During the SAGE III Ozone Loss and Validation Experiment/Third European Stratospheric Experiment on Ozone 2000 mission, the Multiangle Aerosol Spectrometer Probe (MASP) was flown aboard the ER-2 aircraft sampling the Arctic polar vortex. On several of the flights the ER-2 flew directly through polar stratospheric clouds (PSCs), and on six of these flights the MASP sampled large particles (2--22 ¿m diameter). MASP large particle observations are correlated in time with the large particle signal from the NOAA NOy measurements. This is direct evidence supporting the inference of particle size made by the NOy instrument. On 20 January 2000, the day of the most intense PSC encounter, the shapes of the large particle size distributions above 9 ¿m observed by the MASP and simulated based on the NOy instruments are in reasonable agreement. The MASP observations show a dramatic increase in large particle volume as the temperature approaches the nitric acid trihydrate equilibrium temperature. Our study suggests that large particles containing nitric acid are present in the Arctic polar stratosphere and may lead to denitrification.