A diagnostic for denitrification in the winter polar stratospheres

Abstract

A STRIKING negative correlation between in situ measurements of reactive nitrogen (NOy) and nitrous oxide (N2O) has been observed throughout the lower polar stratospheres1,2. This correlation has been extensively used to determine reference values for NOy, from a measurement of N2O and, hence, to quantify the extent of denitrification in high-latitude air parcels1,3. (Denitrification in the atmosphere is defined as the permanent removal of reactive nitrogen.) The removal of NOy 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 NOy 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 NOy 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.