Climate impact of solid ammonium sulfate aerosols as ice nuclei

Laboratory experiments show that solid ammonium sulfate aerosols can act as heterogeneous ice nuclei particles (INPs) in the deposition mode. In this study we used the coupled IMPACT/CAM5 model to track the efflorescence and deliquescence of ammonium sulfate. Liquid pure ammonium sulfate particles effloresce when RHw is below 34% and dissolve when RHw is above 79%. About 1/3 of the total simulated sulfate aerosol mass is in the solid state. When 0.1% of these solid ammonium sulfate aerosols are assumed to be efficient ice nuclei, they act to reduce the bias in simulated ice particle number concentration in cirrus clouds by reducing ice particle number in cold cirrus clouds (T<205 K) and increasing ice particle number in warm cirrus clouds (T>215K). The increased ice particles in warm cirrus clouds lead to an increase in the IWP and a net global warming effect as large as 3 W/m2. The magnitude of the warming effect can vary if a different efficiency of solid ammonium sulfate aerosols acting as ice nuclei particles is assumed.