Supporting the Montreal Protocol: monitoring long-lived halogenated species with atmospheric nadir sounders

Chlorofluorocarbons (CFCs) were first developed in the 1930s as safe, reliable, and non-toxic refrigerants for domestic use. This explosion in use led to a steady increase in their atmospheric abundances. However, while inert in the troposphere, it was this stability which enabled them to reach the stratosphere, where dissociation by ultraviolet (UV) radiation released chlorine atoms catalysing the destruction of the stratospheric ozone layer which protects us from harmful UV radiation.
It will be many years before stratospheric ozone levels return to pre-1980 levels because these species and their replacements, the majority of which are regulated by the Montreal Protocol, are generally very long-lived in the atmosphere. Continued monitoring of these species is crucial to ensure abundances are decreasing as expected. For example, atmospheric monitoring was able to quantify recent illegal emissions of CFC-11 from eastern China [Rigby et al., 2019].
Over recent decades, monitoring these species, which are also very strong greenhouse gases, from orbit has been the domain of infrared (IR) limb sounders. The only active limb sounder now measuring these species regularly is the ACE-FTS (Atmospheric Chemistry Experiment - Fourier Transform Spectrometer) which has been operating since 2004 [Bernath, 2017]. With the golden age of limb sounding coming to an end, one question is whether hyperspectral nadir IR sounders can fill the satellite monitoring gap.
A number of atmospheric nadir sounders, with low radiometric noise and high spectral resolution, measure top-of-atmosphere radiances in the thermal IR; these include IASI (Infrared Atmospheric Sounding Interferometer) and CrIS (Cross-track Infrared Sounder). From the atmospheric spectra recorded by these instruments we can determine the concentrations of a range of trace gases, such as methane, water, and carbon dioxide. These instruments have the potential to provide monitoring of halogenated species, and early work has shown the promise of using IASI for monitoring eight such species [De Longueville et al., 2021]. The advantage of using CrIS is that it possesses superior signal-to-noise, thus providing the prospect of more robust trends and the monitoring of additional species that IASI cannot observe.