Nitric oxide measurements in the Arctic winter stratosphere

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Abstract
Measurements of nitric oxide (NO) from five flights of the NASA ER‐2 aircraft during the Airborne Arctic Stratospheric Expedition (AASE) are presented. The NO values and vertical gradient near 60°N latitude are similar to previous measurements near 50°N in winter [Ridley et al, 1984; 1987]. The NO latitudinal gradient is distinctly negative outside of the polar vortex, approaching zero at the boundary of the vortex, and remaining below the 20 pptv detection limit inside the vortex. The low NO values in the vortex occur at solar zenith angles as low as 82° indicating that NO2 values in the vortex are also low. Steady state NO2 and NOx (NO+NO2) are calculated from measured NO, O3, and ClO, and modeled photodissociation rates. NOx outside the vortex shows a negative dependence on latitude and solar zenith angle. The average ratio of NOx to NOy (at the same relative latitudes from different flight days) shows a strong latitude gradient with values near 0.08 at 12° equatorward of the vortex edge, decreasing to less than 0.02 at the vortex boundary. Low NOx and NOx/NOy inside and near the vortex boundary may be indications of heterogeneous removal of CIONO2 and N2O5.