Abstract
Abstract. Profiles of CFC-11 (CCl3F) and CFC-12 (CCl2F2) of the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) aboard the European satellite Envisat have been retrieved from versions MIPAS/4.61 to MIPAS/4.62 and MIPAS/5.02 to MIPAS/5.06 level-1b data using the scientific level-2 processor run by 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). These profiles have been compared to measurements taken by the balloon-borne cryosampler, Mark IV (MkIV) and MIPAS-Balloon (MIPAS-B), the airborne MIPAS-STRatospheric aircraft (MIPAS-STR), the satellite-borne Atmospheric Chemistry Experiment Fourier transform spectrometer (ACE-FTS) and the High Resolution Dynamic Limb Sounder (HIRDLS), as well as the ground-based Halocarbon and other Atmospheric Trace Species (HATS) network for the reduced spectral resolution period (RR: January 2005–April 2012) of MIPAS. ACE-FTS, MkIV and HATS also provide measurements during the high spectral resolution period (full resolution, FR: July 2002–March 2004) and were used to validate MIPAS CFC-11 and CFC-12 products during that time, as well as profiles from the Improved Limb Atmospheric Spectrometer, ILAS-II. In general, we find that MIPAS shows slightly higher values for CFC-11 at the lower end of the profiles (below ∼ 15 km) and in a comparison of HATS ground-based data and MIPAS measurements at 3 km below the tropopause. Differences range from approximately 10 to 50 pptv ( ∼ 5–20 %) during the RR period. In general, differences are slightly smaller for the FR period. An indication of a slight high bias at the lower end of the profile exists for CFC-12 as well, but this bias is far less pronounced than for CFC-11 and is not as obvious in the relative differences between MIPAS and any of the comparison instruments. Differences at the lower end of the profile (below ∼ 15 km) and in the comparison of HATS and MIPAS measurements taken at 3 km below the tropopause mainly stay within 10–50 pptv (corresponding to ∼ 2–10 % for CFC-12) for the RR and the FR period. Between ∼ 15 and 30 km, most comparisons agree within 10–20 pptv (10–20 %), apart from ILAS-II, which shows large differences above ∼ 17 km. Overall, relative differences are usually smaller for CFC-12 than for CFC-11. For both species – CFC-11 and CFC-12 – we find that differences at the lower end of the profile tend to be larger at higher latitudes than in tropical and subtropical regions. In addition, MIPAS profiles have a maximum in their mixing ratio around the tropopause, which is most obvious in tropical mean profiles. Comparisons of the standard deviation in a quiescent atmosphere (polar summer) show that only the CFC-12 FR error budget can fully explain the observed variability, while for the other products (CFC-11 FR and RR and CFC-12 RR) only two-thirds to three-quarters can be explained. Investigations regarding the temporal stability show very small negative drifts in MIPAS CFC-11 measurements. These instrument drifts vary between ∼ 1 and 3 % decade−1. For CFC-12, the drifts are also negative and close to zero up to ∼ 30 km. Above that altitude, larger drifts of up to ∼ 50 % decade−1 appear which are negative up to ∼ 35 km and positive, but of a similar magnitude, above.
Highlights
Chlorofluorocarbons (CFCs) have been monitored for some decades because of their potential to release catalytically active species that destroy stratospheric ozone, which was first discovered by Molina and Rowland (1974)
The agreement of Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) CFC-11 measurements taken during the full spectral resolution (FR) period with those of ILAS-II is not good as it shows far larger differences at the bottom end of the profile than comparisons with, e.g., Atmospheric Chemistry Experiment Fourier transform spectrometer (ACE-FTS) or Halocarbon and other Atmospheric Trace Species (HATS), that are as big as 50 % and large deviations at the upper end of the profiles that exceed 100 % above 25 km
For the comparison of CFC-12 during the FR period (Fig. 27), collocated MIPAS profiles were found for the Mark IV (MkIV) measurement taken on December 2002, and 25 MIPAS profiles coincide with the MkIV measurement taken on 1 April 2003
Summary
Chlorofluorocarbons (CFCs) have been monitored for some decades because of their potential to release catalytically active species that destroy stratospheric ozone, which was first discovered by Molina and Rowland (1974). Even though there are natural sources of halogens, observations focus on man-made CFCs, such as CFC-11 and CFC-12, because increased release of active chlorine species due to elevated amounts of these substances can significantly alter the equilibrium of stratospheric ozone formation and destruction. Under certain conditions (sufficiently cold temperatures for chlorine activation; polar stratospheric clouds, PSCs), this can lead to severe ozone depletion. Halogen source gases, such as CFC-11 or CFC-12, are photolyzed or otherwise broken up and converted to so-called reservoir gases, hydrogen chloride (HCl) or chlorine nitrate (ClONO2), by chemical reactions and under the influence of solar ultraviolet radiation. While direct reactions of ozone with the reservoir species HCl and ClONO2 are not relevant for ozone depletion, these reservoir species are transformed into active chlorine species (ClOx; mainly ClO, Cl and Cl2O2) under sufficiently cold temperatures. Heterogeneous reactions on the surface of cold aerosols of PSCs occur and, in combination with sunlight, result in the reactivation of chlorine which can destroy ozone catalytically and leads to ozone depletion and the formation of the ozone hole
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