Observations of denitrification and dehydration in the winter polar stratospheres

Abstract

WATER vapour and reactive nitrogen species are removed from the polar stratosphere in both hemispheres in the winter and early spring, probably by the sedimentation of aerosol particles containing water and nitric acid, which co-condense at low stratospheric temperatures. In the Antarctic in 1987, intense dehydration invariably accompanied intense denitrification. However, from the marked difference between water vapour concentrations in the two hemispheres for similar concentrations of reactive nitrogen, we deduce that in the Antarctic some dehydration may have occurred without denitrification. In the Arctic in 1989, despite higher temperatures than in the Antarctic, intense denitrification occurred but without intense dehydration. These results provide important constraints for the uncertain microphysics controlling the growth and sedimentation of aerosol particles effecting the removal. We argue that the Arctic denitrification can be explained by the selective growth and sedimentation of aerosol particles rich in nitric acid. Because reactive nitrogen species moderate the destruction of ozone by chlorine-catalysed reactions, by sequestering chlorine in reservoir species such as ClONO₂, the possibility of the removal of reactive nitrogen without dehydration should be allowed for in attempts to model ozone depletion in the Arctic. Indeed, denitrification along with elevated concentrations of reactive chlorine¹ observed in 1989 indicate that the Arctic was chemically primed for ozone destruction without an extended period of temperatures below the frost point, as is characteristic of the Antarctic.