Activation and growth of cloud condensation nuclei are investigated. A one-dimensional cloud model including detailed microphysics is developed. The system studied consists of NaNO3 particles and condensing water and HNO3 vapors. According to numerical simulations, enhanced concentrations of atmospheric nitric acid vapor affect cloud formation by increasing the number of cloud droplets and decreasing their mean size compared to circumstances where no condensable vapors other than water are present. It is argued that the cloud albedo increases because of these effects. ¿ American Geophysical Union 1993