CFC-11, CFC-12 and HCFC-22 ground-based remote sensing FTIR measurements at Réunion Island and comparisons with MIPAS/ENVISAT data

Abstract

Profiles of CFC-11 (CCl3F), CFC-12 (CCl2F2/ and HCFC-22 (CHF2Cl) have been obtained from Fourier transform infrared (FTIR) solar absorption measurements above the Saint-Denis (St Denis) and Ma'do sites at Runion Island (21- S, 55- E) with low vertical resolution. FTIR profile retrievals are performed by the well-established SFIT4 program and the detail retrieval strategies along with the systematic/random uncertainties of CFC-11, CFC-12 and HCFC-22 are discussed in this study. The FTIR data of all three species are sensitive to the whole troposphere and the lowermost stratosphere, with the peak sensitivity between 5 and 10 km. The ground-based FTIR data have been compared with the collocated Michelson Interferometer for Passive Atmospheric Sounding (MIPAS/ENVISAT) data and found to be in good agreement: the observed mean relative biases and standard deviations of the differences between the smoothed MIPAS and FTIR partial columns (6-30 km) are (-4:3 and 4.4 %), (-2:9 and 4.6 %) and (-0:7 and 4.8 %) for CFC-11, CFC-12 and HCFC-22, respectively, which are within the combined error budgets from both measurements. The season cycles of CFC-11, CFC-12 and HCFC-22 from FTIR measurements and MIPAS data show a similar variation: concentration is highest in February-April and lowest in August-October. The trends derived from the combined St Denis and Mado FTIR time series are -0:86±0.12 and 2.84±0.06% year-1 for CFC-11 and HCFC-22, respectively, for the period 2004 to 2016, and -0:76±0.05% year-1 for CFC-12 for 2009 to 2016. These measurements are consistent with the trends observed by the National Oceanic and Atmospheric Administration (NOAA) Global Monitoring Divisions (GMD) Halocarbons & other Atmospheric Trace Species Group (HATS) measurements at Samoa (14.2 S, 170.5 W) for CFC-11 (-0:87±0.04% year-1), but slightly weaker for HCFC-22 (3.46±0.05 %) year-1 and stronger for CFC-12 (-0:60±0.02% year-1).