Large nitric acid trihydrate particles and denitrification caused by mountain waves in the Arctic stratosphere

Abstract

The contribution of stratospheric mountain waves to the formation of large nitric acid trihydrate (NAT) particles and subsequent denitrification of the Arctic polar vortex is calculated for the 1999/2000 winter using a three‐dimensional (3‐D) model. The model production mechanism involves the formation of NAT clouds with high particle number concentrations downwind of mountain wave ice clouds, as has been previously observed. These wave‐induced NAT clouds then serve as “mother clouds” for the release of low concentrations of sedimenting NAT particles, following the mechanism of Fueglistaler et al. (2002a). Our calculations show that wave‐induced NAT mother clouds can occupy up to 5–10% of the volume of air below the NAT temperature. NAT particles that have sedimented from the mother cloud bases can occupy in excess of 60% of the NAT supersaturated region. Integrated over the entire vortex at the end of the 1999/2000 simulation, it is estimated that denitrification due to this mechanism could potentially be responsible for as much as 80% of that observed. These results show that mountain waves may contribute to the occurrence of solid polar stratospheric clouds more than was previously thought and that they may play a significant role in denitrification.