CCl₄ distribution derived from MIPAS ESA v7 data: intercomparisons, trend, and lifetime estimation

Abstract. 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 3 times lower than its lifetime-limited rate, suggesting persistent atmospheric emissions despite the ban. In this paper, we study CCl₄ vertical and zonal 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 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 to −15 pptv decade⁻¹ (−10 to −30 % decade⁻¹) are found at all latitudes in the upper troposphere–lower stratosphere region, apart from a region in the southern midlatitudes between 50 and 10 hPa where the trend is positive with values around 5–10 pptv decade⁻¹ (15–20 % decade⁻¹). At the lowest altitudes sounded by MIPAS, we find trends consistent with those determined on the basis of long-term ground-based measurements (−10 to −13 pptv decade⁻¹). For higher altitudes, the trend shows a pronounced asymmetry between the 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. 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.