On the consistency of HNO₃ and NO₂ in the Aleutian High region from the Nimbus 7 LIMS Version 6 data set

Abstract. This study uses photochemical calculations along kinematic trajectories in conjunction with Limb Infrared Monitor of the Stratosphere (LIMS) observations to examine the changes in HNO₃ and NO₂ near 30 hPa in the region of the Aleutian High (AH) during the minor warming event of January 1979. An earlier analysis of Version 5 (V5) LIMS data indicated increases in HNO₃ without a corresponding decrease in NO₂ in that region and a quasi-wave 2 signature in the zonal distribution of HNO₃, unlike the wave 1 signal in ozone and other tracers. Version 6 (V6) LIMS also shows an increase of HNO₃ in that region, but NO₂ is smaller than from V5. The focus here is to convey that V6 HNO₃ and NO₂ are of good quality, as shown by a re-examination of their mutual changes in the AH region. Photochemical model calculations initialized with LIMS V6 data show increases of about 2 ppbv in HNO₃ over 10 days along trajectories terminating in the AH region on 28 January. Those increases are mainly a result of the nighttime heterogeneous conversion of N₂O₅ on background stratospheric sulfuric acid aerosols. Changes in the composition of the air parcels depend on the extent of exposure to sunlight and, hence, on the dynamically controlled history of the trajectories. Trajectories that begin in low latitudes and traverse to across the North Pole in a short time lead to the low HNO₃ in the region separating the anticyclone from the polar vortex, both of which contain higher HNO₃. These findings help to explain the observed seasonal evolution and areal extent of both species. V6 HNO₃ and NO₂ are suitable, within their errors, for the validation of stratospheric chemistry–climate models.