Explosive volcanic eruptions inject a large amount of sulfur dioxide and ashes into the stratosphere and the upper troposphere, where they may modify the cirrus microphysics through perturbing ice formation. However, the clear evidence of volcanic aerosol impacts on cirrus clouds and the acting ice nucleation mechanisms remain elusive. Here we analyze satellite observation data of cirrus microphysics and aerosol extinction (DARDAR, CALIPSO level 3 stratospheric aerosol profile product). We show that microphysics of midlatitude cirrus clouds is altered by the enhanced stratospheric aerosol loadings produced by three moderate-magnitude midlatitude explosive eruptions (Kasatochi in 2008, Sarychev in 2009, and Calbuco in 2015). Out of the three eruptions, two (Kasatochi, Sarychev) produce volcanic ash (VA)-rich aerosols that are potent ice nucleating particles (INPs) while the other one (Calbuco) is ash-deficit. In the ash-rich cases, post-eruption cirrus clouds have up to 5 times less ice crystal number concentrations compared to cirrus in volcanically quiescent periods, indicative of heterogeneous freezing on VA suppressing the homogeneous freezing. Conversely, cirrus clouds under the influence of the Calbuco eruption in 2015 are found to have up to 2.2 times more ice crystals with minimal VA, implying a moderately enhanced homogeneous freezing on volcanic sulfate aerosols. This suppression of homogeneous freezing by heterogeneous freezing on VA is supported by process-level cloud microphysical simulations and the global CESM2-WACCM6 simulations of Sarychev volcanic eruption in 2009. Our findings highlight rarely investigated VA aerosol-ice cloud interactions and shed new light on the feasibility of cirrus geoengineering.