Phosgene distribution derived from MIPAS ESA v8 data: intercomparisons and trends

Abstract

Abstract. The Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) measured the middle-infrared limb emission spectrum of the atmosphere from 2002 to 2012 on board ENVISAT, a polar-orbiting satellite. Recently, the European Space Agency (ESA) completed the final reprocessing of MIPAS measurements, using Version 8 of the Level 1 and Level 2 processors, which include more accurate models, processing strategies and auxiliary data. The list of retrieved gases has been extended, it now includes a number of new species with weak emission features in the MIPAS spectral range. The new retrieved trace species include carbonyl chloride (COCl2), also called phosgene. Due to its toxicity, its use has been reduced over the years, however it is still used by chemical industries for sevaeral applications. Besides its direct injection in the troposphere, stratospheric phosgene is mainly produced from the photolysis of CCl4, a molecule present in the atmosphere because of human activity. Since phosgene has a long stratospheric lifetime, it must be carefully monitored as it is involved in the ozone destruction cycles, especially over the winter polar regions. In this paper we exploit the ESA MIPAS Version 8 data in order to discuss the phosgene distribution, variability and trends in the middle and lower stratosphere and in the upper troposphere. The zonal averages show that phosgene volume mixing ratio is larger in the stratosphere, with a peak of 40 pptv between 50 and 30 hPa at equatorial latitudes, while at middle and polar latitudes it varies from 10 to 25 pptv. A moderate seasonal variability is observed in polar regions, mostly between 80 and 50 hPa. The comparison of MIPAS/ENVISAT COCl2 v.8 profiles with the ones retrieved from MIPAS/balloon and ACE-FTS measurements highlights a negative bias of about 2 pptv, mainly in polar and mid-latitude regions. Part of this bias is attributed to the fact that the ESA Level 2 v.8 processor uses an updated spectroscopic database. For the trend computation, a fixed pressure grid is used to interpolate the phosgene profiles and, for each pressure level, VMR monthly averages are computed in pre-defined 10°-wide latitude bins. Then, for each latitudinal bin and pressure level, a regression model has been fitted to the resulting time-series in order to derive the atmospheric trends. We find that the phosgene trends are different in the two hemispheres. The analysis shows that the stratosphere of the Northern Hemisphere is characterised by a negative trend, of about −7 pptv/decade, while in the Southern Hemisphere phosgene mixing ratios increase with a rate of the order of +4 pptv/decade. In the upper troposphere a positive trend is found in both hemispheres.

Long-term validation of MIPAS ESA operational products using MIPAS-B measurements

“…Deviations in the tropics are quite large. The deviations are in line with the findings of Pettinari et al (2021), who compared v8 data not only to MIPAS-B but also to observations from ACE-FTS. Pettinari et al (2021) found that some of the differences between MIPAS-E and MIPAS-B can be attributed to the different spectroscopic data used (Toon et al, 2001, for MIPAS-B andTchana et al, 2015, in the case of MIPAS-E).…”

“…3.9 Additional v8 products: C 2 H 2 , C 2 H 6 , COCl 2 , OCS, and CH 3 Cl Some more target molecules have been operationally processed by the v8 algorithm. To date, an intercomparison study is only available for COCl 2 (Pettinari et al, 2021). Similar to the additional v7 gases, the emissions of spectral lines of the v8 molecules are also weak compared to the spectral signatures of the standard gases (before v7).…”

The ESA MIPAS/Envisat level2-v8 dataset: 10 years of measurements retrieved with ORM v8.22

“…The span of time covered by the database, and the fact that the database has been developed using the last version of MIPAS level-1 data where the calibration errors due to time dependent detector non-linearities have been reduced, will ensure an improvement in the estimation of the trends for the stratospheric and upper tropospheric concentration of the included gases. An example of the studies that can be performed with the MIPAS level2-v8 database is reported in Pettinari et al (2021), where the data of the new retrieved species COCl 2 have been used to study its trends in the UTLS. The data are reported at the vertical grid coincident with the tangent altitudes of the observations, spanning the altitude range from 6 to 68 km in the FR and from 3 to 70 km in the OR.…”