Continuous measurements of the optical and microphysical properties of aerosol particles have been made at the Department of Energy's Atmospheric Radiation Measurement Program Southern Great Plains Cloud and Radiation Testbed (CART) site covering the 4-year period from July 1996 through June 2000. Hourly, daily, and monthly statistics have been calculated that illustrate aerosol variability over a range of timescales. A pronounced peak in total particle number, centered on the midafternoon hours (local time), is evident in the hourly statistics. A broad early morning peak in the concentration of particles >0.1-μm aerodynamic diameter corresponds with a similar peak in aerosol light-scattering coefficient, &sgr;sp. No strong cycles were observed in the daily statistics, suggesting that day of the week has only a minor influence on the observed aerosol variability. The &sgr;sp at a wavelength of 550 nm for the 4-year period showed a median value of 33 Mm-1 and was highest in February and August. The median fraction of aerosol light scattering at 550 nm due to particles <1-μm aerodynamic diameter was 0.85 over the entire record. The median aerosol light absorption coefficient, &sgr;ap, for the 4-year period was ~1.5 Mm-1 and was observed to be highest in late summer and autumn. The &sgr;ap showed an increasing trend of nearly 0.5 Mm-1 yr, possibly due to increased agricultural field burning in the area. The occurrence of an autumn decrease in single-scattering albedo, ω0, was observed and may be caused by regional-scale agricultural or transportation activities or seasonal changes in atmospheric flow patterns. The median value for ω0 over the 4-year period was 0.95, but this value has decreased ~1--2% yr-1 presumably due to increased agricultural burning. Numerous field fires during the second half of 1999 influenced the surface aerosol at the CART site causing substantial variability of aerosol optical properties. The aerosol hygroscopic growth factor (f(RH)), corresponding to a relative humidity increase of 40--85%, showed a median value of 1.83 for 1999, although much lower values were observed during periods that were probably influenced by locally generated smoke and dust aerosols (median f(RH)=1.55 and 1.59, respectively). ¿ 2001 American Geophysical Union