Introduction

In the 1990s and early 2000s, an international team of engineers and scientists designed an integrated observatory for atmospheric composition – a bold endeavor to provide unprecedented detail that was essential to understanding how Earth’s ozone (O₃) layer and air quality respond to changes in atmospheric composition caused by human activities and natural phenomena. This work addressed a key NASA Earth science objective. Originally referred to as Earth Observing System (EOS)–CHEM (later renamed Aura,) the mission would become the third EOS Flagship mission, joining EOS-AM 1 (Terra) launched in 1999 and EOS-PM 1 (Aqua), launched in 2002. The Aura spacecraft – see Figure 1 – is similar in design to Terra and identical to Aqua. Aura and its four instruments were launched on July 15, 2004 from Vandenberg Air Force Base (now Space Force Base) in California – see Photo.

In 2014 The Earth Observer published an article called  “Aura Celebrates Ten Years in Orbit,” [Nov–Dec 2014, 26:6, pp. 4–18] which details the history of Aura and the first decade of science resulting from its data. Therefore, the current article will focus on the science and applications enabled by Aura data in the last decade. It also examines Aura’s future and the legacies of the spacecraft’s instruments. Readers interested in more information on Aura and the scientific research and applications enabled by its data can visit the Aura website.

Recent Science Achievements from Aura’s Instrument (in alphabetical order)

High Resolution Dynamics Limb Sounder

The capabilities of the High Resolution Dynamics Limb Sounder (HIRDLS) were compromised at launch and operations ceased in March 2008 due to an image chopper stall. Nevertheless, the HIRDLS team was able to produce a three-year dataset notable for high vertical resolution profiles of greater than 1 km (0.62 mi) for temperature and O₃ in the upper troposphere to the mesosphere. Though limited, the HIRDLS dataset demonstrated the incredible potential of the instrument for atmospheric research. So much so, that scientists are now in the study phase for a new instrument, part of the proposed Stratosphere Troposphere Response using Infrared Vertically-Resolved Light Explorer (STRIVE) mission, which would have similar capabilities as HIRDLS with advancements in spectral and spatial imaging. (STRIVE is one of four missions currently undergoing one-year concept studies, as part of NASA’s Earth System Explorer Program, which was established in the 2017 Earth Science Decadal Survey. Two winning proposals will be chosen in 2025 for full development and launch in 2030 or 2032.)

Microwave Limb Sounder

The Microwave Limb Sounder (MLS) was developed to study: 1) the evolution and recovery of the stratospheric O₃ layer; 2) the role of the stratosphere, notably stratospheric humidity, in climate feedback processes; and 3) the behavior of air pollutants in the upper troposphere. MLS measures vertical profiles from the upper troposphere at ~10 km altitude (6.2 mi) to the mesosphere at ~90 km (56 mi) of 16 trace gases, temperature, geopotential height, and cloud ice. Its unique measurement suite has made it the “go-to” instrument for most data-driven studies of middle atmosphere composition over the last two decades.

Data collection during the past decade has highlighted the ability of the stratosphere to exhibit surprising and/or envelope-redefining behavior, (Envelope-redefining is a term that is used to refer to an event that greatly exceeded previous observed ranges of this event.) MLS observations have been crucial for the discovery and diagnoses of these extreme events. For example, in 2019, a stratospheric sudden warming over the southern polar cap in September – rare in the Antarctic – curtailed chemical processing, leading to an anomalously weak O₃ hole. As another example, prolonged hot and dry conditions in Australia during the subsequent 2019–2020 southern summer promoted the catastrophic “Australian New Year” (ANY) fires. MLS observations showed that fire-driven pyrocumulonimbus convection lofted plumes of polluted air into the stratosphere to a degree never seen during the Aura mission.

Apart from those individual plumes, smoke pervaded the southern lower stratosphere, leading to unprecedented perturbations in southern midlatitude lower stratospheric composition, with chlorine (Cl) shifting from its main reservoir species, hydrochloric acid (HCl), into the O₃-destroying form, hypochlorite (ClO). Peak anomalies in chlorine species occurred in mid-2020 – months after the fires. State-of-the-art atmospheric chemistry models in which wildfire smoke has properties similar to those of sulfate (SO₄) aerosols were unable to reproduce the observed chemical redistribution. New model simulations assuming that HCl dissolves more readily in smoke than in SO₄ particles under typical midlatitude stratospheric conditions better match the MLS observations.

As extraordinary as these events were, their impacts on the stratosphere were spectacularly eclipsed by the impact of the January 2022 eruption of the Hunga Tonga-Hunga Ha’apai  (Hunga) volcano in the Pacific Ocean. The Hunga eruption lofted about 150 Tg of water vapor into the stratosphere – with initial injections reaching into the mesosphere. The eruption almost instantaneously increased total stratospheric water vapor by about 10%. MLS was the only sensor able to track the plume in the first weeks following the eruption. The Hunga humidity enhancement resulted in an envelope-redefining, low-temperature anomaly in the stratosphere, in turn inducing changes in stratospheric circulation. Repartitioning of southern midlatitude Cl also occurred, though to a lesser degree than following the ANY fires and in a manner broadly consistent with known chemical mechanisms. The Hunga water vapor enhancement has not substantially declined in the 2.5 years since the eruption, and studies indicate that it will likely endure for several more years.

Impacts of the Hunga humidity on polar O₃ loss have also been investigated. The timing and location of the eruption were such that the plume reached high southern latitudes only after the 2022 Antarctic winter vortex had developed. Since the strong winds at the vortex edge present a transport barrier, polar stratospheric cloud (PSC) formation and O₃ hole evolution were largely unaffected. When the vortex broke down at the end of the 2022 Antarctic winter, moist air flooded the southern polar region, increasing humidity in the region. Cold, moist conditions led to unusually early and vertically extensive PSC formation and Cl activation, but chemical processing ran to completion by mid-July, as typically occurs in southern winter. The cumulative chemical O₃ losses ended up being unremarkable throughout the lower stratosphere. The Hunga plume was also largely excluded from the 2022–2023 Arctic vortex. The 2023–2024 Arctic O₃ loss season was characterized by conditions that were dynamically disturbed and not persistently cold, and springtime O₃ was near or above average. The extraordinary stratospheric hydration from Hunga has so far had minimal impact on chemical processing and O₃ loss in the polar vortices in either hemisphere – see Figure 2.

With the end of Aura and MLS, the future for stratospheric limb sounding observations is unclear. While stratospheric O₃ and aerosol will continue to be measured on a daily, near-global basis by the Ozone Mapping and Profiler Suite (OMPS) Limb Profiler (OMPS-LP) instruments on the Suomi National Polar-orbiting Partnership (Suomi NPP) and Joint Polar Satellite System (JPSS-2, -3, and -4) satellites, there are no confirmed plans for daily, near-global observations of either long-lived trace gases or halogenated species – both of which are needed to diagnose observed changes in O₃. The only other sensor making such measurements, the Canadian Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE–FTS), is itself older than MLS and, as a solar occultation instrument, measures only 30 profiles-per-day, taking around a month to cover all latitudes. Similarly, no other sensor is set to provide daily, near-global measurements of stratospheric water vapor until the launch of the Canadian High-altitude Aerosols, Water vapour and Clouds (HAWC) mission in the early 2030s. Some potential new mission concepts are under consideration by both NASA and ESA, but they are subject to competition. Even if both instruments are ultimately selected, gaps in the records of many species measured by MLS are inevitable. The MLS PI is leading an effort to develop new technologies that would allow an instrument that could restart MLS measurements to be built in a far smaller mass/power footprint (e.g., 60 kg, 90 W vs. 500 kg, 500 W for Aura MLS), and technologies exist for yet-smaller MLS-like instruments that could assume the legacy of the highly impactful MLS record at low cost in future decades.

Ozone Monitoring Instrument

The Ozone Monitoring Instrument (OMI) continues the Total Ozone Mapping Spectrometer (TOMS) record for total O₃ and other atmospheric parameters related to O₃ chemistry and climate. It employs hyperspectral imaging in a push-broom mode to observe solar backscatter radiation in the visible and ultraviolet.

OMI is a Dutch–Finnish contribution to the Aura mission, and its remarkable stability and revolutionary two-dimensional (2D) detector (spatial in one dimension and spectral in the other) has produced a two-decade record of science- and trend-quality datasets of atmospheric column observations. OMI continues the long-term record of total column O₃ measurements begun in 1979, and its observations of nitrogen dioxide (NO₂), sulfur dioxide (SO₂), formaldehyde (CH₂O), and absorbing aerosols provided exceptional spatial resolution for study of anthropogenic and natural trends and variations of these pollutants around the world. Its radiometric and spectral stability has made it a valuable contributor for solar spectral irradiance measurements to complement dedicated solar instruments on other satellites. The many achievements made possible with OMI are documented in a review article.

OMI’s multidecade data records have revolutionized the ability to monitor air quality changes around the world, even at the sub-urban level. In particular, OMI NO₂ data have been transformative. Recently, these data were used to track changes in air pollution associated with efforts to control the spread of SARS-CoV-2. OMI’s long, stable data record allowed for changes in pollution levels in 2020 – at the height of global lockdowns – to be put into historical perspective, especially within the envelope of typical year-to-year variations associated with meteorological variability. Many research studies assessed the impact of the pandemic lockdowns on air pollution, supporting novel uses of OMI data for socioeconomic-related research. For example, OMI NO₂ data were shown to serve as an environmental indicator to evaluate the effectiveness of lockdown measures and as a significant predictor for the deceleration of COVID-19 spread. OMI NO₂ data were also used as a proxy for the economic impact of the pandemic as NO₂ is emitted during fossil fuel combustion, which is another proxy for economic activity since most global economies are driven by fossil fuels – see Animation.

OMI’s datasets are being continued by successor 2D detector array instruments, such as the previously mentioned Copernicus Sentinel-5P TROPOMI mission, the Republic of Korea’s Geostationary Environment Monitoring Spectrometer (GEMS), and NASA’s Tropospheric Emissions: Monitoring of Pollution (TEMPO). All of these missions have enhanced spatial resolution relative to OMI, but have benefited from the innovative retrieval algorithms pioneered by OMI’s retrieval teams.

Tropospheric Emission Spectrometer

The Tropospheric Emission Spectrometer (TES) provided vertically-resolved distributions of a number of tropospheric constituents, e.g., O₃, methane (CH₄), and various volatile organic compounds. The instrument was decommissioned in 2018 due to signs of aging associated with a failing Interferometer Control System motor encoder bearing. Nevertheless, TES measurements led to a number of key results regarding changes in atmospheric composition that were published over the past 10 years.

Measurements from TES, OMI, and MLS showed that transport of O₃ and its precursors from East Asia offset about 43% of the decline expected in O₃ over the western U.S., based on emission reductions observed there over the period 2005–2010. TES megacity measurements revealed that the frequency of high-O₃ days is particularly pronounced in South Asian megacities, which typically lack ground-based pollution monitoring networks. TES water vapor and semi-heavy water measurements indicated that water transpired from Amazonian vegetation becomes a significant moisture source for the atmosphere, during the transition from dry to wet season. The increasing water vapor provides the fuel needed to start the next rainy season. Measurements of CH₄ from TES and carbon monoxide (CO) from Measurements of Pollution in the Troposphere (MOPITT) on Terra showed that CH₄ emissions from fires declined at twice the rate expected from changes in burned area from 2004–2014. This finding helped to balance the CH₄ budget for this period, because it offset some of the large increases in fossil fuel and wetland emissions. Through direct measurement of the O₃ greenhouse gas effect, TES instantaneous radiative kernels revealed the impact of hydrological controls on the O₃ radiative forcing and were used to show substantial radiative bias in Intergovernmental Panel on Climate Change (IPCC) chemistry–climate models. The TES team pioneered the retrieval of a number of species, such as peroxyacetyl nitrate, carbonyl sulfide, and ethylene.

The spirit of TES lives on through the NASA TRopospheric Ozone and its Precursors from Earth System Sounding (TROPESS) project, which generates data products of O₃ and other atmospheric constituents by processing data from multiple satellites through a common retrieval algorithm and ground data system. TROPESS builds upon the success of TES and is considered a bridge to allow the development of a continuous record of O₃ and other trace gas species as a follow-on to TES.

Future of Aura

In April 2023, Aura’s mission operations team performed the last series of maneuvers to maintain its position in the A-Train constellation of satellites. Since then, Aura has begun drifting. As of July 2024, Aura has descended ~5 km (3 mi) in altitude from ~700 km (435 mi) and its equator crossing time has increased by ~9 min from ~1:44 PM local time. This amount of drift is small, and the Aura MLS and OMI retrieval teams are ensuring the science- and trend-quality of the datasets.

As Aura continues to drift, the amount of sunlight reaching its solar panels will slowly decrease and will no longer be able to generate sufficient power to operate the spacecraft and instruments by mid-2026. At this point, the amount of local time drift will still be relatively small – less than one hour – so the retrieval teams will be able to ensure quality for most data products until this time.

In the remaining years, Aura’s aging but remarkably stable instruments will continue to add to the unprecedented two decades of science- and trend-quality data of numerous key tropospheric and stratospheric constituents. Aura data will be key for monitoring the evolution of the Hunga volcanic plume and understanding its continued impact on the chemistry and dynamics of the stratosphere. Observations from MLS and OMI will also be used to evaluate data from new and upcoming instruments (e.g., ESA’s Atmospheric Limb Tracker for Investigation of Upcoming Stratosphere (Altius); NASA’s TEMPO, Plankton, Aerosol, Cloud, ocean Ecosystem (PACE), and Total and Spectral Solar Irradiance Sensor-2 (TSIS-2) missions, or at least used to help minimize the gaps between data collections.

Aura’s Scientific Legacy

The Aura mission has been nothing short of transformative for atmospheric research and applied sciences. The multidecade, stable datasets have furthered process-based understanding of the chemistry and dynamics of atmospheric trace gases, especially those critical for understanding the causes of trends and variations in Earth’s protective ozone layer.  

The two decades that Aura has flown have been marked by profound atmospheric changes and numerous serendipitous events, both natural and man-made. The data from Aura’s instruments have given scientists and applied scientists an unparalleled view – including at the sub-urban scale – of air pollution around the world, clearly showing the influence of rapid industrialization, environmental regulations designed to improve air quality, seasonal agricultural burning, catastrophic wildfires, and even a global pandemic, on the air we breathe. The Aura observational record spans the period that includes the decline of O₃-destroying substances, and Aura data illustrate the beginnings of the recovery of the Antarctic O₃ hole, a result of unparalleled international cooperation to reduce these substances.

Aura’s datasets have given a generation of scientists the most comprehensive global view to date of critical gases in Earth’s atmosphere and the chemical and dynamic processes that shape their concentrations. Many, but not all, of these datasets are being/will be continued by successor instruments that have benefited from the novel technologies incorporated into the design of Aura’s instruments as well as the innovative retrieval algorithms pioneered by Aura’s retrieval teams.

Acknowledgements
The author wishes to acknowledge the decades of hard work of the many hundreds of people who have contributed to the success of the international Aura mission. There are too many to acknowledge here and I’m sure that many names from the early days are lost to time. I would like to offer special thanks to those scientists who, back in the 1980s, first dreamed of the mission that would become Aura.

Bryan Duncan
NASA’s Goddard Space Flight Center (GSFC)
bryan.n.duncan@nasa.gov