Carbon tetrachloride in the UTLS: distribution, trend and lifetime estimation using MIPAS ESA v7 data
Carbon tetrachloride (CCl?) is a strong ozone-depleting atmospheric gas regulated by the Montreal protocol. Recently it received increasing interest because it was found that at the surface its atmospheric concentration declines with a rate almost three times smaller than its lifetime-limited rate. Indeed there is a discrepancy between atmospheric observations and the estimated distribution based on the reported production and consumption (the so called "mystery of CCl?").
We use for the first time the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) measurements to estimate CCl? distribution, its trend, and atmospheric lifetime in the upper troposphere / lower stratosphere (UTLS) region. In particular, here we use MIPAS product generated with Version 7 of the Level 2 algorithm operated by the European Space Agency.
The CCl? zonal means show features typical of long-lived species of anthropogenic origin that are destroyed primarily in the stratosphere: larger amounts are found in the troposphere, monotonically decreasing with altitude. We calculate CCl? trends as a function of both latitude and altitude: negative trends are found at all latitudes in the UTLS, apart from a region in the Southern mid-latitudes between 50 and 10 hPa where the trend is slightly positive. At the lowest altitudes sounded by the MIPAS scan we find trend values consistent with those determined on the basis of long-term ground-based measurements. CCl? global average lifetime of 46(38 - 60) years has been estimated using the tracer-tracer linear correlations approach and the CFC-11 as the reference tracer. This estimated value is consistent with the most recent literature result of 44(36 - 58) years.