Clouds & Aerosols
ACE has detected the first spectra of polar mesospheric clouds (PMCs or noctilucent clouds), which prove to be small ice particles with ~50 nm effective radius as expected (Bernath et al., 2005; Eremenko et al., 2005). PMCs form at high latitudes near the mesopause (at about 90 km) around the time of the summer solstice when the temperatures are very low. Decreased temperatures in the upper atmosphere and increased amount of water vapour (from the increase in atmospheric methane) have been observed, leading to more frequent occurrence of PMCs.
ACE is able to measure simultaneously the water vapour VMR, temperature and total ice volume of PMCs. Zasetsky et al. (2009) modelled PMC growth kinetics and showed that visible size changes can occur on a timescale of minutes, much faster than previously estimated. Petelina and Zasetsky (2009, 2011) used the high spectral resolution of the ACE-FTS to infer the ice temperature directly from the observed spectrum. Jones et al. (2019) published a description of a new PMC retrieval using ACE spectra.
ACE continues to have major success in the observation and characterization of clouds and aerosols. This work is based on “residual” ACE-FTS spectra obtained by removing the gas phase features, leaving broad spectral features of solid and liquid particles. Lecours et al. (2022) provides a spectral atlas of these clouds and aerosols: polar mesospheric clouds, three types of polar stratospheric clouds (nitric acid trihydrate, sulfuric/nitric acid ternary solutions, and ice), cirrus clouds, stratospheric smoke from fires and sulfate aerosols. Most of the example spectra have been modelled using the appropriate optical constants and the calculated extinction of sunlight by the particles. ACE infrared spectra characterize these particles by determining composition and size.
Additional work on ACE-FTS spectra of polar stratospheric clouds (PSCs) was carried out by Lecours et al. (2023). Quantitative modelling of these PSC spectra shows that supercooled nitric acid (SNA) is also common, i.e., “STS” with no observable sulfuric acid in their infrared spectra. ACE-FTS observations therefore classify PSCs into 4 basic spectral types, nitric acid trihydrate, supercooled ternary solutions of sulfuric and nitric acid, SNA and ice, as well as their mixtures. Continued work by Lecours et al. (2024) presents an analysis of the polar stratospheric clouds (PSCs) from the Antarctic polar winters from 2005 to 2023, demonstrating that the PSC composition distribution is mostly governed by temperature. Lavy et al. (2024) compares the Antarctic PSC composition as observed by ACE, CALIPSO, and MIPAS showing, in general, the three satellites agree in their identification of PSCs.
Studies on the detection and characterization of stratospheric sulfate aerosols from volcanic eruptions have also continued. Raymond et al. (2024) analyzed the atmospheric chemistry effects of the Tonga volcanic sulfate aerosols. Bernath et al. (2025) studied the SO2 sources in the stratosphere, which includes the photolysis of sulfate aerosols and the increased abundance due to the Hunga Tonga volcanic eruption. Dodangodage et al. (2024) also analyzed the composition of sulfate aerosols from the 2024 Ruang volcanic eruption.
Other studies on volcanic and wildfire aerosols include those by Wang et al. (2024); Zhang et al. (2024).
Atmospheric Dynamics
The ACE mission has been able to provide information on the motion of air by analyzing the distribution of trace gases. For example, descent rates were measured in the Arctic vortex during winter 2004-2005 (Nassar et al., 2005) and the “age of the air” (transport of air from the tropical tropopause into the stratosphere) was studied by Schoeberl et al. (2005). Folkins et al. (2006) used ACE data to study atmospheric motion in the upper troposphere. Manney et al. (2009) looked at the effects of an unusually strong and prolonged stratospheric sudden warming in January 2006.
The Asian summer monsoon creates a strong anticyclone (high pressure region) centred over Tibet that enhances and confines pollution in the upper troposphere. Park et al. (2008) found that the degree of enhancement was inversely related to the atmospheric lifetime as expected for rapid transport of polluted air from the surface. In a paper published with widespread publicity in Science, Randel et al. (2011) used the HCN tracer to show that the Asian monsoon is uniquely able to inject pollution directly into the stratosphere. In 2017, Ploeger et al. used ACE HCN as a tracer in a more detailed modelling study of transport in the Asian monsoon; Glatthor et al. (2015) compared HCN from ACE and MIPAS.
The isotopic composition of water vapour is a powerful tool to study convective processes and cloud formation in the upper troposphere. The HDO isotopologue of water has been used by Nassar et al. (2007) to study dehydration mechanisms in the tropical tropopause layer (TTL). They conclude that in addition to gradual dehydration, lofting of ice particles must be an important part of convection in the TTL. Randel et al. (2012) demonstrated that the degree of deuterium depletion is associated with deep convection and there is a strong regional isotopic enhancement associated with the North American summer monsoon but not the Asian monsoon or the western Pacific warm pool.
ACE isotopologue data have now been used by several research groups to track atmospheric chemistry and dynamics: CO (Beale et al., 2016), CH4 (Buzan et al., 2016) and H2O (Eichinger et al., 2015).
Water vapour is a popular ACE data product. Sioris et al. (2016) published two papers using ACE and MAESTRO water vapour to understand tropospheric water variability at high latitudes; (Sun et al., 2015) looked at transport across the tropopause; ACE water vapour was included in a SPARC assessment (Lossow et al., 2017); used to study double tropopauses (Schwartz et al., 2015) and compared to SCIAMACHY measurements (Weigel et al., 2016).
Recent work includes work by Zhang et al. (2025) that studied the record-high ozone in the Austral mid-latitude tropopause region driven by dynamical and chemical effects of the 2019 sudden stratospheric warming. Saunders et al. (2025) studied age of air using 20 years of ACE measurements of SF6, which can be used to observe changes in the Brewer-Dobson circulation. Pastorek et al. (2025) analyzed the effect of atmospheric rivers using ACE data. Yan et al. (2025) studied transport into the polar stratosphere from the Asian monsoon region using ACE SF6 data.