NOy Measurements in Polar Stratospheric Clouds and the Gas Phase - Results of the EuPLEx project

A PSC has been detected in February 2003 in the Arctic stratosphere by in-situ instruments onboard a high-altitude research aircraft. Small nitric acid (HNO3) containing particles - probably consisting of solid nitric acid trihydrate (NAT) – with diameters up to 6 micrometers and number densities of 3.5 10-4 per cubic centimeter have been measured at altitudes between 18.3 and 19.5 kilometers. Upon growing to larger sizes, these particles have the potential to remove HNO3 from the stratosphere, thereby enhancing polar ozone loss. Interestingly, these NAT particles formed in less than a day at temperatures 0 to 3 Kelvin below the NAT equilibrium temperature TNAT. For the first time, this unique experimental situation at threshold PSC formation conditions allows to constrain current hypotheses of NAT formation. We find that NAT nucleation on meteoritic smoke might explain the present observations, and that this process might be the origin of low number density NAT PSCs