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

Abstract

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, 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 in the 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 mid-latitude and polar regions, the volume mixing ratios (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.

Phosgene in the Upper Troposphere and Lower Stratosphere: A Marker for Product Gas Injection Due to Chlorine‐Containing Very Short Lived Substances

“…It was noted in the original ACE‐FTS study (Fu et al, ) that the phosgene retrieval uncertainty is dominated by spectroscopic errors, assumed to be ∼30%, resulting from uncertainties in line intensities and the lack of hot bands in the linelist. A recent MIPAS phosgene data set (Valeri et al, ) used a new and improved spectroscopic linelist for phosgene. The upper tropospheric values are comparable to those of the ACE‐FTS, although it needs to be recognized that this was an optimal‐estimation scheme making use of ACE‐FTS profiles as the a priori.…”

“…The first global distribution of phosgene was derived from measurements by the Atmospheric Chemistry Experiment Fourier transform spectrometer (ACE‐FTS)—in total 5,614 vertical profiles between February 2004 and May 2006 were used (Fu et al, ). More recently Valeri et al () used Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) measurements, in particular 28,000 vertical profiles from the 18th and 20th of each month of 2008, to investigate the seasonality and latitudinal distribution of phosgene in the UTLS region.…”