Ozone (O3), a gas made up of three oxygen atoms, is something of a double-edged compound. In the upper stratosphere, where it occurs naturally, ozone protects life on Earth from the Sun's ultraviolet radiation. This is the so-called “good” ozone. Yet, in the troposphere, the lowest level atmosphere near Earth's surface, ozone can be created by reactions between airborne pollutants, and at ground level, high ozone concentrations can be toxic to humans. This is the “bad” ozone.
The natural concentration of ozone in the troposphere is about 10 parts per billion (10 molecules for every billion air molecules), much lower than the 2-8 parts per million in the ozone layer where most (85-90%) of the planet's ozone is located. However, the levels of tropospheric ozone can rise when nitrogen oxide gases from vehicle and industrial emissions react with volatile organic compounds (e.g., carbon-containing chemicals such as paint thinners) that evaporate easily into the air. According to the Environmental Protection Agency (EPA), exposure to ozone levels of greater than 70 parts per billion for 8 hours or longer is unhealthy. Such concentrations occur in or near cities during periods where the atmosphere is warm and stable, and the harmful effects of prolonged exposure can result in throat and lung irritation, aggravation of asthma, or emphysema.
Fortunately, the presence of ozone and other trace (small percentage) gasses in the atmosphere, including pollutants such as nitrogen dioxide (NO2) and sulfur dioxide (SO2), can be detected by their unique spectral signatures. This is precisely what the Ozone Monitoring Instrument (OMI) aboard NASA's Aura satellite was created to do.
NASA launched the Aura satellite, the third of its flagship Earth Observing System satellite missions behind Terra and Aqua, in 2004. Flying about 15 minutes behind Aqua as part of the Afternoon Constellation (or A-train) of Earth-observing satellites, Aura's four instruments—the Microwave Limb Sounder (MLS), the High-Resolution Dynamics Limb Sounder (HIRDLS), the Tropospheric Emission Spectrometer (TES), and OMI—were designed to collect data about the chemistry and dynamics of Earth's atmosphere, enabling scientists to investigate the presence and trends in atmospheric trace gas concentrations, the trends and processes that create various atmospheric conditions, and the impact of climate change on air quality.
One of the most important aims of the Aura mission was to assess and monitor the stratospheric ozone layer. Concern about ozone depletion in the 1970s led to a congressional mandate that charged NASA and NOAA with developing ways to monitor and track the amounts of ozone and ozone-depleting gases in the atmosphere. Following the discovery of the ozone hole over the South Pole by NASA and British researchers in the early 1980s and the implementation of the Montreal Protocol on Substances that Deplete the Ozone Layer, which sought to end the use of ozone-depleting chlorofluorocarbons (CFCs) beginning in 1987, NASA has routinely monitored the state of stratospheric ozone levels around the globe to assess the ozone hole's recovery. Monitoring serves to gauge the effect of regulations designed to curb the use of ozone-depleting chemicals.
To help it perform these critical roles, NASA partnered with the Royal Netherlands Meteorological Institute (KNMI) and the Finnish Meteorological Institute (FMI) to develop and operate the OMI instrument. Built by Dutch Space (now Airbus Defense and Space Netherlands) and aerospace company TNO, OMI is a nadir-viewing wide-field-imaging spectrometer designed to provide high-resolution observations of O3, NO2 (total as well as tropospheric columns), and SO2, as well as other trace gases and aerosols like dust and smoke by measuring mid-range ultraviolet (UV) and visible infrared radiation from multiple fields of view simultaneously. As such, it is an improvement over legacy sensors such as the Total Ozone Mapping Spectrometer (TOMS) aboard NASA’s Earth Probe Satellite and the European Space Agency's (ESA) Global Ozone Monitoring Experiment (GOME) instrument aboard the ESA ERS-2 satellite
“OMI was the first to obtain daily global coverage with very high spatial resolution, measuring ultraviolet and visible spectra at the same time,” said OMI Principal Investigator Dr. Pieternel Levelt. “So, there was an expectation that we could improve on the space-based ozone and air quality measurements with OMI. Its potential to measure air pollution and emission sources in the troposphere turned out to be unprecedented”
Those expectations were spot on.
“One of the very interesting things that came out of our research work with OMI was the ability to determine tropospheric ozone with techniques like cloud slicing,” said U.S. OMI science team leader Dr. Joanna Joiner. “By measuring the total column ozone above thick clouds we can determine the stratospheric ozone amount accurately in the tropics. Defining stratosphere ozone, which of course lies between the instrument and troposphere, is important for being able to determine trends in tropospheric ozone.”
OMI is not the only instrument aboard Aura that measures ozone and trace gases. MLS provides high vertical resolution profiles of the upper atmosphere (stratosphere) that are nearly simultaneous with those from OMI. By combining these observations with meteorological data, scientists have developed algorithms that separate out the ozone observed in the stratosphere from ozone in the atmospheric column to derive measurements of tropospheric ozone. OMI and MLS observations have also been combined with atmospheric profiles of O3 from the TES instrument, giving atmospheric scientists a better understanding of the stratospheric and tropospheric contributions to ozone as well as the physical and chemical processes that effect their distributions in these layers of the atmosphere.
Beyond ozone, OMI has improved the detection of atmospheric SO2 from erupting and outgassing volcanoes, oil and gas production sites, power plants, and other industrial operations. “Detecting individual sources of SO2—sources that were previously undetected and not in the emissions databases used by modelers—quantifying them, and tracing them over time has been an exciting challenge,” said Joiner. “Trends in SO2 and NO2 have been surprising, both upwards and downwards and in different parts of the world. So, being able to measure and track them accurately has been important.”
OMI has also enhanced the detection of aerosols and provided insight into their effects on climatological phenomena.
“When you have more smoke over clouds the sunlight gets absorbed by the smoke and the dust, and the clouds reflect less light,” said Joiner. “For example, we've seen recent fires, very intense fires, that create a large amount of absorbing material that gets very high in the atmosphere, and it can remain for long periods of time and impact the radiation balance. It's important to have a long-term record so that when such events happen in the future we can get an idea of how frequent they are.”
These improvements are a direct result of OMI's high-resolution measurements.
"With small pixels one can measure in-between clouds more often, which is important for penetrating to the lower kilometers in the troposphere for measuring air pollution," said Levelt. "With an instrument like OMI that measures across the ultraviolet and visible spectrum, one cannot measure through the clouds, just as we cannot see through a cloud with our own eyes. Therefore, it is important to have small pixels so that the instrument measures the pollution residing in the lower layers of the atmosphere. Moreover, the smaller the pixels, the better one can measure and localize small emission sources."
OMI also advanced the use of satellites for air quality observations by offering measurements for never-before-seen areas.
"People thought you couldn't see pollution in sparsely populated areas at high latitudes like Finland. It was thought to be clean and nothing there," said FMI's Dr. Johanna Tamminen, an OMI Co-Principal Investigator. "But when we started to average data, zoom, and adjust display scales, OMI started to show us interesting things. It was a real eye-opener that we could also use satellites to look at the air quality in Finland."
Of course, OMI's most significant contribution to the atmospheric research community, as noted in this 2018 paper by Levelt, et al., is its long-term, 18-year data record.
"Being able to observe atmospheric conditions at the city scale is allowing us to understand their impacts on human health and even health inequities depending on where people live. To do this you need a long-term data record," Joiner said. "Air quality can change significantly based on the local weather. OMI's coverage allows you to compare a day from a week in February with the same week averaged over several previous years to draw reasonable conclusions by minimizing the impact local weather has on air pollution. You need an assessment of what normal NO2 emissions look like to be able to see the impact."
Identifying those long-term trends is essential for determining whether regulations, like those enacted to curb the use of CFCs or reduce emissions of NO2, are having an effect. OMI’s 18-year data record shows that they are.
“NASA's Aura satellite detects gasses like NO2, a common pollutant from cars and power plants. The data show us that the U.S. and Europe have some of the highest emissions in the world,” said former U.S. President Barack Obama in a 2016 video from DNews. “But it also shows us that, in the last 25 years, NO2 levels have dropped by up to 50% in both regions, thanks in large part to new rules that protect our air. Imagery like this can help us see what actions are working, and where we need to focus additional international efforts.”
Having a consistent long-term data record allows researchers to establish local and seasonal baseline trends, so they can assess both normal conditions and the effects of out-of-the-ordinary events, when conditions may be changing rapidly. For example, by comparing data from the previous five years to measurements taken during COVID lockdowns, scientists were able to assess the lockdown's impact on transportation using OMI NO2 observations.
Beyond clarifying long-term atmospheric trends, scientists have used OMI's lengthy data record to devise new applications for the instrument's data, such as retrieving new trace gas species, helping evaluate physical models of the Sun, and experimenting with new inverse modelling techniques to estimate pollution emissions, such as along the coast of China.
As a case in point, a recent paper published in Frontiers of Remote Sensing discussed the use of OMI data to help fill gaps in the record of ocean color measurements, or the spectral composition of the visible light field that emanates from the ocean, which provides clues to the substances (e.g., phytoplankton and suspended sediments) that are dissolved and suspended in the water column.
“Even after 18 years in space we're still learning new things that we can do with OMI and similar instruments,” said Joiner. “When we learn how to do something new, we can always go back and reprocess existing data to create a long record of new and improved products.”
OMI's long-term data record is a consequence of the instrument's remarkable durability and stability. During its nearly two decades in space, OMI has experienced only minor optical problems, unusually low optical degradation over its lifespan, and detector degradation well within the expected range. This is significant as observational errors typically increase with sensor degradation.
“Over its 18-year life, the detector degradation has reached the point where retrievals for trace gases like formaldehyde, which typically has very low spectral signatures, are difficult,” said Levelt. “As far as the other OMI products, the detector degradation may impact NO2 and SO2 products, but not to the level that it significantly challenges the retrievals. It's just amazing.”
Of course, no instrument or satellite orbits Earth forever. Therefore, it likely came as little surprise when, after 18 years, NASA's Aura Project began reporting decommissioning scenarios, plans, and schedules at Mission Operations Working Group meetings. Various de-orbiting scenarios are under consideration, but eventually Aura's instruments will need to be turned off as part of spacecraft passivation, which is projected to occur in 2025 (if maximally extended). In the interim, Aura will drift from its traditional orbit, but the change is expected to be small and the impact on data quality is forecast to be minimal.
Next Generation Instruments
After Aura has been decommissioned, three newer instruments already in orbit—the Joint Polar Satellite System’s (JPSS) Ozone Mapping and Profiler Suite (OMPS), the ESA GOME-2 instrument, and the TROPOspheric Monitoring Instrument (TROPOMI)—will become the primary sources for daily global hyperspectral observations of ozone and trace gases, including SO2 and NO2.
The OMPS instruments aboard the joint NASA-NOAA Suomi National Polar-orbiting Partnership (Suomi NPP) and NOAA-20 satellites are now the operational instruments for NOAA's ozone monitoring program. OMPS was designed to measure atmospheric total column ozone with configurable nadir temporal (along-track) and spatial (across-track) resolutions. OMPS on Suomi NPP collects data at a larger 50x50 km pixel size, while NOAA-20 OMPS observations are collected at a smaller pixel size. OMPS also offers SO2 and other trace gas measurements but lacks spectral coverage in the violet-blue wavelength typically used for NO2 retrievals. (Note: NO2 observations are possible from OMPS, but they are less optimal as compared with OMI.)
In addition to OMPS, TROPOMI (the next generation instrument after OMI) was launched in 2017 aboard the European Sentinel-5P satellite, which circles Earth in an afternoon orbit slightly higher than Aura. Compared to OMI, TROPOMI has a higher spatial resolution (3.5 x 5.5 km2) and a higher signal-to-noise ratio-per-ground pixel. It provides measurements of the same trace gases that can be monitored with OMI plus methane and carbon monoxide. TROPOMI's spectral requirements, calibration, and stability are about the same as OMI.
These improvements are noteworthy, said Levelt, who is the scientific initiator of the TROPOMI instrument.
“Regardless of TROPOMI's smaller pixel size, we get at least the same and up to five times more radiation depending on the wavelength,” she said. “Another big improvement is the addition of the oxygen bands to improve information about clouds. With the extra bands, we can get more parameters enabling us to determine cloud fraction, cloud height, surface albedo, and cloud albedo. The short-wave infrared channel (SWIR) that was added for carbon monoxide and methane monitoring is performing extremely well. The methane measurements are especially newsworthy, showing many methane sources, such as those from oil and gas exploration and landfills.”
The TROPOMI mission has a seven-year lifespan, although it could remain in orbit much longer. ESA is planning three follow-on instruments, called Sentinel-5/UVNS, that will be similar to TROPOMI with 7-km pixel size and an extra shortwave infrared channel. Sentinel-5/UVNS is the follow-up of the GOME-2 instrument and will fly in a morning, rather than afternoon, orbit.
“We find ourselves in the position of going to a morning orbit with a similar spatial resolution and more spectral coverage, but we lose the afternoon orbit,” said Levelt, who is a member of ESA's Sentinel-4 and -5 Mission Advisory Group, which is planning three Sentinel-5 (TROPOMI-like) instruments over the next 20 years. She and her colleagues are aware that a potential gap in afternoon coverage could impact long-term data continuity after TROPOMI, especially for NO2 and methane observations.
Air quality scientists and modelers have expressed similar concern about the impact of losing early afternoon observations of trace gas satellite retrievals. In the morning, the boundary layer is not well mixed and resides mainly in the first few hundred meters above the surface. During the day the Sun warms the atmosphere and the boundary layer gets well-mixed, which makes interpretation and comparisons between modelling and measurements easier. Therefore, the best time to make NO2 observations is in the early afternoon, as OMI and TROPOMI do.
OMI-like spectrograph instruments are also being planned for inclusion on geostationary satellites, such as the Tropospheric Emissions Monitoring of Pollution (TEMPO) mission, the Korean GEMS instrument, the European Sentinel-4 light imaging spectrometer instruments, and NOAA's GeoXO system. OMI-like sensors aboard these spacecraft would potentially give atmospheric scientists the opportunity to acquire measurements 8 to 10 times during the day and follow the diurnal cycle of air pollutants. To intercalibrate the geostationary TEMPO, GEMS, and Sentinel-4 satellites, it is also important to keep the afternoon orbit next to the morning orbit so that there are two points in the diurnal cycle to use in intercalibrating the GEO atmospheric measurements, instead of having only one point in the morning.
New instruments aside, there's no question that the eventual end of OMI observations will result in a lot of work to create long-term data records that connect OMI data to that from OMPS and TROPOMI, as well as further back to the GOME and TOMS instruments.
"At the moment, when we see changes, we can compare them to what happened almost 20 years ago, thanks to the instrument being so stable," said Tamminen. "It is not easy to simply put another satellite instrument up and expect it to work like OMI does now. We will need to figure out how to combine and how to continue reliable and consistent datasets from OMI though to the next generation instruments."
Differences in instrument resolution and calibration among the instruments will need to be examined and accounted for to combine observations made in 2030 with those from 2010. Nevertheless, Tamminen is hopeful FMI will be able to develop long term records of UV index products extending from TOMS, through OMI, to TROPOMI in the hopes of achieving a 40-year record.
OMPS data are already being used to advance the total ozone long-term data record that started with TOMS and continued with OMI. Even though OMPS has a larger footprint, the global total ozone data product is expected to be of similar quality. Long-term data records pertaining to trace gasses have proven more challenging, as the differences between the OMI and OMPS instruments are more significant.
For now, Levelt advises OMI data users to begin working with OMPS and TROPOMI data while there is still a period of overlap. However, given the differences among products, users are advised to take care in determining which OMPS or TROPOMI data products are best for their application or research.
Users can find the information on OMI, OMPS, and TROPOMI data products from NASA’s Goddard Earth Sciences Data and Information Services Center (GES DISC) Distributed Active Archive Center (DAAC), which archives and distributes data from NASA's Earth-observing satellites and field measurement programs and provides services to users within the discipline communities. These services include providing complete documentation for available datasets as well as access to subject matter experts who can advise users on research needs, data applications, and accessing, viewing, and manipulating NASA Earth observing data. Products can be accessed as well through the EOSDIS Earthdata Search application by searching with keywords such as "ozone."
In addition, near real-time (NRT) OMI and NRT OMPS data products are available through the EOSDIS Land, Atmosphere Near real-time Capability for EOS (LANCE). While NRT products do not have the extensive processing, quality assurance, or validation required for use in scientific research, they are available within three hours of a sensor observation and are an excellent resource for tracking on-going events or monitoring daily ozone concentrations. Experience with OMI and TROPOMI NRT data has found that the data quality is very good and these data are regularly used for scientific research. (OMI NRT data are also available from KNMI's TEMIS website.)
"OMI and OMPS science quality data products are available usually within 24 hours," says James Johnson, GES DISC Scientist for the OMI and OMPS missions. "Right now, we are receiving Version 3 and the new Version 4 OMI products. Version 4 data is in a new format, netCDF4, which is designed to be compatible with TROPOMI (which is also in netCDF4) and make the transition easier."
The OMI Version 4 Level 1B products—the calibrated radiances that are the inputs to trace gas retrieval algorithms—are nearly fully reprocessed and should be completed in the next month. Once completed, reprocessing to Level 2 and 3 products can begin. Version 3 is in an older format, but maintains the ability to process consistently from the start of the mission.
According to Johnson, there is generally a good mapping between OMI and OMPS products, especially ozone, SO2, NO2, and aerosols. "OMPS consists of multiple instruments, a nadir mapper similar to OMI and two vertical profilers on Suomi NPP, and just one vertical profiler on NOAA-20, each with products archived by GES DISC," he said. "In addition, NASA scientists and researchers generate products that combine instrument data from multi-satellites that are also archived by GES DISC. With product names and titles somewhat similar, this can make it confusing to know which products are suitable for your application or research. GES DISC has been developing guidance information and is available to help."
TROPOMI data are available from ESA's Copernicus Open Access Hub, and through GES DISC, courtesy of an agreement wherein NASA assists with archiving and distributing the instrument's data. A limited amount of TROPOMI L2 NRT data can also be found within three hours after an observation on the TEMIS website.
"NASA's Sentinel Gateway facility retrieves TROPOMI products from ESA and sends the products to the GES DISC," said Dr. Feng Ding, GES DISC Data System Engineer specializing in TROPOMI data and services. "Products are normally available within two to three days of observation."
ESA released the first TROPOMI ozone profile product on November 16, 2021. Users can access it through the GES DISC website, get it through GES DISC data downloading and subsetting services, and display it with popular visualization tools.
"The best way to find and download these data products is go to the GES DISC website or Earthdata Search, which provides users with product lists, landing pages, and documentation, including data resolution and data coverage information," said Ding.
Ding also advises users to review the product documentation that GES DISC provides along with any updates, as they may offer important information about changes to the data. For example, ESA changed the instrument's operating configuration in August 2019 to start taking higher-resolution measurements. As of August 6, 2022, there are new versions of the mission's data products. To combine the observations for a long-term record, researchers will need to update data processing to account for the change.
As Johnson's and Ding's statements suggest, efforts to extend OMI's 18-year data record to the next generation of instruments are well underway. As they continue, and as scientists find new ways to use and benefit from OMI's long-term observations, the instrument's legacy of helping scientists characterize and identify long-term atmospheric trends in the interest of public health and clean air will live on.