Subsidence, mixing, and denitrification of Arctic polar vortex air measured During POLARIS

We determine the degree of denitrification that occurred during the 1996-19 97 Arctic winter using a technique that is based on balloon and aircraft bo rne measurements of NOy, N2O, and CH4. The NOy/N2O relation can undergo sig nificant change due to isentropic mixing of subsided vortex air masses with extravortex air due to the high nonlinearity of the relation. These transp ort related reductions in NOy can be difficult to distinguish from the effe cts of denitrification caused by sedimentation of condensed HNO3. In this s tudy, high-altitude balloon measurements are used to define the properties of air masses that later descend in the polar vortex to altitudes sampled b y the ER-2 aircraft (i.e., similar to 20 km) and mix isentropically with ex travortex air. Observed correlations of CH4 and N2O are used to quantify th e degree of subsidence and mixing for individual air masses. on the basis o f these results the expected mixing ratio of NOy resulting from subsidence and mixing, defined here as NOy**, is calculated and compared with the meas ured mixing ratio of NOy. Values of NOy and NOy** agree well during most pa rts of the flights. A slight deficit of NOy versus NOy** is found only for a limited region during the ER-2 flight on April 26, 1997. This deficit is interpreted as indication for weak denitrification (similar to 2-3 ppbv) in that air mass. The small degree of denitrification is consistent with the general synoptic-scale temperature history of the sampled air masses, which did not encounter temperatures below the frostpoint and had relatively bri ef encounters with temperatures below the nitric acid trihydrate equilibriu m temperature. Much larger degrees of denitrification would have been infer red if mixing effects had been ignored, which is the traditional approach t o diagnose denitrification. Our analysis emphasizes the importance of using other correlations of conserved species to be able to accurately interpret changes in the NOy/N2O relation with respect to denitrification.