[1] Nitric acid-containing particles with diameters of 10-20 mm were detected inside the Arctic polar vortex in the period January to March 2000. We present the results of a unique three-dimensional microphysical simulation of these large HNO 3 -containing particles covering the entire Arctic vortex. The model describes the simultaneous growth, evaporation, sedimentation, and advection of several thousand individual nitric acid hydrate particles over their complete lifetime. We compare modeled and observed particle size distributions as a test of different particle nucleation mechanisms. The model is able to produce particles with sizes typical of those observed and broadly reproduces the change in particle characteristics through the winter assuming nitric acid trihydrate (NAT) particle growth. The possibility that the observed large nitric acid-containing particles were composed of nitric acid dihydrate (NAD) cannot be excluded within the uncertainty of the HNO 3 field above the aircraft. The formation of nitric acid hydrate particles on synoptic ice clouds may be a source of some of the observed large nitric acid-containing particles. However, a direct, but highly selective, nucleation of NAT or NAD particles over wide regions appears to be necessary to explain the observations.

Abstract.A PSC was detected on 6 February 2003 in the Arctic stratosphere by in-situ measurements onboard the high-altitude research aircraft Geophysica. Low number densities (∼10 −4 cm −3 ) of small nitric acid (HNO 3 ) containing particles (d<6 µm) were observed at altitudes between 18 and 20 km. Provided the temperatures remain below the NAT equilibrium temperature T NAT , these NAT particles have the potential to grow further and to remove HNO 3 from the stratosphere, thereby enhancing polar ozone loss. Interestingly, the NAT particles formed in less than a day at temperatures just slightly below T NAT (T >T NAT −3.1 K). This unique measurement of PSC formation at extremely low NAT saturation ratios (S NAT 10) constrains current NAT nucleation theories. We suggest, that the NAT particles have formed heterogeneously, but for certain not on ice. Conversely, meteoritic particles may be favorable candidates for triggering NAT nucleation at the observed low number densities.

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We have used a three‐dimensional chemical transport model to quantify Arctic ozone loss in 2004/2005 and compare it to other winters through 2006. Relative to Arctic stratospheric variability, 2004/05 was a very cold winter with large regions of possible NAT (nitric acid trihydrate) and ice polar stratospheric cloud formation. These areas were the largest during the past 12 years in January and the vortex area was similarly the largest in early March. Accordingly, the model produces strong denitrification, extensive chlorine activation and large chemical ozone loss of up 75% locally and ∼140 DU in the vortex‐averaged column, which slightly overestimates that derived from observations. Compared with similar calculations for recent years, 2004/05 compares with 1999/2000 as one of maximum modelled loss. Sensitivity experiments show that small regions of extreme ozone loss, near 100% at some altitudes, could have happened if the winter of 2004/05 was followed by a spring like 1997 with a long‐lasting cold polar vortex.

[1] We have used the SLIMCAT three-dimensional chemical transport model together with observations from the Stratospheric Aerosol and Gas Experiment (SAGE III) Ozone Loss and Validation Experiment (SOLVE) and the Third European Stratospheric Experiment on Ozone (THESEO 2000) to quantify the effect of denitrification on Arctic ozone loss. We have used two different denitrification schemes in the model: one based on the sedimentation of ice particles containing cocondensed nitric acid trihydrate (NAT) and the other based on large NAT particles. The model was forced using both UK Meteorological Office (UKMO) and European Centre for Medium-Range Weather Forecasts (ECMWF) analyses. In the Arctic lower stratosphere the UKMO analyzed temperatures are similar to the ECMWF, except at temperatures near the ice point where the UKMO analyses are colder by over 2 K. Consequently, the UKMO analyses predicted large regions of ice clouds, in contrast to the ECMWF. The denitrification scheme based on large NAT particles gives the best agreement with ER-2 NO y observations for both sets of meteorological analyses. Although the ice scheme and UKMO analyses also produce denitrification, the vertical extent of denitrification and renitrification does not agree as well with the observed NO y . Uncertainties in the budget of ClO y observations from the ER-2 prevent an indirect validation of the best model denitrification scheme based on these data. The denitrified model runs give the best agreement with the observed HCl and ClONO 2 reservoirs in mid March. However, UKMO-forced runs generally overestimate the observed ClO x during the same period. The denitrified model runs indicate that by late March 56-74% O 3 loss had occurred at 460 K and that denitrification contributed 21-30% of this loss. The model runs showing the largest O 3 depletion (forced by UKMO analyses) agree well with ER-2 and ozone sonde data, although these runs overestimated ClO x .