Assessing the losses of HCFC-22 using ACE-FTS measurements

The annual springtime minimum in stratospheric ozone over the Antarctic is primarily caused by catalytic reactions of ozone and chlorine. The Montreal Protocol on Substances that Deplete the Ozone Layer (with its subsequent amendments) restricts the emissions of ozone depleting substances. HCFC-22 has been the primary replacement for both CFC-11 and CFC-12, which has led to an increase in its atmospheric abundance. The Atmospheric Chemistry Experiment (ACE) is a mission on-board the Canadian satellite SCISAT. The primary instrument on SCISAT is a highresolution infrared Fourier Transform Spectrometer (ACE-FTS). With its wide spectral range, the ACE-FTS is capable of measuring an extensive range of gases including key CFC and HCFC species. The altitude distribution from the ACE-FTS profiles provides information that is complementary to the ground-based measurements that have been used to monitor these species.
The ACE-FTS measurements compare well with surface in situ and balloon measurements. A preliminary validation of HCFC-22 using ground-based FTSs is discussed. The zonal mean distribution of HCFC-22 as observed by ACE-FTS is presented. The global distributions of HCFC-22 have been compared to the Global Modelling Initiative (GMI) Combined Stratospheric-Tropospheric Model, a chemistry and transport model. Large differences between the model and ACE-FTS measurements of HCFC-22 reveal issues with the boundary value mixing ratios. The comparison of stratospheric measurements with GMI suggest that there may be a missing loss process in the stratosphere, some issues with transport circulation and polar cap averaging in the current run, or a combination of the two processes. We propose the reaction of HCFC-22 with atomic chlorine as a potentially important loss process in the lowermost stratosphere and the lower stratosphere.