The model presented predicts visibility degradation due to aerosol, using thermodynamic equilibrium calculations and Mie theory. Observed scattering coefficients bsca at Mead View, Nevada, are successfully predicted. The model is used to examine the dependence of bsca on certain changes in the ambient sulfate concentrations, as well as the sensitivity of bsca to the ambient relative humidity. The model predicts that aerosol water plays an important role in the visibility degradation at relative humidities higher than the deliquescent point of the aerosol particles. We have also shown that reduction in the ambient sulfate levels does not necessarily result in improvements in the ambient visibility, unless it is accompanied by reductions in the ambient nitrate concentrations. ¿ American Geophysical Union 1989