Mesospheric N₂O enhancements as observed by MIPAS on Envisat during the polar winters in 2002–2004

Abstract. N₂O abundances ranging from 0.5 to 6 ppbv were observed in the polar upper stratosphere/lower mesosphere by the MIPAS instrument on the Envisat satellite during the Arctic and Antarctic winters in the period July 2002 to March 2004. A detailed study of the observed N₂O-CH₄ correlations shows that such enhancements cannot be explained by dynamics without invoking an upper atmospheric chemical source of N₂O. The N₂O enhancements observed at 58 km occurred in the presence of NO_x intrusions from the upper atmosphere which were related to energetic particle precipitation. Further, the inter-annual variability of mesospheric N₂O correlates well with observed precipitating electron fluxes. The analysis of possible chemical production mechanisms shows that the major part of the observed N₂O enhancements is most likely generated under dark conditions by the reaction of NO₂ with atomic nitrogen at altitudes around 70–75 km in the presence of energetic particle precipitation (EPP). A possible additional source of N₂O in the middle and upper polar atmosphere is the reaction of N₂(A³Σ_u⁺), generated by precipitating electrons, with O₂, which would lead to N₂O production peaking at altitudes around 90–100 km. N₂O produced by the latter mechanism could then descend to the mesosphere and upper stratosphere during polar winter. The estimated fraction of EPP-generated N₂O to the total stratospheric N₂O inside the polar vortex above 20 km (30 km) never exceeds 1% (10%) during the 2002–2004 winters. Compared to the global amount of stratospheric N₂O, the EPP-generated contribution is negligible.