Measurements of HCFC-22 in the upper troposphere and lower stratosphere from the MIPAS-E instrument
In this thesis, the potential for retrieval of HCFC-22 volume mixing ratios in the upper troposphere and lower stratosphere (UTLS) from a spaceborne infrared limb-sounding spectrometer is assessed. Hydrochlorofluorocarbon-22 (HCFC-22) is an important trace constituent in the atmosphere through its role as a greenhouse gas and its potential influence on stratospheric ozone chemistry. Current estimates of its vertical distribution are limited to infrequent and sparse balloon and aircraft campaigns, giving poor spatial coverage. Atmospheric limb-emission spectra measured by the Michelson Interferometer for Passive Atmospheric Sounding onboard the ENVISAT satellite (MIPAS-E) have been used to successfully detect and retrieve HCFC-22 volume mixing ratios (vmrs) between 9 and 15 km. Global and seasonal means are presented from 2003. This is achieved using the Optimal Estimation Retrieval Algorithm (OPERA) developed as part of this thesis and designed specifically to invert measurements from MIPAS-E data to trace gas concentrations. The scheme performs a joint retrieval of the target gas and total extinction from clouds or aerosols. High extinctions are observed globally at 9 km. In January 2003, global mean HCFC-22 vmr data at 9 km agreed to within 8% of the measured mean surface value. Mean agreement for all seasons in 2003 was 15%. Indications are that mean HCFC-22 data at 9 km are accurate to better than 10%. Vertical profiles were distinguished between 9 and 15 km. OPERA CFC-12 retrievals were used as a consistency check on the likely accuracy of HCFC-22 data. These data were validated against independent measurements. Agreement with HAGAR measurements in spring 2003 was better than 7%. The results of this thesis demonstrate that HCFC-22 (and CFC-12) can be detected and retrieved from MIPAS-E data in the UTLS based on current concentrations. In addition, they show the potential to obtain other vertical profiles of particle extinction on a global basis throughout the UTLS.