Abstract
We review current knowledge about the annual cycle of transport of nitrogen oxides to, and removal from, the polar stratosphere, with particular attention to Antarctica where the annual winter denitrifi cation process is both regular in occurrence and severe in effect. Evidence for a large downward fl ux of NOy from the mesosphere to the stratosphere, fi rst seen briefl y in the Limb Infrared Monitor of the Stratosphere (LIMS) data from the Arctic winter of 1978-1979, has been found during the 1990s in both satellite and ground-based observations, though this still seems to be omitted from many atmospheric models. When incorporated in the Stony Brook -St. Petersburg two dimensional (2D) transport and chemistry model, more realistic treatment of the NOy fl ux, along with sulfate transport from the mesosphere, sulfate aerosol formation where temperature is favorable, and the inclusion of a simple ion-cluster reaction, leads to good agreement with observed HNO3 formation in the mid-winter middle to upper stratosphere. To further emphasize the importance of large fl uxes of thermospheric and mesospheric NOy into the polar stratosphere, we have used observations, supplemented with model calculations, to defi ne new altitude dependent correlation curves between N2O and NOy. These are more suitable than those previously used in the literature to represent conditions within the Antarctic vortex region prior to and during denitrifi cation by Polar Stratospheric Cloud (PSC) particles. Our NOy -N2O curves lead to a 40% increase in the average amount of NOy removed during the Antarctic winter with respect to estimates calculated using NOy -N2O curves from the Atmospheric Trace Molecule Spectroscopy (ATMOS)/ATLAS-3 data set.
Highlights
The production and transport of oxides of nitrogen in the atmosphere and their subsequent cryogenic removal by Polar Stratospheric Clouds (PSCs) over polar regions constitutes one of the most fascinating atmospheric cycles traced out in recent years
Transport from the winter mesosphere into the stratosphere is well established over both polar regions, denitrification of the lower stratosphere is quite variable from year to year in the Arctic under current climatic conditions, and we concentrate here on the more regular and severe denitrification process of the Antarctic
Measurements made with the Stony Brook Ground-Based Millimeter-wave Spectrometer (GBMS) during the decade of the 1990s, along with measurements by the Cryogenic Limb Array Etalon Spectrometer (CLAES) and the Microwave Limb Sounder (MLS) onboard the Upper Atmospheric Research Satellite (UARS), have revealed much more detail about the formation of HNO3 in fall and winter over a large vertical range of the southern polar stratosphere, its descent as part of winter vortex dynamics, and its freeze-out in PSC particles (Kawa et al, 1995; de Zafra et al, 1997; Santee et al, 1998, 1999; McDonald et al, 2000)
Summary
The production and transport of oxides of nitrogen in the atmosphere and their subsequent cryogenic removal by Polar Stratospheric Clouds (PSCs) (denitrification) over polar regions constitutes one of the most fascinating atmospheric cycles traced out in recent years. Measurements made with the Stony Brook Ground-Based Millimeter-wave Spectrometer (GBMS) during the decade of the 1990s, along with measurements by the Cryogenic Limb Array Etalon Spectrometer (CLAES) and the Microwave Limb Sounder (MLS) onboard the UARS, have revealed much more detail about the formation of HNO3 in fall and winter over a large vertical range of the southern polar stratosphere, its descent as part of winter vortex dynamics, and its freeze-out in PSC particles (Kawa et al, 1995; de Zafra et al, 1997; Santee et al, 1998, 1999; McDonald et al, 2000). Transport of large concentrations of NOy from the upper atmosphere to the lower stratosphere inside the vortex adds to this discrepancy, increasing the error when computing Antarctic winter denitrification using NOy-N2O curves obtained outside the southern vortex as a reference.
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