NO x descent in the Arctic middle atmosphere in early 2009

Measurements by the Atmospheric Chemistry Experiment show that the amount of NO x (NO + NO 2 ) produced by energetic particle precipitation (EPP) that descended from the Arctic mesosphere and lower thermosphere into the stratosphere in early 2009 was up to ∼50 times higher than average in 2005, 2007 and 2008. This is of note because the level of EPP in the preceding months was very low, suggesting that excess production of NO x was not the cause of the enhancements. Rather, the enhancements are attributed to unusually strong descent in the middle atmosphere. This is the third time on record that extraordinary meteorology contributed to descent of excess NO x . The results confirm that EPP impacts on the middle atmosphere can be large even in the absence of exceptional EPP, and highlight the need to continually measure NO x throughout the polar region from the stratosphere to the lower thermosphere.