On the consistency of HNO 3 and NO 2 in the Aleutian High region from the Nimbus 7 LIMS Version 6 data set
This study uses photochemical calculations along kinematic trajectories in conjunction with Limb Infrared Monitor of the Stratosphere (LIMS) observations to examine the changes in HNO 3 and NO 2 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 HNO 3 without a corresponding decrease in NO 2 in that region and a quasi-wave 2 signature in the zonal distribution of HNO 3, unlike the wave 1 signal in ozone and other tracers. Version 6 (V6) LIMS also shows an increase of HNO 3 in that region, but NO 2 is smaller than from V5. The focus here is to convey that V6 HNO 3 and NO 2 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 HNO 3 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 N 2O 5 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 HNO 3 in the region separating the anticyclone from the polar vortex, both of which contain higher HNO 3. These findings help to explain the observed seasonal evolution and areal extent of both species. V6 HNO 3 and NO 2 are suitable, within their errors, for the validation of stratospheric chemistry–climate models.