The hygroscopicity, cloud condensation nucleation (CCN) activity and infrared optical extinction of CaCO3, a reactive component of mineral dust aerosol, and Ca(NO3)2, a product of atmospherically-aged CaCO3 through reaction with nitrogen oxides, have been measured. The hygroscopic growth and CCN activity of Ca(NO3)2 are orders of magnitude greater than CaCO3 and more similar to ammonium sulfate. Ca(NO3)2 particles also reflect a greater amount of near infrared radiation at higher relative humidity. These measurements provide the first quantitative assessment of the important changes in climate forcing that can occur as mineral dust aerosol is transported, reacted and aged in the atmosphere.