CCl4 distribution derived from MIPAS ESA V7 data: intercomparisons, trend and lifetime estimation

Atmospheric emissions of carbon tetrachloride (CCl4) are regulated by the Montreal Protocol due to its role as a strong ozone-depleting substance. The molecule has been the subject of recent increased interest as a consequence of the so called "mystery of CCl4", the discrepancy between atmospheric observations and reported production and consumption. Surface measurements of CCl4 atmospheric concentrations have declined at a rate almost three times smaller than its lifetimelimited rate, suggesting persistent atmospheric emissions despite the ban. In this paper, we study CCl4 vertical and zonal 5 distributions in the upper troposphere and lower stratosphere (including the photolytic loss region, 70-20 hPa), its trend, and its stratospheric lifetime using measurements from the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS), which operated onboard the ENVISAT satellite from 2002 to 2012. Specifically, we use the MIPAS data product generated with Version 7 of the Level 2 algorithm operated by the European Space Agency. The CCl4 zonal means show features typical of long-lived species of anthropogenic origin that are destroyed primarily in the 10 stratosphere, with larger quantities in the troposphere and a monotonic decrease with increasing altitude in the stratosphere. MIPAS CCl4 measurements have been compared with independent measurements from other satellite and balloon-borne remote sounders showing a good agreement between the different datasets. CCl4 trends are calculated as a function of both latitude and altitude. Negative trends of about −10/− 15 pptv/decade (−10/− 30 %/decade) are found at all latitudes in the upper-troposphere / lower-stratosphere region, apart from a region 15 in the Southern mid-latitudes between 50 and 10 hPa where the trend is positive with values around 5/10 pptv/decade (15/20 %/decade). At the lowest altitudes sounded by MIPAS, we find trends consistent with those determined on the basis of long-term ground-based measurements (−10/− 13 pptv/decade). For higher altitudes, the trend shows a pronounced asymmetry between Northern and Southern hemispheres, and the magnitude of the decline rate increases with altitude. We use a simplified model assuming tracer-tracer linear correlations to determine CCl4 lifetime in the lower stratosphere. 20 The calculation provides a global average lifetime of 47(39 61) years considering CFC-11 as the reference tracer. This value is consistent with the most recent literature result of 44(36 58) years.