Measurements of chemical, physical, and optical properties of ambient aerosol particles were obtained at Bondville, Illinois. This research was completed to increase the spatial and temporal resolution of measured aerosol. Results from measurements describe (1) the physical and chemical characteristics of the aerosol and (2) the dependence of light scattering and backscattering on wavelength of light λ, controlled relative humidity RH, and aerosol particle chemical composition. Formulations for the hygroscopic growth factor f(RH), and backscatter ratio b, as functions of λ and RH and estimates of the upscatter fraction β¿, and ¿ngstr¿m exponent ¿ are also provided. For aerosol sampled at the site from January to December 1995, the mean gravimetric mass concentration for particles with aerodynamic diameter (dpa)≤1 μm had an arithmetic mean and standard deviation of 10.6¿6.5 μg/m3, respectively. Ion chromatography (IC) speciated 53% of the total gravimetric mass for particles with dpa≤1 μm. Most of the IC-identified material (88.0¿13.5%) consisted of NH4+ and SO42-. Material not identified with IC was primarily elemental and organic carbon. The total aerosol light-scattering coefficient at λ=550 nm and RH≤40% was 51.6¿43.2 Mm-1 for dpa≤10 μm and 42.0¿34.9 Mm-1 for dpa≤1 μm. Mean values of f(RH=82.5%,λ) for total scattering ranged between 1.4 and 1.5 and for back scattering between 1.1 and 1.2. Mean values of b(λ) ranged from 0.11 to 0.18 for RH<40% and from 0.09 to 0.14 for RH=82.5%. Mean values of β¿ ranged from 0.21 to 0.30 depending on λ, RH, and the particle size distribution. Mean values of ¿ rangedbetween 1.8 and 2.4 for RH<40% and 1.8 and 2.1 for RH=82.5%. These measured properties are now available for use in models to reduce uncertainties when quantifying direct aerosol radiative forcing at a continental site influenced by aerosol with anthropogenic origin. ¿ 2001 American Geophysical Uni