Anomalously high levels of NOx in the polar upper stratosphere during April, 2004: Photochemical consistency of HALOE observations

Significant perturbations to the odd nitrogen mixing ratios in the high latitude upper stratosphere are seen in the Halogen Occultation Experiment (HALOE) data during April, 2004. Sunrise observation of 99.1 ppbv of NOx (NO + NO2) at 68N represents the largest NOx measured near 2 hPa. Examination of potential vorticity maps indicates that these high NOx mixing ratios correspond to measurements made within the polar vortex. Photochemical model results show large reductions in ozone in response to the enhancements in NOx, and HALOE ozone data are in good agreement with the model results. Probable cause of these anomalous values of NOx in the upper stratosphere is the enhanced production of NO in the high latitude upper mesosphere/thermosphere regions due to the powerful solar flares and the associated energetic particle precipitation that took place during October–November, 2003, followed by downward transport in the polar winter.