Denitrification of the polar stratosphere project
People: Prof Ken Carslaw, Stewart Davies and Dr Graham Mann
Funded by: 2 EU Framework 5 grants MAPSCORE and EUPLEX.
Abstract
Simulation of large nitric acid particles in our 3-D model of polar denitrification.
Denitrification is the removal of reactive nitrogen species from the stratosphere. This occurs through gravitational settling of large polar stratospheric cloud particles containing nitric acid. The effect of denitrification is to increase ozone depletion in the polar stratosphere since it hinders an important gas phase chemical reaction that deactivates ozone destroying chlorine species.
In Waibel et al. (Science, 1999), we used a computer model to simulate denitrification in the Arctic stratosphere for the winter 1994/95. The results showed that the temperature of the 1994/95 Arctic vortex was just cold enough to allow significant polar stratospheric cloud formation and subsequent denitrification. We showed that our calculated denitrification agreed well with the limited observations that were then available. In addition, chemical model simulations indicated that a significant fraction of the observed ozone depletion in that winter could be attributed to denitrification.
Simulating denitrification is not easy. First of all, we don't understand polar stratospheric cloud formation well enough to know which particles are causing it. We therefore don't understand what conditions in the Arctic stratosphere will predispose it to denitrification.
Particle observations from the NASA ER-2 aircraft during the SOLVE campaign have led to a step-change in our understanding of denitrification. Particles as large as 20 micrometres (christened 'rocks') were observed over large regions of the Arctic stratosphere. These particles are much larger than previously thought possible, and require us to completely rethink the way we simulate them in models. The first report of these particles and our preliminary modelling calculations can be viewed in this downloadable publication in Science. In addition, this NASA press release describes the background to the discovery of the rocks. Based on results from the SOLVE campaign, we are developing more sophisticated models of denitrification. These will account for the growth kinetics of the cloud particles and the formation and evaporation cycles of particles as they circle the pole. Our eventual aim is to understand the processes well enough to be able to make reliable predictions of denitrification for future Arctic winters, and hence to more accurately predict ozone losses. Development and application of the model is described in some recent publications [Carslaw et al., Mann et al.]
As part of the EU project MAPSCORE (Mapping of Polar Stratospheric Clouds and Ozone Levels Relevant to Europe) we will compare model calculations of denitrification in recent winters with observations. The model denitrification fields will then be used in chemical transport models to investigate the effect on ozone depletion.
In EUPLEX (European Polar Stratospheric Cloud and Leewave Experiment) we will evaluate our model against new observations from the M55 Geophysica aircraft in the Arctic winter 2002/3. Members of the group will also participate in the field campaign in Kiruna, Sweden, to predict the occurrence of large NAT particles for flight-planning purposes.