A diagnostic for denitrification in the winter polar stratospheres

Abstract

A STRIKING negative correlation between in situ measurements of reactive nitrogen (NO_y) and nitrous oxide (N₂O) has been observed throughout the lower polar stratospheres^1,2. This correlation has been extensively used to determine reference values for NO_y, from a measurement of N₂O and, hence, to quantify the extent of denitrification in high-latitude air parcels^1,3. (Denitrification in the atmosphere is defined as the permanent removal of reactive nitrogen.) The removal of NO_y from the Antarctic winter stratosphere maintains high concentrations of reactive chlorine, thereby priming the atmosphere for catalytic ozone destruction⁴. Here we present the pairwise correlation of the NO_y and N₂O data from the Southern and Northern Hemispheres. Both datasets show a linear correlation region, defined as a reference state, and regions of denitrification, where the correlation breaks down. Using two-dimensional photochemical model simulations of the atmosphere, we find a similar linear correlation between NO_y and N₂O, thereby establishing a theoretical framework for the reference state. This general approach, which can be extended to other pairs of molecules, should prove to be powerful in further comparisons of aircraft data with numerical models.