Phosgene in the UTLS: Vertical Distribution from MIPAS Observations Using New Spectroscopic Data

Carbonyl chloride (COCl2), better known as phosgene, is a toxic gas and it was mainly used by chemical industry in the preparation of insecticides, pharmaceuticals and herbicides. Its usage has been reduced over the years due to its high toxicity. To study possible seasonal and latitudinal variations of COCl2 in the upper troposphere / lower stratosphere (UTLS) we used the measurements from the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS). MIPAS is a Fourier Transform Spectrometer that measured atmospheric limb emission spectra from a polar orbit on board the ESA ENVISAT satellite in the time span from June 2002 to April 2012. Retrievals of phosgene were carried-out using the ?5 band of COCl2 in the 830-860 cm-1 spectral region. In the same spectral region we find the ?4 band of CCl3F (CFC-11), that is much stronger than the COCl2 signal. Therefore phosgene spectral features are hidden by the CFC-11 absorbing band. To cope with this strong interference, CFC-11 and phosgene abundances have been retrieved simultaneously. The retrieval scheme used in this work is based on the so called Optimized Retrieval Model (ORM), the scientific version of ESA level 2 processor, upgraded with the Multi-Target Retrieval (MTR) functionality and with the possibility to use Optimal Estimation (OE) to apply external constraints to the state vector. We carry-out the forward simulations using state-of-the art spectroscopic line data calculated at LISA-CNRS, not yet included in the HITRAN database. We show the global distribution of phosgene in the UTLS region and an analysis of its seasonal and latitudinal variability.