A long-term local experiment was designed with the purpose to accurately quantify aerosol parameters needed in order to estimate aerosol climate forcing at an anthropogenically perturbed continental site. Total light-scattering &sgr;λ,sp and backscattering &sgr;λ,bsp coefficients at wavelength λ, the hygroscopic growth factors with respect to scattering, f(RH)λ,s, and the backscatter ratio bλ are the parameters considered in the paper. Reference and controlled relative humidity nephelometry measurements were taken at a ground level field sampling station, located near Bondville Illinois (40¿03'12″N, W 88¿22'19″W). Aerosol particle chemical composition and mass particle size distributions were also measured. The target parameters were also estimated from models. The modeling approach involved a two-step process. In the first step, aerosol properties were parameterized with an approach that made use of a modified thermodynamic equilibrium model, published laboratory measurements of single hygroscopic particle properties, and empirical mixing rules. In the second step, the parameterized aerosol properties were used as inputs into a code that calculate &sgr;λ,sp and &sgr;λ,bsp as functions of λ, RH, particle size, and composition. Comparison between the measured and the modeled results showed that depending on the assumptions, the differences between the modeled and observed results were within 5 to 28% for f(RH)λ,s and within 22--35% for bλ at low RH and 0--20% for bλ at high RH. The temporal variation of the particle size distribution, the equilibrium state of the particles, and the hygroscopicity of the material characterized as residual were the major factors limiting the predictive ability of the models. ¿ 1998 American Geophy