CCl<sub>4</sub> distribution derived from MIPAS ESA V7 data: validation, trend and lifetime estimation

Abstract

<p><strong>Abstract.</strong> Atmospheric emissions of Carbon tetrachloride CCl<sub>4</sub> 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 CCl<sub>4</sub>,'' the discrepancy between atmospheric observations and reported production and consumption. Surface measurements of CCl<sub>4</sub> atmospheric concentrations have declined at a rate almost three times smaller than its lifetime-limited rate, suggesting persistent atmospheric emissions despite the ban. In this paper, we study CCl<sub>4</sub> 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. <br><br> The CCl<sub>4</sub> 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. In the troposphere, the largest concentrations are observed at the latitudes of major industrial countries (20/50°N). The good agreement we find between MIPAS CCl<sub>4</sub> and independent measurements from other satellite and balloon-borne remote sounders proves the reliability of the MIPAS dataset. <br><br> CCl<sub>4</sub> trends are calculated as a function of both latitude and altitude. Negative trends are found at all latitudes in the upper-troposphere / lower-stratosphere region, apart from a region in the Southern mid-latitudes between 50 and 10 hPa where the trend is positive. At the lowest altitudes sounded by MIPAS, we find trends consistent with those determined on the basis of long-term ground-based measurements. For higher altitudes, the trend shows a pronounced asymmetry between Northern and Southern Hemispheres, and the magnitude of the decline rate increases with altitude. At 50 hPa the decline is about 30–35 %/decade, close to the lifetime-limited trend. <br><br> We use a simplified model assuming tracer-tracer linear correlations to determine CCl<sub>4</sub> lifetime in the lower stratosphere. The calculation provides a global average lifetime of 46(38–60) years considering CFC-11 as the reference tracer. This value is consistent with the most recent literature result of 44(36–58) years.</p>

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

“…A maximum in the tropics connected with higher values of CCl 4 below the northern extratropical tropopause is a feature also seen in HCFC-22 (Chirkov et al, 2016), where it was associated with the uplift in the Asian monsoon, so CCl 4 distributions in this region might have a similar explanation. Trends of the entire measurement period from July 2002 to April 2012 show good agreement with trends estimated by Valeri et al (2017). Comparisons with ACE-FTS and MIPAS-B2 show very good agreement and historical measurements of MIPAS-B2 and ATMOS are coherent with MIPAS Envisat CCl 4 results using the new spectroscopic data.…”

“…The data set used for trend calculation covers the entire MIPAS Envisat measurement period from July 2002 to April 2012. The distribution of the trends agrees well with the trends estimated by Valeri et al (2017), who calculated trends from MIPAS Envisat V7 data they formerly retrieved and displayed them on a pressure-latitude grid. The most likely cause of differences between their and our trend es- timates are the underlying MIPAS spectra.…”

“…Since these temperature drifts are expected to propagate onto the retrieved CCl 4 mixing ratios, it is not clear if version 5 or version 7 is more adequate for trend analysis. In spite of these differences and technical differences in the level-2 data processing, the trends inferred by Valeri et al (2017) and ours show important common features. In both data sets a hemispheric asymmetry is found, with positive trends in the Southern Hemisphere around 25 km (although the region is larger in our data set) and negative trends in the Northern Hemisphere in almost the whole altitude range.…”

“…We use MIPAS V5 spectra which were found to be subject to an instrument drift due to detector aging . Valeri et al (2017) use version 7 spectra, where an attempt was made to tackle the problem of detector aging during the level-1 processing. However, Hubert et al (2016) show that there is still a drift problem in the version 7 MIPAS temperatures.…”