Extensive populations of large HNO3-containing particles were observed in situ in the 1999--2000 Arctic winter vortex. Assuming nitric acid trihydrate (NAT) composition, the associated values of instantaneous gravitational flux of HNO3 are between 1¿108 and 5¿1010 molec cm-3 km day-1 and are the largest ever derived from in situ measurements. The relationship between these observed population and observed denitrification in the 1999--2000 Arctic vortex developed using constraints of NAT formation and sedimentation. This analysis of the 1999--2000 winter provides a useful guide to interpreting future observations of polar stratospheric clouds and denitrification in the polar regions.