are obtained every hour. This paper describes the data collected in the first 13 month period. The year-long data provide detailed information on temporal variations of aerosol size distributions. Our measurements show that particle number concentrations tend to be higher on weekdays than on weekends. Concentrations of particles in the 10-100 nm and 100-2000 nm diameter ranges are higher at night than during the daytime and tend to reach their highest values during morning rush hour. Concentrations of 4-10 nm particles were elevated during rush hour when temperatures were < 10 ° C. Annual average concentrations of particles in the 3-10 nm diameter range peak between 11 a.m. and 2 p.m. due to the appearance of very high concentrations at those times on a few days. We believe these high concentrations resulted from nucleation. We have identified three types of "ultrafine particle" events. On 23 days during August and April we observed events where pronounced peaks in the 3-10 nm size range occurred. These events typically occurred around noon, when solar radiation was high. During winter months, we occasionally observed significantly elevated concentrations in the 10-35 nm diameter range. These events occurred during early morning and late afternoon hours. Relatively high number concentrations in the 35-45 nm diameter range were also detected several times. Elevated concentrations of SO[sub 2] were observed during all three types of events. NO was typically depleted during the 3-10 nm events and was more likely to be elevated during the 10-35 and 35-45 nm events. The sources of these particles are not yet known. [ABSTRACT FROM AUTHOR]