The potential impact of volcanic aerosols on nucleation of ice crystals in upper tropospheric cirrus clouds is examined from a microphysical perspective. The sulfuric acid aerosols which form in the stratosphere are presumably transported into the troposphere by sedimentation and tropopause folding. The tropospheric volcanic aerosol size distribution is estimated from 10 &mgr;m lidar backscatter measurements [Post, 1986> and in situ measurements [Pueschel et al., 1992b>. Microphysical simulations suggest that at temperatures below about -50 ¿C the concentration of ice crystals which nucleate may be as much as a factor of 5 larger when volcanic aerosols are present. Our simulations suggest that the presence of volcanic aerosols may increase the net radiative forcing (surface warming) of certain types of cirrus near the tropopause by as much as 8 W/m2. Further observations are required to determine whether these effects actually occur, and their global impact. ¿ American Geophysical Union 1992