The uptake of HNO3 by volcanic sulfuric acid droplets is investigated using a one dimensional sulfate aerosol model and recent laboratory data. HNO3 uptake by volcanic sulfate particles coupled with the sedimentation of the particles may lead to irreversible transfer of HNO3 from higher to lower altitudes in the polar stratosphere during winter. This denitrification mechanism is found to be strongly dependent on the sulfate aerosol loading and temperature. Model calculations suggest that anomalously large volcanic eruptions have caused a severe denitrificaton of the Arctic winter lower stratosphere. There is conflicting evidence for this mechanism in ice core nitrate records. ¿ American Geophysical Union 1994