Polar vortex concentricity as a controlling factor in Arctic denitrification

[1] Recent in situ observations in the Arctic stratosphere have detected nitric acid-containing particles with sizes up to 10-mum radius and number concentrations between 10(-5) and 10(-3) cm(-3). Here we quantify the effect of these particles on Arctic denitrification by using a new three-dimensional (3-D) model which can couple particle growth and sedimentation with the full dynamics of the Arctic polar vortex. We show that the very long growth times of large nitric acid trihydrate (NAT) particles lead to a highly nonlinear dependence of Arctic denitrification on the growth and evaporation cycles of individual particles, thus making denitrification dependent on the precise meteorological conditions in a given winter. Using 3-D wind and temperature fields from December 1999, we identify a period that was optimum for denitrification, in which the cold pool and vortex were nearly concentric and in which a large proportion of the particles were able to sediment over about 8 days through the full depth of the cold pool without evaporating. We then show that small departures from concentric conditions can lead to substantial reductions in denitrification. A case is presented in which denitrification was completely shut off even with over half of the cold pool area remaining within the vortex. Under the same conditions, a model in which the particles were assumed to be in continuous equilibrium with the gas phase caused extensive denitrification. Our results show that low Arctic vortex temperatures in themselves are unlikely to be a reliable indicator of potential denitrification if large NAT particles are involved.