Ground-based FTIR Measurements of Stratospheric Trace Species from Aberdeen During Winter and Spring 1993/94 and 1994/95 and Comparison with a 3D Model

Vertical column abundances of HCl, ClONO2, HF and HNO3 have been obtained from infrared solar absorption measurements made at Aberdeen, UK (57°N, 2°W) during the periods January 13 1994 - May 8 1994 and November 23 1994 - April 19 1995. The measurements reveal the partitioning of inorganic chlorine (Cly) inside and outside the polar vortex during these two winter and spring periods. Stratospheric temperatures within the northern polar vortex during 1993/94 were not cold throughout January and most of February. The measurements reported here suggest that following a brief period of chlorine activation in late February and early March, the active chlorine within the vortex recovered rapidly to form ClONO2 resulting in in-vortex ClONO2 columns of 7 × 1015 molecules cm-2. In contrast, measurements during January 1995 suggest extensive invortex activation with in-vortex HCl + ClONO2 as low as 3.6×1015 molecules cm-2. High day-to-day variability in the ClONO2 columns observed during February is evidence for the transport of ClONO2 rich air from high to mid latitudes during the late winter. The implications for mid latitude O3 loss are discussed. A preliminary comparison of the HCl, ClONO2, and HNO3 column data from winter 94/95 with a three-dimensional chemical transport model shows that the model generally reproduces well the day-to-day variability and absolute magnitude of the observed columns, especially for HNO3 outside of the vortex.