Articles | Volume 9, issue 9
Research article
20 Sep 2016
Research article |  | 20 Sep 2016

Phosgene in the UTLS: seasonal and latitudinal variations from MIPAS observations

Massimo Valeri, Massimo Carlotti, Jean-Marie Flaud, Piera Raspollini, Marco Ridolfi, and Bianca Maria Dinelli

Abstract. The Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) is a Fourier transform spectrometer that measured mid-infrared atmospheric limb emission spectra from July 2002 to April 2012 on board the polar-orbiting satellite ENVISAT. We have used MIPAS data to study the latitudinal variations of phosgene (COCl2 or carbonyl chloride) and, for the first time, its seasonal variation in the upper troposphere/lower stratosphere region (UTLS). Retrievals of phosgene were made using the 830–860 cm−1 region, corresponding to the ν5 bands of COCl2. Unfortunately, in that region, the ν4 band of CFC-11, which is much stronger than COCl2 ν5, hides the phosgene emission. In order to evaluate seasonality and latitudinal distribution of phosgene we have analysed all the measurements made by MIPAS on days 18 and 20 of each month of 2008 with the optimized retrieval model (ORM) recently upgraded with the multi-target retrieval technique and with the optimal estimation functionality to apply external constraints to the state vector. Average seasonal profiles of phosgene show an evident latitudinal variability with the largest values observed in the tropical regions (maximum  ≈  35 parts per trillion by volume (pptv) at about 300 hPa). In the midlatitude and polar regions, the volume mixing ratio (VMR) values do not exceed 30 pptv and the vertical distributions are less peaked. Our analysis highlights that COCl2 seasonal variability is fairly low, apart from the polar regions.

Short summary
Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) measurements have been analysed to measure the vertical distribution of phosgene (COCl2) in the UTLS using new spectroscopic data. To highlight its seasonal and latitudinal variability, MIPAS measurements of 2 days/month during 2008 have been analysed. The results show a strong latitudinal variability of COCl2 with the largest VMR values observed in the tropical regions. The seasonality is fairly low, apart from the polar regions.