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Phosgene in the atmosphere is produced via the degradation of carbon tetrachloride, methyl chloroform, and a number of chlorine-containing very short lived substances (VSLS). These VSLS are not regulated by the Montreal Protocol even though they contribute to stratospheric ozone depletion. While observations of VSLS can quantify direct stratospheric source gas injection, observations of phosgene in the upper troposphere/lower stratosphere can be used as a marker of product gas injection of chlorine-containing VSLS. In this work we report upper troposphere/lower stratosphere measurements of phosgene made by the ACE-FTS (Atmospheric Chemistry Experiment Fourier Transform Spectrometer) instrument and compare with results from the TOMCAT/SLIMCAT three-dimensional chemical transport model to constrain phosgene trends over the 2004-2016 period. The 13-year ACE-FTS time series provides the first observational evidence for an increase in chlorine product gas injection. In 2016, VSLS accounted for 27% of modeled stratospheric phosgene, up from 20% in the mid-2000s.

Plain Language Summary Atmospheric phosgene, a chlorine-containing molecule, is produced via the degradation of the reasonably long lived species carbon tetrachloride and methyl chloroform, and a number of very short lived substances (VSLS), including dichloromethane, chloroform, and tetrachloroethene. Whereas the former are regulated by the Montreal Protocol because they contribute to stratospheric ozone depletion, these latter species are not. It is therefore important that we continue to monitor VSLS and the degradation products to determine how much additional chlorine, which catalyzes the destruction of ozone, is reaching the stratosphere. VSLS can either reach the stratosphere directly, via so-called source gas injection, or degrade in the troposphere into products such as phosgene, which are then delivered into the stratosphere via so-called product gas injection (PGI). Monitoring phosgene in the upper troposphere/lower stratosphere is therefore a marker for PGI due to chlorine-containing VSLS. In this work we report measurements of phosgene in the upper troposphere/lower stratosphere made by the ACE-FTS (Atmospheric Chemistry Experiment Fourier Transform Spectrometer) instrument. The 13-year ACE-FTS time series provides the first observational evidence for an increase in chlorine PGI, which has been predicted by atmospheric models.