MIPAS IMK/IAA carbon tetrachloride (CCl₄) retrieval and first comparison with other instruments

Abstract. MIPAS thermal limb emission measurements were used to derive vertically resolved profiles of carbon tetrachloride (CCl₄). Level-1b data versions MIPAS/5.02 to MIPAS/5.06 were converted into volume mixing ratio profiles using the level-2 processor developed at Karlsruhe Institute of Technology (KIT) Institute of Meteorology and Climate Research (IMK) and Consejo Superior de Investigaciones Científicas (CSIC), Instituto de Astrofísica de Andalucía (IAA). Consideration of peroxyacetyl nitrate (PAN) as an interfering species, which is jointly retrieved, and CO₂ line mixing is crucial for reliable retrievals. Parts of the CO₂ Q-branch region that overlap with the CCl₄ signature were omitted, since large residuals were still found even though line mixing was considered in the forward model. However, the omitted spectral region could be narrowed noticeably when line mixing was accounted for. A new CCl₄ spectroscopic data set leads to slightly smaller CCl₄ volume mixing ratios. In general, latitude–altitude cross sections show the expected CCl₄ features with highest values of around 90 pptv at altitudes at and below the tropical tropopause and values decreasing with altitude and latitude due to stratospheric decomposition. Other patterns, such as subsidence in the polar vortex during winter and early spring, are also visible in the distributions. The decline in CCl₄ abundance during the MIPAS Envisat measurement period (July 2002 to April 2012) is clearly reflected in the altitude–latitude cross section of trends estimated from the entire retrieved data set.