Homogeneous and heterogeneous growth mechanisms for cirrus cloud formation are considered in light of recent measurements showing the presence of NH4+ ions in the aerosol of the upper troposphere. Low-temperature ternary phase diagrams of (NH4)2SO4-H2SO4-H2O are presented, and the thermodynamic and the kinetic data of the phase transitions are employed to consider the implications of NH4+ ions on the mechanisms of cirrus cloud formation. The analysis concludes that changes in the degree of ammoniation can lead to a crossover in the cloud growth system from homogeneous to heterogeneous nucleation. A corresponding change in the cloud's particle distribution alters the cloud's radiative properties, persistence, and surface area available for heterogeneous chemistry. ¿ 1998 American Geophysical Union