Phosgene distribution derived from MIPAS ESA v8 data: intercomparisons and trends
Authors: Pettinari Paolo , Barbara Flavio , Ceccherini Simone , Dinelli Bianca Maria , Gai Marco , Raspollini Piera , Sgheri Luca , Valeri Massimo , Wetzel Gerald , Zoppetti Nicola , Ridolfi Marco 
Published: December 2021

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, and 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 several 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 (parts per trillion by volume) 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 v8 profiles with the ones retrieved from
MIPAS balloon and ACE-FTS (Atmospheric Chemistry
Experiment – Fourier Transform Spectrometer) 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 v8 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 (volume mixing ratio)
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 characterized by a negative trend of about
−7 pptv per decade, while in the Southern Hemisphere phosgene
mixing ratios increase with a rate of the order of +4 pptv per decade. This behavior resembles the stratospheric trend
of CCl4, which is the main stratospheric source of COCl2. In
the upper troposphere a positive trend is found in both hemispheres.