Stratospheric Ozone Chemistry
The Antarctic “ozone hole” and springtime Arctic ozone declines are caused primarily by the chemical conversion of the inactive chlorine reservoir compounds HCl and ClONO2 to the active ClO radical catalyzed by polar stratospheric clouds (PSCs). These decreases in stratospheric ozone led to increases in UV radiation on the ground, which damages ecosystems and increases skin cancer rates.
ACE is the only satellite that monitors all the main components involved in activated polar chlorine chemistry: O3, HCl, ClONO2, ClO, HNO3, H2O and PSCs. It is necessary to have this broad range of measurements to be able to attribute the changes in ozone observed in the polar regions. For example, the ACE-FTS measurements were used to study chlorine partitioning during the very cold 2005 Arctic winter and spring (Dufour et al., 2006b). Santee et al. (2008) combined Microwave Limb Sounder (MLS) and ACE data in a more detailed study of four polar winters. For all winters, the SLIMCAT (Single Layer Isentropic Model of Chemistry And Transport) chemical transport model over-predicted the amount of chlorine activation. ACE-FTS is currently the only satellite instrument that measures the important ClONO2 reservoir compound and, for example, Rex et al. (2014) have used ACE-FTS ClONO2 to develop a new fast model that accurately estimates the degree of stratospheric ozone depletion during polar winters.
Recently, ACE data contributed to a study that incorporates a chlorine peroxide reaction into chemistry transport models which improves agreement with observed levels of chlorine compounds in the Antarctic vortex (Grooβ et al., 2025). ACE data was also used to study the lower stratosphere interhemispheric asymmetry, where trends of O3, N2O, and HCl had opposite signs between the Southern Hemisphere and Northern Hemisphere (Chrysanthou et al., 2025). ACE ClONO2, HCl, and O3 data were used to study the buffering of ozone depletion in the middle stratosphere by a smoke-charged vortex caused by the Australian wildfires in the summer of 2019-2020 (Ma et al., 2024). ACE data were used in other related studies, including Zou, 2024; Millán et al., 2024; Zhu et al., 2024; and Dubé et al. 2025.
Halogenated Gases
Long-lived chlorine- and bromine-containing gases are responsible for ozone depletion in the stratosphere and have been banned by the Montreal Protocol and its subsequent amendments. As a temporary measure, CFCs (chlorofluorocarbons) had been replaced by HCFCs (hydrochlorofluorocarbons). HCFCs still destroy atmospheric ozone so they are in turn being replaced by HFCs (hydrofluorocarbons) that contain no chlorine. Halogenated molecules are also very powerful greenhouse gases, so their control under the Montreal Protocol has had the added benefit of reducing their contribution to global warming. The Kigali amendment (2016) targets the reduction of long-lived HFCs that do not destroy ozone but are powerful greenhouse gases.
Twenty-two halogen-containing gases are retrieved from ACE spectra including the main CFC, HCFC and HFC source gases. These species are destroyed by UV photolysis in the stratosphere and lead to the ClONO2, COF2, COClF and COCl2 intermediates, and ultimately result in the formation of HCl and HF (which are all measured by ACE). Using ACE data, Nassar et al. (2006a,b) and Brown et al. (2013a,b) have determined stratospheric fluorine and chlorine budgets. Updates to the fluorine budget (Raymond et al., 2025) and chlorine budget (Raymond et al., submitted) were created using ACE data comparing total abundances and percent compositions over a 20-year period.
The longevity of ACE allows trends in the concentrations of halogenated gases (Froidevaux et al., 2006, 2015; Lary et al., 2007; Brown et al., 2011) to be measured throughout the atmosphere, not just at surface sites. Only ACE observes the entire process of emission of source gases in the troposphere, destruction in the stratosphere and nearly complete conversion to HCl/HF products in the mesosphere on a global scale. CFC-113, HCFC-142b, HFC-134a, COClF, COCl2, HFC-23 were all first measured from orbit by ACE. ACE is currently the only satellite instrument that measures profiles of halogenated gases, except for HCl, which is also measured by the MLS instrument on the Aura satellite. It is important to note that ACE is also the only satellite instrument that can measure the dominant HFCs (HFC-134a, HFC-23, HFC-32, and now HFC-125) as well as CF4 and SF6.
In recent work, ACE measurements of ClONO2 and HCl were used to assess the contribution of “very short-lived substances” to long-term stratospheric chlorine reduction (Dubé et al. 2025). ACE data for halogenated gases were also used by Pardo Cantos et al., 2024; Thornhill et al., 2024; Zhou et al., 2024; Bourguet et al., 2025 as well as many other projects.