MODELING OF AEROSOL PROPERTIES RELATED TO DIRECT CLIMATE FORCING

A long-term local experiment was designed with the purpose to accurate ly quantify aerosol parameters needed in order to estimate aerosol cli mate forcing at an anthropogenically perturbed continental site. Total light-scattering sigma(lambda,sp) and backscattering sigma(lambda,bsp ) coefficients at wavelength ii, the hygroscopic growth factors with r espect to scattering, f(RH)(lambda,s), and the backscatter ratio bn ar e the parameters considered in the paper. Reference and controlled rel ative humidity nephelometry measurements were taken ant a ground level field sampling station, located near Bondville Illinois (40 degrees 0 3'12'' N, W 88 22'19'' W). Aerosol particle chemical composition and m ass particle size distributions were also measured. The target paramet ers were also estimated from models. The modeling approach involved a two-step process. In the first step, aerosol properties were parameter ized with an approach that made use of a modified thermodynamic equili brium model, published laboratory measurements of single hygroscopic p article properties, and empirical mixing rules. In the second step, th e parameterized aerosol properties were used as inputs into a code tha t calculate sigma(lambda,sp) and sigma(lambda,bsp) as functions of lam bda, RH, particle size, and composition. Comparison between the measur ed and the modeled results showed that depending on the assumptions, t he differences between the modeled and observed results were within 5 to 28% for f(RH)(lambda,s) and within 22-35% for b(lambda) at low RH a nd 0-20% for b(lambda) at high RH. The temporal variation of the parti cle size distribution, the equilibrium state of the particles, and the hygroscopicity of the material characterized as residual were the maj or factors limiting the predictive ability of the models.