Validation of ACE-FTS HCFC-22 concentrations in the upper troposphere – lower stratosphere

Abstract. The Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) is currently providing the only measurements of vertically resolved chlorodifluoromethane (HCFC-22) from space. This study assesses the ACE-FTS HCFC-22 v5.2 product in the upper troposphere – lower stratosphere, as well as the simulated concentrations of HCFC-22 from a 39-year specified dynamics run of the Canadian Middle Atmosphere Model (CMAM39) in the same region. In general, ACE-FTS HCFC-22 observations tend to agree with subsampled CMAM39 data to within ±5 %, except for between ~15 and 25 km in the extratropical regions where ACE-FTS exhibits a negative bias of 5–30 %, and near 6 km in the tropics where ACE FTS exhibits a bias of 15 %. When comparing against correlative satellite, aircraft, and balloon data, ACE-FTS typically exhibits a low bias on the order of 0–10 % between ~5–15 km and is within ±15 % between ~15–25 km. ACE-FTS, CMAM39, and surface flask measurements from the NOAA Global Monitoring Laboratory’s surface air-sampling network, all exhibit consistent tropospheric HCFC-22 trends ranging between 6.8 and 7.8 pptv/year (within 95 % confidence) for 2004–2012, and between 3.1 and 4.7 pptv/year (within 95 % confidence) for 2012–2018. Interhemispheric differences (IHD) of HCFC-22 concentrations were also derived using ACE-FTS, NOAA, and CMAM39 data, and all three yielded consistent and correlated (r≥0.42) IHD timeseries, with the results indicating that surface IHD values decreased at a rate of 2.2±1.1 pptv/decade between 2004 and 2018.