Evaluating satellite-based measurements of halogenated gases

Industrial compounds that contain halogens are potent greenhouse gases, and those that contain chlorine and bromine substantially reduce springtime ozone over the poles. As a result, these gases have been phased out by the 1987 Montreal Protocol and its subsequent amendments. However, many persist in the atmosphere due to their long lifetimes and gradual elimination from industry. Chlorofluorocarbons (CFCs) and carbon tetrachloride (CCl₄) were scheduled to be fully phased out by 2010 and replaced by hydrochlorofluorocarbons (HCFCs), which will be banned worldwide by 2030. Some halogenated gases, such as perfluorocarbons (PFCs), have global warming potentials thousands of times greater than carbon dioxide but are not controlled by the Montreal Protocol or any other agreement. With so many halogen-containing compounds remaining in the atmosphere at different levels of regulation, it is crucial to continue carefully monitoring their abundances and trends. This can be accomplished using global satellite-based measurements, which have been continuously available for many of these gases as of the early 2000s. To maximize the reliability of these measurements, it is important to validate them through comparisons with other observations. In this study, we compare collocated measurements from three different satellite instruments (ACE-FTS on SCISAT, HIRDLS on Aura, and MIPAS on Envisat) with each other and with independent reference data from balloon-based instruments for CFC-11, CFC-12, CFC-113, HCFC-22, CCl₄, and CF₄. We find that overall, the satellite instruments perform well, but for certain regions and time periods, there are significant biases that need to be considered throughout any monitoring activities.