Aura Mission (EOS/Chem-1)
Aura (Latin for breeze, formerly EOS/Chem-1) is the chemistry mission of NASA with the overall objective to study the chemistry and dynamics of Earth's atmosphere from the ground through the mesosphere. The goal is to monitor the complex interactions of atmospheric constituents from both natural sources, such as biological activity and volcanoes, and man-made sources, such as biomass burning, are contributing to global change and effect the creation and depletion of ozone. The Aura mission will provide global surveys of several atmospheric constituents which can be classified into anthropogenic sources (CFC types), radicals (e.g., ClO, NO, OH), reservoirs (e.g., HNO, HCl), and tracers (e.g., N2O, CO2, H2O). Temperature, geopotential heights, and aerosol fields will also be mapped. 1) 2) 3) 4) 5) 6)
In many ways, Aura is a follow-on to the very successful UARS (Upper Atmosphere Research Satellite) mission of NASA. UARS made stratospheric constituent measurements from 1991-2005. Unlike UARS, however, Aura is designed to focus on the lower stratosphere and the troposphere.
Figure 1: Artist's rendition of the Aura spacecraft )image credit: NGST)
The Aura spacecraft, like Aqua, is based on TRW's modular, standardized AB1200 bus design with common subsystems. [Note: As of Dec. 2002, TRW was purchased by NGST (Northrop Grumman Space Technology) of Redondo Beach, CA]. The S/C dimensions are: 2.68 m x 2.34 m x 6.85 m (stowed) and 4.71 m x 17.03 m x 6.85 m (deployed). Aura is three-axis stabilized, with a total mass of 2,967 kg at launch, S/C mass of 1,767 kg, payload mass =1,200 kg. The S/C design life is six years. 7)
Figure 2: Illustration of the Aura spacecraft (image credit: NGST)
The spacecraft structure is a lightweight 'eggcrate' compartment construction made of graphite epoxy composite over honeycomb core, providing a strong but light base for the science instruments (referred to as T330 EOS common spacecraft design). A deployable flat-panel solar array with over 20,000 silicon solar cells provides 4.6 kW of power.
Spacecraft attitude is maintained by stellar-inertial, and momentum wheel-based attitude controls with magnetic momentum unloading, through interaction with the magnetic field of the Earth that provide accurate pointing for the instruments. Typical pointing knowledge of the line of sight of the instruments to the Earth is on the order of one arcminute (about 0.02º).
A propulsion system of four small-thrust hydrazine monopropellant thrusters gives the spacecraft a capability to adjust its orbit periodically to compensate for the effects of atmospheric drag, so that the orbit can be precisely controlled to maintain altitude and the assigned ground track.
Figure 3: Alternate view of the Aura spacecraft (image credit: NASA)
Launch: A launch of Aura on a Delta-2 7920 vehicle from VAFB, CA, took place on July 15, 2004.
Orbit: Sun-synchronous circular orbit, altitude = 705 km, inclination = 98.7º, with a local equator crossing time of 13.45 (1:45 PM) on the ascending node. Repeat cycle of 16 days.
RF communications: Onboard storage capacity of 100 Gbit of payload data. The payload data are downlinked in X-band. The spacecraft can also broadcast scientific data directly to ground stations over which it is passing. The ground stations also have an S-band uplink capability for spacecraft and science instrument operations. The S-band communication subsystem also can communicate through NASA's TDRSS synchronous satellites in order to periodically track the spacecraft, calculate the orbit precisely, and issue commands to adjust the orbit to maintain it within defined limits.
The Aura spacecraft is part of the so-called "A-train" (Aqua in the lead and Aura at the tail, the nominal separation between Aqua and Aura is about 15 minutes) or "afternoon constellation" (formation flight starting sometime after the Aura launch). The objective is to coordinate observations and to provide a coincident set of data on aerosol and cloud properties, radiative fluxes and atmospheric state essential for accurate quantification of aerosol and cloud radiative effects. The orbits of Aqua and CALIPSO are tied to WRS (Worldwide Reference System) and have error boxes associated with their orbits. The overall mission requirements are written such that CALIPSO is required to be no greater than 2 minutes behind Aqua. The OCO mission of NASA is a late entry into the A-train sequence. The satellites are required to control their along-track motions and remain within designated "control boxes." Member satellites will exchange orbital position information to maintain their orbital separations. 8) 9) 10) 11) 12)
The A-train is part of GOS (Global Observing System), an international network of platforms and instruments, to support environmental studies of global concern. A draft implementation plan for GOS, also referred to as GEOSS ((Global Earth Observation System of Systems), was approved at the fourth Earth Observation summit in Tokyo in April 28, 2004.
Figure 4: Illustration of Aura spacecraft in the A-train (image credit: NASA)
The PARASOL spacecraft of CNES (launch on Dec. 18, 2004) is part of the A-train as of February 2005. The OCO mission (launch in 2009) will be the newest member of the A-train. Once completed, the A-train will be led by OCO, followed by Aqua, then CloudSat, CALIPSO, PARASOL, and, in the rear, Aura. 13)
• February 2018: NASA ended the operation of the TES (Tropospheric Emission Spectrometer) instrument on 31 January 2018, after almost 14 years of discovery. TES was the first instrument designed to monitor ozone in the lowest layers of the atmosphere directly from space. Its high-resolution observations led to new measurements of atmospheric gases that have altered our understanding of the Earth system. 14)
- TES was planned for a five-year mission but far outlasted that term. A mechanical arm on the instrument began stalling intermittently in 2010, affecting TES's ability to collect data continuously. The TES operations team adapted by operating the instrument to maximize science operations over time, attempting to extend the data set as long as possible. However, the stalling increased to the point that TES lost operations about half of last year. The data gaps hampered the use of TES data for research, leading to NASA's decision to decommission the instrument. It will remain on the Aura satellite, receiving enough power to keep it from getting so cold it might break and affect the two remaining functioning instruments.
- "The fact that the instrument lasted as long as it did is a testament to the tenacity of the instrument teams responsible for designing, building and operating the instrument," said Kevin Bowman of NASA/JPL (Jet Propulsion Laboratory) in Pasadena, CA, the TES principal investigator.
- A True Earth System Sounder: TES was originally conceived to measure ozone in the troposphere, the layer of atmosphere between the surface and the altitude where intercontinental jets fly, using high-spectral-resolution observations of thermal infrared radiation. However, TES cast a wider net, capturing signatures of a broad array of other atmospheric gases as well as ozone. That flexibility allowed the instrument to contribute to a wide range of studies — not only atmospheric chemistry and the impacts of climate change, but studies of the cycles of water, nitrogen and carbon.
- One of the surprises of the mission was the measurement of heavy water: water molecules composed of deuterium, an isotope of hydrogen that has more neutrons than normal hydrogen. The ratio of deuterium to "normal" water in water vapor gives clues to the vapor's history — how it evaporated and fell as precipitation in the past — which in turns helps scientists discern what controls the amount in the atmosphere.
- Heavy water data have led to fundamental advances in our understanding of the water cycle that were not possible before, such as how tropical thunderstorms keep the troposphere hydrated, how much water in the atmosphere is evaporated from plants and soil as compared to surface water, and how water "exhaled" from southern Amazon vegetation jump-starts the rainforest's rainy season. JPL scientist John Worden, the pioneer of this measurement, said, "It's become one of the most important applications of TES. It gives us a unique window into Earth's hydrological cycle."
- While the nitrogen cycle isn't as well measured or understood as the water cycle, nitrogen makes up 78 percent of the atmosphere, and its conversion to other chemical compounds is essential to life. TES demonstrated the first space measurement of a key nitrogen compound, ammonia. This compound is a widely used fertilizer for agriculture in solid form, but as a gas, it reacts with other compounds in the atmosphere to form harmful pollutants.
- Another nitrogen compound, peroxyacetyl nitrate (PAN), can be lofted into the troposphere from fires and human emissions. Largely invisible in data collected at ground level, this pollutant can travel great distances before it settles back to the surface, where it can form ozone. TES showed how PAN varied globally, including how fires influenced its distribution. "TES really paved the way in our global understanding of both PAN and [ammonia], two keystone species in the atmospheric nitrogen cycle," said Emily Fischer, an assistant professor in the department of atmospheric science at Colorado State University, Fort Collins.
Figure 5: TES collected spectral "signatures," illustrated here, of ozone and other gases in the lower atmosphere (image credit: NASA)
- The Three Faces of Ozone: Ozone, a gas with both natural and human sources, is known for its multiple "personalities." In the stratosphere ozone is benign, protecting Earth from incoming ultraviolet radiation. In the troposphere, it has two distinct harmful functions, depending on altitude. At ground level it's a pollutant that hurts living plants and animals, including humans. Higher in the troposphere, it's the third most important human-produced greenhouse gas, trapping outgoing thermal radiation and warming the atmosphere.
- TES data, in conjunction with data from other instruments on Aura, were used to disentangle these personalities, leading to a significantly better understanding of ozone and its impact on human health, climate and other parts of the Earth system.
- Air currents in the mid- to upper troposphere carry ozone not only across continents but across oceans to other continents. A 2015 study using TES measurements found that the U.S. West Coast's tropospheric ozone levels were higher than expected, given decreased U.S. emissions, partly because of ozone that blew in across the Pacific Ocean from China. The rapid growth in Asian emissions of precursor gases — gases that interact to create ozone, including carbon monoxide and nitrogen dioxide — changed the global landscape of ozone.
- "TES has borne witness to dramatic changes in which the gases that create ozone are produced. TES's remarkably stable measurements and ability to resolve the layers of the troposphere allowed us to separate natural changes from those driven by human activities," said JPL scientist Jessica Neu, a coauthor of the study.
- Regional changes in emissions of ozone precursor gases alter not only the amount of ozone in the troposphere, but its efficiency as a greenhouse gas. Scientists used TES measurements of ozone's greenhouse effect, combined with chemical weather models, to quantify how the global patterns of these emissions have altered climate. "In order to both improve air quality and mitigate climate change, we need to understand how human pollutant emissions affect climate at the scales in which policies are enacted [that is, at the scale of a city, state or country]. TES data paved the way for how satellites could play a central role," said Daven Henze, an associate professor in the department of mechanical engineering at the University of Colorado at Boulder.
- A Pathfinder Mission: "TES was a pioneer, collecting a whole new set of measurements with new techniques, which are now being used by a new generation of instruments," Bowman said. Its successor instruments are used for both atmospheric monitoring and weather forecasting. Among them are the National Oceanic and Atmospheric Administration's Cross-track Infrared Sounder (CrIS) instrument on the NOAA-NASA Suomi-NPP satellite and the IASI (Infrared Atmospheric Sounding Interferometer) series, developed by the French space agency in partnership with EUMETSAT, the European meteorological satellite organization.
- Cathy Clerbaux, a senior scientist with CNES (Centre National de la Recherche Scientifique) who is the leading scientist on the IASI series, said, "TES's influence on later missions like ours was very important. TES demonstrated the possibility of deriving the concentration of atmospheric gases by using interferometry to observe their molecular properties. Although similar instruments existed to sound the upper atmosphere, TES was special in allowing measurements nearer the surface, where pollution lies. The scientific results obtained with IASI greatly benefited from the close collaboration we developed with the TES scientists."
- TES scientists have been pioneers in another way: by combining the instrument's measurements with those of other instruments to produce enhanced data sets, revealing more than either original set of observations. For example, combining OMI (Ozone Monitoring Instrument) on Aura's measurements in ultraviolet wavelengths with TES's thermal infrared measurements gives a data set with enhanced sensitivity to air pollutants near the surface.
- The team is now applying that capability to measurements by other instrument pairs - for example, enhanced carbon monoxide (CO) from CrIS with CO and other measurements from TROPOMI (TROPOspheric Monitoring Instrument) on the European Space Agency's Copernicus Sentinel-5 Precursor satellite. "The application of the TES algorithms to CrIS and TROPOMI data will continue the 18-year record of unique near-surface carbon monoxide measurements from MOPITT (Measurement of Pollution in the Troposphere instrument) on NASA's Terra' satellite into the next decade," said Helen Worden, a scientist at the National Center for Atmospheric Research in Boulder, Colorado, who is both the principal investigator of MOPITT and a TES science team member.
- These new techniques developed for TES along with broad applications throughout the Earth System assure that the mission's legacy will continue long after TES's final farewell.
• January 2018: Using Aura's OMI (Ozone Monitoring Instrument) sulfur dioxide data collected between 2005-2015, a multi-decade record was produced using prior SO2 emission data from 1980 to the present. 15) (see also Ref. 24)
- This study uses satellite measurements of vertical column densities (VCD: the total number of molecules per unit area) of sulfur dioxide (SO2), a criteria air pollutant, to establish a link between reported SO2 emissions, OMI SO2 VCDs measured by satellites, and surface SO2 concentration measurements.
- The results highlight the value of long-term, consistent satellite observations in detecting changes on both global and regional scales. It has been demonstrated that the PCA (Principal Component Analysis) algorithm is capable of producing highly consistent SO2 data between Aura/OMI and the new generation operational NASA-NOAA polar partnership Suomi-NPP/OMPS UV spectrometer. The algorithm can also be implemented with the NASA/NOAA JPSS-1/OMPS and ESA Sentinel 5 Precursor TROPOMI UV spectrometer, as well as future geostationary sensors such as NASA/TEMPO and Korean GEMS.
Figure 6: These animated gif maps (1980 - 2015) show annual mean SO2 VCD (DU) calculated using the plume model applied to the reported bottom-up emission data. Annual emission data from ~380 SO2 sources (black dots) that emitted 1 kt yr-1 at least once in 2005-2015 were included in the calculations (image credit: NASA)
• January 4, 2018: For the first time, scientists have shown through direct satellite observations of the ozone hole that levels of ozone-destroying chlorine are declining, resulting in less ozone depletion. Measurements show that the decline in chlorine, resulting from an international ban on chlorine-containing manmade chemicals called chlorofluorocarbons (CFCs), has resulted in about 20 percent less ozone depletion during the Antarctic winter than there was in 2005 — the first year that measurements of chlorine and ozone during the Antarctic winter were made by NASA's Aura satellite. 16) 17)
- "We see very clearly that chlorine from CFCs is going down in the ozone hole, and that less ozone depletion is occurring because of it," said lead author Susan Strahan, an atmospheric scientist from NASA's Goddard Space Flight Center in Greenbelt, Maryland.
- CFCs are long-lived chemical compounds that eventually rise into the stratosphere, where they are broken apart by the Sun's ultraviolet radiation, releasing chlorine atoms that go on to destroy ozone molecules. Stratospheric ozone protects life on the planet by absorbing potentially harmful ultraviolet radiation that can cause skin cancer and cataracts, suppress immune systems and damage plant life.
- Two years after the discovery of the Antarctic ozone hole in 1985, nations of the world signed the Montreal Protocol on Substances that Deplete the Ozone Layer, which regulated ozone-depleting compounds. Later amendments to the Montreal Protocol completely phased out production of CFCs.
- Past studies have used statistical analyses of changes in the ozone hole's size to argue that ozone depletion is decreasing. This study is the first to use measurements of the chemical composition inside the ozone hole to confirm that not only is ozone depletion decreasing, but that the decrease is caused by the decline in CFCs. The study was published Jan. 4 in the journal Geophysical Research Letters. 18)
- The Antarctic ozone hole forms during September in the Southern Hemisphere's winter as the returning sun's rays catalyze ozone destruction cycles involving chlorine and bromine that come primarily from CFCs. To determine how ozone and other chemicals have changed year to year, scientists used data from the MLS (Microwave Limb Sounder) aboard the Aura satellite, which has been making measurements continuously around the globe since mid-2004. While many satellite instruments require sunlight to measure atmospheric trace gases, MLS measures microwave emissions and, as a result, can measure trace gases over Antarctica during the key time of year: the dark southern winter, when the stratospheric weather is quiet and temperatures are low and stable.
- The change in ozone levels above Antarctica from the beginning to the end of southern winter — early July to mid-September — was computed daily from MLS measurements every year from 2005 to 2016. "During this period, Antarctic temperatures are always very low, so the rate of ozone destruction depends mostly on how much chlorine there is," Strahan said. "This is when we want to measure ozone loss."
- They found that ozone loss is decreasing, but they needed to know whether a decrease in CFCs was responsible. When ozone destruction is ongoing, chlorine is found in many molecular forms, most of which are not measured. But after chlorine has destroyed nearly all the available ozone, it reacts instead with methane to form hydrochloric acid, a gas measured by MLS. "By around mid-October, all the chlorine compounds are conveniently converted into one gas, so by measuring hydrochloric acid we have a good measurement of the total chlorine," Strahan said.
- Nitrous oxide is a long-lived gas that behaves just like CFCs in much of the stratosphere. The CFCs are declining at the surface but nitrous oxide is not. If CFCs in the stratosphere are decreasing, then over time, less chlorine should be measured for a given value of nitrous oxide. By comparing MLS measurements of hydrochloric acid and nitrous oxide each year, they determined that the total chlorine levels were declining on average by about 0.8 percent annually.
- The 20 percent decrease in ozone depletion during the winter months from 2005 to 2016 as determined from MLS ozone measurements was expected. "This is very close to what our model predicts we should see for this amount of chlorine decline," Strahan said. "This gives us confidence that the decrease in ozone depletion through mid-September shown by MLS data is due to declining levels of chlorine coming from CFCs. But we're not yet seeing a clear decrease in the size of the ozone hole because that's controlled mainly by temperature after mid-September, which varies a lot from year to year."
- Looking forward, the Antarctic ozone hole should continue to recover gradually as CFCs leave the atmosphere, but complete recovery will take decades. "CFCs have lifetimes from 50 to 100 years, so they linger in the atmosphere for a very long time," said Anne Douglass, a fellow atmospheric scientist at Goddard and the study's co-author. "As far as the ozone hole being gone, we're looking at 2060 or 2080. And even then there might still be a small hole."
• November 14, 2017: A new study by researchers from the University of Maryland and NASA indicates that China has greatly reduced its emissions of sulfur dioxide, while India's emissions have risen dramatically. Sulfur dioxide (SO2) is an air pollutant that leads to acid rain, haze, and many health-related problems. It is primarily produced today through the burning of coal to generate electricity. 19)
- Although China and India remain the world's largest consumers of coal, the new research found that China's sulfur dioxide emissions have fallen by 75 percent since 2007, while India's emissions have increased by 50 percent. The results suggest that India may soon become, if it is not already, the world's top emitter of sulfur dioxide. Previous research has shown that sulfur dioxide emissions in the United States have been steadily dropping.
- "The rapid decrease of sulfur dioxide emissions in China far exceeds expectations and projections," said Can Li, an atmospheric chemist in the University of Maryland's Earth System Science Interdisciplinary Center and at NASA's Goddard Space Flight Center. "This suggests that China is implementing sulfur dioxide controls beyond what climate modelers have taken into account."
- The maps of Figures 7 and 8 show regional views of sulfur dioxide emissions as observed by the Dutch-Finnish OMI (Ozone Monitoring Instrument ) on NASA's Aura spacecraft. The values are yearly averages of sulfur dioxide concentrations over India and China in 2005 and 2016. The data come from the study that was published on November 9, 2017, in the journal Scientific Reports. 20)
- China and India are now the world's top consumers of coal, which typically contains up to 3 percent sulfur. Most of the sulfur dioxide emissions come from coal-fired power plants and coal-burning factories. In particular, Beijing suffers from severe haze problems because of the factories and power plants located nearby and upwind.
- Starting in the early 2000s, China began implementing policies such as fining polluters, setting emission reduction goals, and lowering emissions limits. According to the new study, these efforts are paying off. "Sulfur dioxide levels in China declined dramatically even though coal usage increased by approximately 50 percent and electricity generation grew by over 100 percent," explained Li. "This suggests that much of the reduction is coming from controlling emissions." Previous studies, which relied on ground-based inventories and published policies, projected that China's sulfur dioxide emissions would not fall to current levels until 2030 at the earliest.
- Despite the 75 percent drop in sulfur dioxide emissions, recent work by other scientists has shown that the country's air quality remains poor and still causes significant health problems. This may be because sulfur dioxide contributes just 10 to 20 percent of the particles that cause haze. "If China wants to bring blue skies back to Beijing," Li said, "the country needs to also control other air pollutants."
- By contrast, India's sulfur dioxide emissions increased by 50 percent over the past decade. The country opened its largest coal-fired power plant in 2012 and has yet to implement emission controls like China. "Right now, India's increased sulfur dioxide emissions are not causing as many health or haze problems as they do in China, because the largest emission sources are not in the most densely populated area of India," Li said. "However, as demand for electricity grows in India, the impact may worsen."
Figure 7: Changes in SO2 observations over China between 2005 and 2016 with OMI on Aura of NASA, expressed in Dobson Units (1 DU = 2.69 x 1016 molecules cm-2). The values are yearly averages of SO2 concentrations [image credit: NASA Earth Observatory, images by Jesse Allen, using OMI data courtesy of Chris McLinden (Environment Canada), story by Irene Ying (University of Maryland), with Mike Carlowicz (NASA Earth Observatory)]
Figure 8: Changes in SO2 observations over India between 2005 and 2016 with OMI on Aura of NASA, expressed in Dobson Units (1 DU = 2.69 x 1016 molecules cm-2). The values are yearly averages of SO2 concentrations [image credit: NASA Earth Observatory, images by Jesse Allen, using OMI data courtesy of Chris McLinden (Environment Canada), story by Irene Ying (University of Maryland), with Mike Carlowicz (NASA Earth Observatory)]
• November 7, 2017: Ozone pollution near Earth's surface is one of the main ingredients of summertime smog and a primary cause of poor air quality. Yet it is not directly measurable from space because of the abundance of ozone higher in the atmosphere, which obscures measurements of surface ozone. Now NASA-funded researchers have devised a way to use satellites to measure the precursor gases that contribute to ozone formation. By differentiating among three possible conditions that lead to ground-level ozone production, the observations may assist air quality managers in assessing the most effective approaches to emission reduction and air quality improvements. 21)
- At high altitude, ozone acts as Earth's sunscreen from harmful ultraviolet radiation. At low altitudes, ozone is a health hazard contributing to respiratory problems like asthma and bronchitis. Near the ground, the gas is formed through complex chemical reactions initiated by sunlight and involving VOCs (Volatile Organic Compounds) and NOx (Nitrogen Oxides). It turns out that formaldehyde (a VOC) and nitrogen dioxide (NO2), are measurable from space by the Dutch-Finnish OMI (Ozone Monitoring Instrument) aboard NASA's Aura satellite.
- We are using satellite data to analyze the chemistry of ozone from space," said lead author Xiaomeng Jin of the Lamont-Doherty Earth Observatory, Columbia University. The research was published in the Journal of Geophysical Research: Atmospheres. 22)
- Through a combination of computer models and space-based observations, Jin and colleagues used the concentrations of the precursor molecules to infer whether ozone production at a given location increases more in the presence of NOx, VOCs, or a mix of the two. Their study regions focused on North America, Europe, and East Asia during the summer, when abundant sunlight triggers the highest rates of ozone formation. To understand their impact on ozone formation, the team investigated whether VOC or NOx was the ingredient that most limited ozone formation. If emissions of that molecule can be reduced, then ozone formation will be reduced—critical information for air quality managers.
- "We are asking: ‘If I could reduce either VOCs or NOx, which one is going to get me the biggest bang for my buck in terms of the amount of ozone that we can prevent from being formed in the lower atmosphere?'" said co-author and atmospheric chemist Arlene Fiore of Lamont-Doherty, who is also a member of NASA's Health and Air Quality Applied Sciences Team.
- The researchers found that the urban regions that they studied (shown in Figure 9) are more often VOC-limited or in a transitional state between VOC and NOx-limited. Looking at 12 years of Aura measurements, they also found that circumstances can change. For instance, New York City's ozone production in the summer of 2005 was limited by VOCs; by 2015, it had transitioned to a NOx-limited system thanks to pollution controls put into place at regional and national levels. This transition means future NOx reductions will likely further decrease ozone production, Jin said.
- Volatile organic compounds occur naturally; they are most often emitted by trees in the form of formaldehyde. They can also arise from paint fumes, cleaning products, and pesticides, and they are a by-product of burning fossil fuels in factories and automobiles. Nitrogen oxides are a byproduct of burning fossil fuels and are abundant in cities, where they are emitted by power plants, factories, and cars. Because VOCs have a large natural source (trees) in the eastern United States, for example, emission reduction plans have focused on NOx, which is overwhelmingly produced by human activities and therefore more controllable.
Figure 9: OMI on NASA's Aura satellite acquired these data in the timeframe 2005-2015 (image credit: NASA Earth Observatory, image by Joshua Stevens, using data from Xiaomeng Jin, et al. (2017). Story by Ellen Gray, NASA's Earth Science News Team, with Mike Carlowicz, Earth Observatory)
Legend to Figure 9: The top row shows OMI HCHO/NO2 for 3 world regions in 2005, while the bottom row shows the same ratio for 2015. In the U.S. and Europe, major decreases in NOx emissions have caused this ratio to increase, indicating that ozone production is more sensitive now vs. a decade ago to further reductions in NOx emissions. The changes in the ratios over East Asia indicate a patchwork of emission changes and concomitant changes in ozone production sensitivities. 23)
• October 24, 2017: Reported sulfur dioxide (SO2/ emissions from US and Canadian sources have declined dramatically since the 1990s as a result of emission control measures. Observations from OMI (Ozone Monitoring Instrument) on NASA's Aura satellite and ground-based in situ measurements are examined to verify whether the observed changes from SO2 abundance measurements are quantitatively consistent with the reported changes in emissions. To make this connection, a new method to link SO2 emissions and satellite SO2 measurements was developed. The method is based on fitting satellite SO2 VCDs (Vertical Column Densities) to a set of functions of OMI pixel coordinates and wind speeds, where each function represents a statistical model of a plume from a single point source. 24)
- The concept is first demonstrated using sources in North America and then applied to Europe. The correlation coefficient between OMI-measured VCDs (with a local bias removed) and SO2 VCDs derived here using reported emissions for 1º by 1º gridded data is 0.91 and the best-fit line has a slope near unity, confirming a very good agreement between observed SO2 VCDs and reported emissions. Having demonstrated their consistency, seasonal and annual mean SO2 VCD distributions are calculated, based on reported point-source emissions for the period 1980–2015, as would have been seen by OMI. This consistency is further substantiated as the emission-derived VCDs also show a high correlation with annual mean SO2 surface concentrations at 50 regional monitoring stations.
Figure 10: Annual mean OMI SO2 VCDs from the PCA (Principal Component Analysis) algorithm (column I), mean OMI SO2 VCDs with a large-scale bias removed (column II), results of the fitting of OMI data by the set of functions that represent VCDs near emission sources using estimated emissions (see text) (column III), and SO2 VCDs calculated using the same set of functions but using reported emission values (column IV). Point sources that emitted 20 kt yr-1 at least once in the period 2005–2015 were included in the fit (they are shown as the black dots). Results of the fitting of OMI data by the set of functions that represent "sources" as 0.5º by 0.5º grid cells (shown as the black dots) using estimated emissions (see text) are shown in column V. The maps are smoothed by the pixel averaging technique with a 30 km radius (Fioletov et al., 2011). Averages for four multi-year periods – 2005–2006, 2007–2009, 2010–2012, and 2013–2015 – over the area 32.5º to 43º N and 75º to 89ºW are shown (image credit: collaborative Aura/OMI team)
Figure 11: The same as Figure 10, columns I–IV, but for the part of Europe where the majority of SO2 point sources are located. Point sources that emitted 10 kt yr-1 at least once in the period 2005–2014 were included in the fit (they are shown as the black dots). High SO2 values related to the Mt. Etna volcano in Sicily are excluded from the OMI plots. The area 35.6º to 56.6º N and 10ºW to 28.4º E is shown (image credit: collaborative Aura/OMI team)
• May 26, 2017: For more than a decade, the OMI (Ozone Monitoring Instrument) on NASA's Aura satellite has observed changes in a critical air pollutant: sulfur dioxide (SO2). In addition to harming human health, the gas reacts with water vapor to produce acid rain. Sulfur dioxide also can react in the atmosphere to form aerosol particles, which can contribute to outbreaks of haze and influence the climate. 25)
- Natural sources (volcanoes, fires, phytoplankton) produce sulfur dioxide, but burning sulfur-rich fossil fuels—primarily coal, oil, and petroleum—is the main source of the gas. Smelter ovens, which are used to concentrate metals found in ore, also produce it.
- Since Aura launched in 2004, scientists at Environment and Climate Change Canada, NASA Goddard Space Flight Center, University of Maryland, Michigan Technological University, and other research centers have been refining their analytical techniques and developing increasingly accurate ways of identifying and monitoring major sources of sulfur dioxide. This has produced an increasingly clear picture of where the gas comes from and how the amount entering the atmosphere has changed over time.
- The story of change that OMI has witnessed over North America is particularly varied and interesting. In some areas, emissions have dropped significantly; in others, they rose. In many areas, human activities played the dominant role; in others, natural processes did.
- In 2016, scientists published a global catalog of large sulfur dioxide sources as observed by OMI. Of the 92 major "hot spots" of sulfur dioxide in North America, 9 of them are volcanoes, 71 are power plants, 4 are smelters, and 8 are oil refineries. Note that some of the hot spots may represent multiple sources. The bulk of emissions in North America come from human activity; volcanoes represent about 30 percent of the total emissions.
- As seen in the maps of Figures 12 and 13, one of the most noticeable changes occurred in the Ohio Valley of the eastern United States. Sulfur dioxide in that region comes mainly from coal-fired power plants. According to OMI, emissions dropped by more than 80 percent between 2005 and 2016, mainly because of new technologies, laws, and regulations that promoted cleaner burning.
- Major sulfur dioxide sources in Mexico include several power plants in western Mexico, oil infrastructure in and along the Bay of Campeche, and active volcanoes. In particular, Popocatépetl, which is just 70 km from Mexico City, is near several power plants and oil refineries. "There are very few places in the world where such strong anthropogenic and volcanic sulfur dioxide sources are so close together," noted Simon Carn of Michigan Tech.
- OMI detected more sulfur dioxide over Mexico in 2016 than in 2005, according to the researchers. However, the mix of sources in Mexico makes it more difficult to understand the change. In 2017, Carn authored a study indicating that degassing at Popocatépetl had roughly doubled the volcano's emissions. (Note: Degassing is a constant process that occurs even when a volcano is not actively erupting).
- Meanwhile, other research based on OMI observations suggests that emissions from gas and oil facilities in and around the Gulf of Campeche also increased. The 10 to 15 percent increase OMI observed from industrial sources in Mexico, the researchers pointed out in a 2016 study, is not reflected in ground-based emissions inventories, which report a significant decline in emissions. The hotspots from power plants in western Mexico show more mixed trends. Some have shown decreases, while others have stayed roughly the same.
- There are fewer sulfur dioxide sources in the Caribbean, but OMI has observed a subtle increase over some smelters and power plants. A much more notable shift occurred around the volcanic island of Montserrat. Though the volcano was a significant source of the gas in 2005, its emissions have declined significantly since 2010.
Figure 12: Sulfur Dioxide emissions in North America observed by OMI on NASA's Aura satellite in 2005 (image credit: NASA Earth Observatory, image by Jesse Allen, story by Adam Voiland)
Figure 13: Sulfur Dioxide emissions in North America observed by OMI on NASA's Aura satellite in 2016 (image credit: NASA Earth Observatory, image by Jesse Allen, story by Adam Voiland)
• March 22, 2017: When the oil refinery in San Nicolas, Aruba, shut its doors in 2009, a silent change took place. Above the azure seas and chimney stacks, the air began to clear as sulfur dioxide (SO2) emissions plummeted (Figure 14). 26)
- Invisible to the human eye, SO2 can disrupt human breathing and harm the environment when it combines with water vapor to make acid rain. It tends to concentrate in the air above power plants, volcanoes, and oil and gas infrastructure, such as the refinery in Aruba. Using data collected by OMI (Ozone Monitoring Instrument) on the Aura satellite, the researchers mapped the noxious gas as observed over a decade.
- "Previous papers focused on particular regions; this is the first real global view," said Nickolay Krotkov of NASA/GSFC (Goddard Space Flight Center), an author of the study published in Atmospheric Chemistry and Physics. The data gives a mixed outlook across the planet, with emissions rising in some places (India) and falling in others (China and the United States).
- As a regional example of the changes, the researchers examined the fluctuations in SO2 levels around Aruba. The prevailing winds, or trade winds, naturally blow air to the west of the refineries. Light purple patches over the ocean indicate areas where SO2 has decreased over time.
- After the San Nicolas refinery halted operations in late 2009, its SO2 output dropped. But by mid-2011, the refinery was reopened and SO2 emissions were back to their previous levels of roughly 350 kilotons per year. When low oil prices caused the refinery to cut back production, emissions follow a downward curve until mid-2013, when the San Nicolas plant was converted to a product terminal. The graph of Figure 15 plots its emissions over the years.
- The same downturns in the global oil market that have put the fate of the San Nicolas operation in limbo have also affected the nearby Paraguaná Refinery Complex, which is the largest of its kind in Latin America. The 2010 and 2011 images show dark spots in the Gulf of Venezuela—likely, emissions from Paraguaná (Figure 14).
Figure 14: Plots of SO2 emissions acquired with OMI on Aura in various time slots over Aruba and Venezuela (image credit: NASA Earth Observatory, images by Joshua Stevens, using emissions data courtesy of Fioletov, Vitali E., et al. (2017))
Figure 15: SO2 emission history at San Nicolas Refinery, Aruba, acquired by OMI in the timeframe 2004-2014 (image credit: NASA Earth Observatory, images by Joshua Stevens, using emissions data courtesy of Fioletov, Vitali E., et al. (2017))
• March 10, 2017: Volcanoes erupt, spewing ash and rock. Their scarred flanks sometimes run with both lava and landslides. Such eruptions are dramatic, but sporadic. - A less dramatic but important volcanic process is the continuous, mostly quiet emission of gas. A number of volcanoes around the world continuously exhale water vapor laced with heavy metals, carbon dioxide, hydrogen, sulfide and sulfur dioxide, among many other gases. Of these, sulfur dioxide (SO2) is the easiest to detect from space. 27)
- In a new study published in Scientific Reports this week, a team of researchers reported on a new global inventory of sulfur dioxide emissions from volcanoes. They compiled emissions data gathered by the Dutch-Finnish OMI (Ozone Monitoring Instrument) on NASA's Aura satellite, and then produced annual emissions estimates for 91 active volcanoes worldwide. The maps of Figures 16 and 17 depict the emissions from volcanoes in the Aleutian Island chain off Alaska and in the islands of Indonesia. 28)
- "Many people may not realize that volcanoes are continuously releasing quite large amounts of gas, and may do so for decades or even centuries," said Michigan Technological University volcanologist Simon Carn, the lead author of the study. "Because the daily emissions are smaller than a big eruption, the effect of a single plume may not seem noticeable. But the cumulative effect of all volcanoes can be significant. In fact, on average, volcanoes release most of their gas when they are not erupting."
- Carn and his team found that volcanoes collectively emit 20 to 25 million tons of sulfur dioxide (SO2) into the atmosphere each year. This number is higher than the previous estimate (made from ground measurements in the 1990s) because the new research includes data on more volcanoes, including some that scientists have never visited.
- The sulfur dioxide released by volcanoes is half as much as the amount released by human activities, according to co-author Vitali Fioletov, an atmospheric scientist at Environment and Climate Change Canada. He has been working with satellite observations and wind data to catalog SO2 emissions sources (human and volcanic) and trace them back to their sources.
- Manmade emissions of sulfur dioxide have been declining in many countries due to stricter pollution and technological advances. As those emissions decrease, the relative importance of persistent volcanic emissions rises. These new data will help refine climate and atmospheric chemistry models and provide more insight into human and environmental health risks. Atmospheric processes convert SO2 into sulfate aerosols—small suspended particles in the atmosphere that reflect sunlight back into space and can cause a cooling effect on climate. Sulfate aerosols near the land surface are harmful to breathe. Higher in the atmosphere, they become the primary source of acid rain.
- Tracking sulfur dioxide emissions via satellite also could help with eruption forecasting, as noticeable increases in SO2 gas releases may precede eruptions. "It is complementary to ground-based monitoring," Carn said. "Ground-based measurements are crucial, but the satellite data could allow us to target new measurements at unmonitored volcanoes more effectively, leading to better estimates of volcanic carbon dioxide emissions."
- Field measurements of SO2 emissions are improving, but they are still too sparse for a cohesive global picture. That's where the new inventory is handy: it gathers data from remote volcanoes and provides consistent measurements over time from the world's biggest emitters, including Ambrym in Vanuatu and Kilauea in Hawaii.
- "Satellites provide us with a unique big-picture view of volcanic emissions that is difficult to obtain using other techniques," Carn said. "We can use this to look at trends in sulfur dioxide emissions on the scale of an entire volcanic arc."
Figure 16: Mean SO2 columns (in Dobson Units [DU]; 1 DU = 2.69 x 1016 molecules cm-2) acquired with OMI (Ozone Monitoring Instrument) on NASA's Aura satellite during the period 2014-2016 over the Aleutian Islands (image credit: NASA Earth Observatory, maps created by Jesse Allen using OMI data provided by Chris McLinden, caption by Allison Mills)
Figure 17: Mean SO2 columns acquired with OMI (Ozone Monitoring Instrument) on NASA's Aura satellite during the period 2014-2016 over the Indonesian Islands (image credit: NASA Earth Observatory, maps created by Jesse Allen using OMI data provided by Chris McLinden, caption by Allison Mills)
• Instrument status after more than 12 years on orbit in December 2016: Three of Aura's four original instruments continue to function nominally; however, TES (Tropospheric Emissions Spectrometer) is about to reach the end of its expected life. Aura data are being widely used by the science and applications community, and in many instances Aura data are being applied in conjunction with data from other instruments that fly onboard satellites that make up the A-Train (Afternoon Constellation). 29)
- OMI (Ozone Monitoring Instrument) of KNMI has been extremely stable, making it highly suitable for ozone and solar irradiance trend analysis. OMI data have been used to study global and regional air-quality trends. 30)
- OMI and TES continue to yield significant science results that are being applied by operational environmental protection agencies for air quality assessments, regulations, and forecasts, both in the U.S. (EPA) and Europe. Although Aura does not measure carbon directly, it is making substantial contributions to understanding climate change by measurements of other climate forcing factors, such as, water vapor, solar irradiance, and aerosols. OMI and MLS (Microwave Limb Sounder) continue their crucial observations in the stratosphere that are needed for monitoring compliance of the Montreal Protocol.
• October 27, 2016: The size and depth of the ozone hole over Antarctica was not remarkable in 2016. As expected, ozone levels have stabilized, but full recovery is still decades away. What is remarkable is that the same international agreement that successfully put the ozone layer on the road to recovery is now being used to address climate change. 31)
- The stratospheric ozone layer protects life on Earth by absorbing ultraviolet light, which damages DNA in plants and animals (including humans) and leads to health issues like skin cancer. Prior to 1979, scientists had never observed ozone concentrations below 220 Dobson Units. But in the early 1980s, through a combination of ground-based and satellite measurements, scientists began to realize that Earth's natural sunscreen was thinning dramatically over the South Pole. This large, thin spot in the ozone layer each southern spring came to be known as the ozone hole.
- The image of Figure 18 shows the Antarctic ozone hole on October 1, 2016, as observed by the OMI (Ozone Monitoring Instrument) on NASA's Aura satellite. On that day, the ozone layer reached its average annual minimum concentration, which measured 114 Dobson Units. For comparison, the ozone layer in 2015 reached a minimum of 101 Dobson Units. During the 1960s, long before the Antarctic ozone hole occurred, average ozone concentrations above the South Pole ranged from 260 to 320 Dobson Units.
- The area of the ozone hole in 2016 peaked on September 28, 2016, at about 23 million km2. "This year we saw an ozone hole that was just below average size," said Paul Newman, ozone expert and chief scientist for Earth Science at NASA's Goddard Space Flight Center. "What we're seeing is consistent with our expectation and our understanding of ozone depletion chemistry and stratospheric weather."
- The image of Figure 19 was acquired on October 2 by the OMPS (Ozone Mapping Profiler Suite) instrumentation during a single orbit of the Suomi-NPP satellite. It reveals the density of ozone at various altitudes, with dark orange areas having more ozone and light orange areas having less. Notice that the word hole isn't literal; ozone is still present over Antarctica, but it is thinner and less dense in some areas.
- In 2014, an assessment by 282 scientists from 36 countries found that the ozone layer is on track for recovery within the next few decades. Ozone-depleting chemicals such as chlorofluorocarbons (CFCs) — which were once used for refrigerants, aerosol spray cans, insulation foam, and fire suppression — were phased out years ago. The existing CFCs in the stratosphere will take many years to decay, but if nations continue to follow the guidelines of the Montreal Protocol, global ozone levels should recover to 1980 levels by 2050 and the ozone hole over Antarctica should recover by 2070.
- The replacement of CFCs with hydrofluorocarbons (HFCs) during the past decade has saved the ozone layer but created a new problem for climate change. HFCs are potent greenhouse gases, and their use — particularly in refrigeration and air conditioning — has been quickly increasing around the world. The HFC problem was recently on the agenda at a United Nations meeting in Kigali, Rwanda. On October 15, 2016, a new amendment greatly expanded the Montreal Protocol by targeting HFCs, the so-called "grandchildren" of the Montreal Protocol.
- "The Montreal Protocol is written so that we can control ozone-depleting substances and their replacements," said Newman, who participated in the meeting in Kigali. "This agreement is a huge step forward because it is essentially the first real climate mitigation treaty that has bite to it. It has strict obligations for bringing down HFCs, and is forcing scientists and engineers to look for alternatives."
Figure 18: Image of the Antarctic Ozone Hole acquired with OMI on Aura on October 1, 2016 (image credit: NASA Earth Observatory, Aura OMI science team)
Figure 19: An edge-on (limb) view of Earth's ozone layer, acquired with OMPS on the Suomi-NPP on October 2, 2016 (image credit: NASA Earth Observatory, image by Jesse Allen, using Suomi-NPP OMPS data)
• June 1, 2016: Using a new satellite-based method, scientists at NASA, Environment and Climate Change Canada, and two universities have located 39 unreported and major human-made sources of toxic sulfur dioxide emissions. Data from NASA's Aura spacecraft were analyzed by scientists to produce improved estimates of sulfur dioxide sources and concentrations worldwide between 2005 and 2014. 32)
- A known health hazard and contributor to acid rain, sulfur dioxide (SO2) is one of six air pollutants regulated by the U.S. Environmental Protection Agency. Current, sulfur dioxide monitoring activities include the use of emission inventories that are derived from ground-based measurements and factors, such as fuel usage. The inventories are used to evaluate regulatory policies for air quality improvements and to anticipate future emission scenarios that may occur with economic and population growth.
- But, to develop comprehensive and accurate inventories, industries, government agencies and scientists first must know the location of pollution sources.
- "We now have an independent measurement of these emission sources that does not rely on what was known or thought known," said Chris McLinden, an atmospheric scientist with Environment and Climate Change Canada in Toronto and lead author of the study published this week in Nature Geosciences. "When you look at a satellite picture of sulfur dioxide, you end up with it appearing as hotspots – bull's-eyes, in effect — which makes the estimates of emissions easier." 33)
- The 39 unreported emission sources, found in the analysis of satellite data from 2005 to 2014, are clusters of coal-burning power plants, smelters, oil and gas operations found notably in the Middle East, but also in Mexico and parts of Russia. In addition, reported emissions from known sources in these regions were — in some cases — two to three times lower than satellite-based estimates.
- Altogether, the unreported and underreported sources account for about 12 percent of all human-made emissions of sulfur dioxide – a discrepancy that can have a large impact on regional air quality, said McLinden.
- The research team also located 75 natural sources of sulfur dioxide — non-erupting volcanoes slowly leaking the toxic gas throughout the year. While not necessarily unknown, many volcanoes are in remote locations and not monitored, so this satellite-based data set is the first to provide regular annual information on these passive volcanic emissions.
- "Quantifying the sulfur dioxide bull's-eyes is a two-step process that would not have been possible without two innovations in working with the satellite data," said co-author Nickolay Krotkov, an atmospheric scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland.
- First was an improvement in the computer processing that transforms raw satellite observations from the Dutch-Finnish Ozone Monitoring Instrument aboard NASA's Aura spacecraft into precise estimates of sulfur dioxide concentrations. Krotkov and his team now are able to more accurately detect smaller sulfur dioxide concentrations, including those emitted by human-made sources such as oil-related activities and medium-size power plants.
- Being able to detect smaller concentrations led to the second innovation. McLinden and his colleagues used a new computer program to more precisely detect sulfur dioxide that had been dispersed and diluted by winds. They then used accurate estimates of wind strength and direction derived from a satellite data-driven model to trace the pollutant back to the location of the source, and also to estimate how much sulfur dioxide was emitted from the smoke stack.
- "The unique advantage of satellite data is spatial coverage," said Bryan Duncan, an atmospheric scientist at Goddard. "This paper is the perfect demonstration of how new and improved satellite datasets, coupled with new and improved data analysis techniques, allow us to identify even smaller pollutant sources and to quantify these emissions over the globe." - The University of Maryland, College Park, and Dalhousie University in Halifax, Nova Scotia, contributed to this study.
• June 2015: The Aura satellite was launched in July 2004 as part of the A-Train. The three operating instruments on-board Aura MLS (Microwave Limb Sounder), OMI (Ozone Monitoring Instrument), and TES (Tropospheric Emissions Spectrometer) provide profiles and column measurements of atmospheric composition in the troposphere, stratosphere, and mesosphere. OMI is contributed from the Netherlands Space Office and the Finnish Meteorological Institute. The suite of observations from MLS, OMI and TES is very rich, with nearly 30 individual chemical species relevant for stratospheric chemistry (O3, HCl, HOCl, ClO, OClO, BrO, NO2, N2O, HNO3, etc.), tropospheric pollutants (O3, NO2, CO, PAN, NH3, SO2, aerosols), and climate-related quantities (CO2, H2O, CH4, clouds, aerosol optical properties). The Aura spacecraft is healthy and is expected to operate until at least 2022, likely beyond. There is great value in continuing the mission to: 34)
1) extend the unique 10-year record of stratospheric composition, variability, and trends as well as the chemical and dynamical processes affecting ozone recovery and polar ozone chemistry
2) continue to map-out rapidly changing anthropogenic emissions of NO2, SO2, and aerosol products influencing air quality
3) continue to develop greater vertical sensitivity by combining radiances from separate sensors
4) use Aura data to further evaluate global chemistry-climate, climate, and air quality models
5) extend observations of short-term solar variability overlapping with SORCE and providing a bridge to future measurements (GOME-2 TROPOMI)
6) continue the development of new synergetic products combining multiple Aura instruments and instruments from the A-Train
7) provide continuity and comparison to current and future satellite missions (Suomi-NPP, SAGE-III, TROPOMI)
8) deliver operational products: volcanic monitoring, aviation safety, operational ozone assimilation at NOAA for weather and UV index forecasting, OMI Aerosol Index and NO2 products for air quality forecasting. As such, the Panel concludes that Aura mission be continued as currently baselined.
The Aura spacecraft flight systems are operating on primary hardware with redundant systems intact and are expected to continue to perform very well through the proposed mission extension period. Aura Mission Operations have been very successful (Ref. 34).
• April 29, 2015: Late on April 22, 2015, the Calbuco volcano in southern Chile awoke from four decades of slumber with an explosive eruption. Ash and pumice particles were lofted high into the atmosphere, and the debris has been darkening skies and burying parts of Chile, Argentina, and South America for nearly a week. Along with 210 million cubic meters of ash and rock, the volcano has been spewing sulfur dioxide (SO2) and other gases. 35)
- Near the land surface, sulfur dioxide is a acrid-smelling gas that can cause respiratory problems in humans and animals. Higher in the atmosphere, it can have an effect on climate. When SO2 reacts with water vapor, it creates sulfate aerosols that can linger for months or years. Those small particles can have a cooling effect by reflecting incoming sunlight.
- The images of Figure 20 show the average concentration of sulfur dioxide over South America and surrounding waters between April 23–26, 2015. The maps were made with data from OMI (Ozone Monitoring Instrument) on NASA's Aura satellite. Like ozone, atmospheric sulfur dioxide is sometimes measured in Dobson Units. If you could compress all the sulfur dioxide in a column of atmosphere into a single layer at the Earth's surface at 0º Celsius, one Dobson Unit would be 0.01 mm thick and would contain 0.0285 grams of sulfur dioxide/m2.
- "Satellite sulfur dioxide data are critical for understanding the impacts of volcanic eruptions on climate," said Simon Carn, a part of the OMI team and professor at the Michigan Technological University. "Climate modelers need estimates of SO2 mass and altitude to run their models and accurately predict the atmospheric and climate impacts of volcanic eruptions. The SO2 plume images also provide unique insights into the atmospheric transport and dispersion of trace gases in the atmosphere, and on upper atmospheric winds."
- So far, Calbuco has released an estimated 0.3 to 0.4 teragrams (0.3 to 0.4 million tons) of SO2 into the atmosphere. The gas was injected into the stratosphere (as high as 21 km), where it will last much longer and travel much farther than if released closer to the surface. The SO2 will gradually convert to sulfate aerosol particles. However, it is not clear yet if there will be a cooling effect from this event.
Figure 20: The OLI measurements were acquired on four days in April 2015. On the maps, data appear in stripes or swaths, revealing the areas observed (colored) or not observed (clear) by the Aura spacecraft on a given day. Note how the plume moves north and east with the winds. By April 28, the plume of SO2 had reached the Indian Ocean (image credit: NASA Earth Observatory, Jesse Allen)
Figure 21: The natural color image, acquired on April 25, 2015 by the ALI (Advanced Land Imager) instrument on NASA's EO-1 (Earth Observing-1) satellite, shows Calbuco's plume rising above the cloud deck over Chile (image credit: NASA, EO-1 team)
• In early 2015, Aura is operating "nominally" in its extended mission phase. Although HIRDLS is no longer operational and the TES instrument shows signs of wear that have limited its operations, OMI and MLS continue to operate well. OMI's highly successful advanced technology has been and will continue to be employed by new NASA satellite instruments, such as OMPS (Ozone Mapper Profiling Suite) on the Suomi-NPP (Suomi National Polar-orbiting Partnership) and on the TEMPO (Tropospheric Emissions: Monitoring of Pollution) missions. Overall, the Aura mission continues to operate satisfactorily and there is enough fuel reserve for Aura to operate safely in the A-Train until 2023. 36)
- To date, the mission has met or surpassed nearly all mission success criteria. Aura data are being used by the U.S. GCRP (Global Change Research Program) and for three international assessments, including the AR5 (Fifth Assessment Report) of the IPCC (Intergovernmental Panel on Climate Change), the WMO (World Meteorological Organization) and UNEP/SAOD (United Nations' Environmental Program/Scientific Assessments of Ozone Depletion), and the TF HTAP (Task Force on Hemispheric Transport of Air Pollution).
- Aura's data have proven to be valuable for air quality applications such as identifying the trends that result from regulation of emissions on decadal time scales, and shorter time scale applications are being assessed. The original Aura science questions have surely been addressed or answered and serendipitous discoveries have been realized.
• On July 15, 2014, the Aura Spacecraft of NASA was 10 years on orbit. Aura has provided vital data about the cause, concentrations and impact of major air pollutants. With its four instruments measuring various gas concentrations, Aura gives a comprehensive view of one of the most important parts of Earth - the atmosphere. 37)
Figure 22: Nitrogen dioxide pollution, averaged yearly from 2005-2011, has decreased across the United States (image credit: NASA Goddard's Scientific Visualization Studio, T. Schindler)
Legend to Figure 22: The OMI (Ozone Monitoring Instrument) on the Aura satellite began monitoring levels of nitrogen dioxide worldwide shortly after its launch. OMI data show that nitrogen dioxide levels in the United States have decreased at a rate of 4% per year from 2005 to 2010 — a time period when stricter government policies on power plant and vehicle emissions came into effect. As a result, ground-level ozone concentrations also decreased. OMI data also showed a 2.5% decrease of nitrogen dioxide per year during the same time period in Europe, which had enacted similar legislation.
While air quality in the United States has improved, the issue still persists nationwide. Since bright sunlight is needed to produce unhealthy levels of ozone, ozone pollution is largely a summertime issue. As recent as 2012, about 142 million people in America— 47 % of the population— lived in counties with pollution levels above the National Ambient Air Quality Standards, according to the EPA (Environmental Protection Agency). The highest levels of ozone tend to occur on hot, sunny, windless days.
Air pollution also remains an issue worldwide. The WHO (World Health Organization) reported that air pollution still caused one in eight deaths worldwide in 2012. Outside of the United States and Europe, OMI showed an increase in nitrogen dioxide levels. Data from 2005 to 2010 showed China's nitrogen dioxide levels increased at about 6 % and South East Asia increased levels at 2% per year. Globally, nitrogen dioxide levels increased a little over half a percent per year during that time period (Ref. 37).
• Figure 23, released on June 8, 2014 in NASA's Earth Observatory program, was assembled from observations made by the OMI (Ozone Monitoring Instrument) on the Aura satellite. The map shows the concentration of stratospheric ozone over the Arctic—63º to 90º North—on April 1, 2014. Ozone is typically measured in Dobson Units, the number of molecules required to create a layer of pure ozone 0.01 mm thick at a temperature of 0º Celsius and an air pressure of 1 atmosphere (the pressure at the surface of the Earth). Reaching 470 Dobson Units, April 1 marked the highest average concentration of ozone over the region so far this year. The average amount of ozone in Earth's atmosphere is 300 Dobson Units, equivalent to a layer 3 mm in thickness. 38)
Figure 23: OMI map of Arctic ozone in spring acquired on April 1, 2014 (image credit: NASA Earth Observatory)
• The Aura spacecraft and three of its four instruments are operating nominally in 2014.
Sulfur dioxide (SO2), emitted from both man-made and volcanic activities, significantly impact air quality and climate. Advanced sensors including OMI (Ozone Monitoring Instrument) flying on NASA's Aura spacecraft have been employed to measure SO2 pollution. This however, remains a challenging problem owing to relatively weak signals from most anthropogenic sources and various interferences such as ozone absorption and stray light.
The project has developed a fundamentally different approach for retrieving SO2 from satellites. Unlike existing methods that attempt to model different interferences, we directly extract characteristic features from satellite radiance data to account for them, using a principal component analysis technique. This proves to be a computationally efficient way to use hundreds of wavelengths available from OMI, and greatly decreases modeling errors.
The new approach has the following features:
- 50% noise reduction compared with the operational OMI algorithm.
- Reduction of unrealistic features in the operational product.
- Computation efficiency (10 times faster than comparable methods relying on online radiative transfer calculation).
- Applicability to many instruments such as GOME-2 and the Suomi National Polar Partnership (NPP) Ozone Mapping and Profiler Suite (OMPS).
This new algorithm will significantly improve the SO2 data quality from the OMI mission. Once applied to other sensors, it will enable the production of consistent long-term global SO2 records essential for climate and air quality studies.
Figure 24: Monthly mean SO2 maps over the Eastern U.S. for August 2006 generated using (a) the new algorithm and (b) the operational algorithm (image credit: NASA/GSFC)
Legend to Figure 24: The circles represent large SO2 sources (e.g., coal-fired power plants) that emit more than 70,000 tons of SO2 annually. The colors represent the amount of SO2 in the atmospheric column above the surface in Dobson Unit (DU). 1 DU means 2.69 x 1016 SO2 molecules above a surface of 1 cm2 . Because the SO2 signals from anthropogenic sources are relatively weak, small errors in the estimated interferences (e.g., ozone absorption) may lead to substantial biases in the retrieved SO2. Negative values can arise when, for example, the contribution from ozone absorption in the SO2 spectral window is only slightly overestimated. As shown above, the negative retrieval biases become much smaller in the new algorithm.
Relevance for future science and NASA missions: The new algorithm has been proposed to reprocess data from the Aura OMI mission. It can be applied to Suomi-NPP and future JPSS OMPS instruments to ensure SO2 data continuity from the EOS era. It can also help extend satellite SO2 data records if applied to other current and future NASA and European missions such as TEMPO, GEO-CAPE, TROPOMI and GOME-2. TEMPO is the first selected NASA EVI (Earth Venture Instrument) and will be launched on a geostationary platform near the end of the decade (Ref. 39).
• June 2013: The 2013 Senior Review evaluated 13 NASA satellite missions in extended operations: ACRIMSAT, Aqua, Aura, CALIPSO, CloudSat, EO-1, GRACE, Jason-1, OSTM, QuikSCAT, SORCE, Terra, and TRMM. The Senior Review was tasked with reviewing proposals submitted by each mission team for extended operations and funding for FY14-FY15, and FY16-FY17. Since CloudSat, GRACE, QuikSCAT and SORCE have shown evidence of aging issues, they received baseline funding for extension through 2015. 41)
- The satellite is in excellent health. The Aura MLS, OMI and TES instruments are showing signs of aging, but are still producing science data of excellent quality, and there is an excellent chance of extending measurements beyond the current proposal cycle. The data are highly utilized in the research and operational communities.
- The reasons for extending the Aura mission include: (1) to allow current scientific and applied benefits to continue; (2) to increase the value of the Aura data for climate studies through increasing the length of the Aura data sets; (3) to allow continued collection of data that are unique since the loss of the European Envisat satellite; and (4) to continue to generate synergistic products by combining different Aura and measurements from other A-Train satellite missions.
• The Aura spacecraft and two of its four instruments (MLS and OMI) are operating nominally in 2013. The TES (Tropospheric Emission Spectrometer) instrument continues to make special observations in targeted regions, but no longer makes global measurements. A moving part of TES is suffering from lubricant degradation. 42)
• The Aura spacecraft and three of its four instruments are operating nominally in 2011.
• The Aura spacecraft and three of its four instruments are operating nominally in 2010. Aura entered its extended mission phase in October 2010 (extension until the end of 2012).
• The Aura spacecraft and three of its four instruments are operating nominally in 2009 (MLS, OMI and TES Operations are nominal while HIRDLS is collecting limited science data). 43)
• The spacecraft has been declared "operational" by NASA as of Oct. 14, 2004 (ending the commissioning phase). 44)
• Shortly after launch, the HIRDLS science team discovered, that a piece of plastic was blocking 80% of the optical instrument aperture. Engineers concluded that the plastic was torn from the inside of the instrument during the explosive outgassing on spacecraft ascent at launch. This plastic remains caught on the scan mirror despite efforts to free it. In spite of these setbacks, the HIRDLS team has shown that it can use the remaining 20% of the aperture to produce their promised data products at high vertical resolution. Unfortunately, the instrument no longer has its azimuthal scanning capability. 45) 46) 47)
Sensor complement: (HIRDLS, MLS, OMI, TES)
The Aura instrument package provides complementary observations from the UV to the microwave region of the EMS (Electromagnetic Spectrum) with unprecedented sensitivity and depth of coverage to the study of the Earth's atmospheric chemistry from its surface to the stratosphere. MLS is on the front of the spacecraft (the forward velocity direction) while HIRDLS, TES, and OMI are mounted on the nadir side. 48)
Figure 25: Auro atmospheric profile measurements (image credit: NASA)
Legend to Figure 25: OMI also measures UVB flux, cloud top/cover, and column abundances of O3, NO2, BrO, aerosol and volcanic SO2. TES also measures several additional 'spectral products' such as ClONO2, CF2Cl2, CFCl3, NO2, and volcanic SO2.
Figure 26: Instrument field-of-view accommodation (image credit: NASA)
HIRDLS (High-Resolution Dynamics Limb Sounder):
HIRDLS is a joint instrument between the University of Colorado at Boulder and Oxford University, Oxford, UK. PIs: J. Gille of the University of Colorado and J. Barnett, Oxford University; prime contractors are Lockheed Martin and Astrium Ltd., UK. The instrument is a mid-infrared limb-scanning spectroradiometer designed to sound the upper troposphere, stratosphere, and mesosphere emissions within the spectral range of 6 - 18 µm (21 channels). The instrument measures infrared thermal emissions from the atmosphere which are used to determine vertical profiles as functions of pressure of the temperature and concentrations of several trace species in the 8-100 km height range. The HIRDLS design is of LRIR (Nimbus-6), LIMS and SAMS (Nimbus-7), ISAMS and CLAES (UARS) heritage.
HIRDLS observes global distributions of temperature and trace gas concentrations of O3, H2O, CH4, N2O, HNO3, NO2, N2O5, CFC11, CFC12 ClONO2, and aerosols in the upper troposphere, stratosphere, and mesosphere plus water vapor, aerosol, and cloud tops. The swath width is 2000-3000 km (typically six profiles across swath). Complete Earth coverage (including polar night) can be obtained in 12 hours. High horizontal resolution is obtained with a commandable azimuth scan which, in conjunction with a rapid elevation scan, provides profiles up to 3,000 km apart in an across-track swath. Spatial resolution: standard profile spacing is 500 x 500 km horizontally (equivalent to 5º longitude x 5º latitude) x 1 km vertically; averaging volume for each data sample 1 km vertical x 10 km across x 300 km along line-of-sight. 49) 50)
Figure 27: Illustration of the HIRDLS instrument (image credit: Oxford University)
HIRDLS performs limb scans in the vertical at multiple azimuth angles, measuring infrared emissions in 21 channels (temperature distribution) ranging from 6.12 - 17.76 µm. Four channels measure the emission of CO2. Taking advantage of the known mixing ratio of CO2, the transmittance is calculated, and the equation of radiative transfer is inverted to determine the vertical distribution of the Planck black body function, from which the temperature is derived as a function of pressure. Winds and potential vorticity are determined from spatial variations of the height of geopotential surfaces. These are determined at upper levels by integrating the temperature profiles vertically from a known reference base. The HIRDLS instrument will improve knowledge in data-sparse regions by measuring the height variations of the reference surface with the aid of a gyro package. This level (near the base of the stratosphere) can also be integrated downward using nadir temperature soundings to improve tropospheric analyses. 51)
FOV (scan range): elevation from 22.1º to 27.3º below horizontal, azimuth: -21º (sun side) to +43º (anti-sun side). The instrument has 21 photoconductive HgCdTe detectors cooled to 65 K; each detector has a separate bandpass interference filter. Thermal control by paired Stirling cycle coolers, heaters, sun baffle, radiator panel; the thermal operating range is 20-30º C.
Table 1: Overview of some HIRDLS parameters
Status 2005: The instrument is performing correctly except for a problem with radiometric views out from the main aperture. A series of tests using the (i) the in-orbit instrument, (iii) the Engineering Model, (iii) purpose-built ground rigs, has led to the conclusion that the optical beam is obstructed between the scan mirror and the entrance aperture by what is believed to be a piece of Kapton film that became detached during the ascent to orbit. This film was intended to prevent movement of contamination, but itself moved from behind the scan mirror to in front. The lines along which that film tore can only be deduced from the in-orbit behavior.
Extensive tests have been performed on the HIRDLS instrument to understand the form of the optical blockage and how it occurred. A clear picture has emerged of the geometry, and this adds to the confidence in the approach to extracting atmospheric profiles which appears to be giving good results. All other aspects of the instrument are performing as well as or better than expected and there is every reason to expect that a long series of valuable atmospheric data will be obtained. 52) 53) 54) 55)
Figure 28: Illustration of the HIRDLS instrument and its components (image credit: UCAR, Ref. 51)
Figure 29: Internal view of the HIRDLS instrument (image credit: UCAR)
MLS (Microwave Limb Sounder):
The MLS instrument is of UARS MLS heritage; PI: J. W. Waters, NASA/JPL. The instrument measures thermal emissions from the atmospheric limb in submillimeter and millimeter wavelength spectral bands and is intended for studies of the following processes and parameters: 56) 57) 58) 59) 60) 61)
• Chemistry of the lower stratosphere and upper troposphere. Measurement of lower stratospheric temperature and concentrations of: H2O, O3, ClO, BrO, HCl, OH, HO2, HNO3, and HCN, and N2O. Measurement of upper tropospheric H2O and O3 (radiative forcing on climate change).
• Chemistry of the middle and upper stratosphere. Monitoring of ozone chemistry by measuring radicals, reservoirs, and source gases in chemical cycles which destroy ozone
• The effects of volcanoes on global change. MLS measures SO2 and other gases in volcanic plumes.
Measurements are performed continuously, at all times of day and night, altitude range from the upper troposphere to the lower thermosphere. The vertical scan is chosen to emphasize the lower stratosphere and upper troposphere. Complete latitude coverage is obtained each orbit. Pressure (from O2 lines) and height (from a gyroscope measuring small changes in the FOV direction) are measured to provide accurate vertical information for the composition measurements. 62) 63) 64)
The MLS instrument consists of three modules (Figure 31):
9) GHz module: This module contains the GHz antenna system, calibration targets, switching mirror, optical multiplexer, and 118, 190, 240, and 640 GHz radiometers
10) THz module: contains the THz scan and switching mirror, calibration target, telescope, and 2.5 THz radiometers at both polarizations. Measurement of the OH emissions near 2.5 THz (119 µm).
11) Spectrometer module: that contains spectrometers, command and data handling systems, and power distribution systems.
Measurement approach: passive limb sounder; thermal emission spectra collected by offset Cassegrain scanning antenna system; Limb scan = 0 - 120 km; spatial resolution = 3 x 300 km horizontal x 1.2 km vertical. MLS contains heterodyne radiometers in five spectral bands.
Table 2: MLS instrument parameters
Figure 30: Schematic view of the MLS instrument (image credit: NASA)
FOV: boresight 60-70º relative to nadir; IFOV = ±2.5º (half-cone, along-track); spatial resolution: measurements are performed along the suborbital track; the resolution varies for different bands, at 640 GHz the spatial resolution is: 1.5 km vertical x 3 km cross-track x 300 km along-track at the limb tangent point; a typical resolution is: 3 km vertical x 5 km cross-track x 500 km along-track. Spectral bands at millimeter and submillimeter wavelengths. Instrument mass = 490 kg, power = 550 W; duty cycle = 100%; data rate = 100 kbit/s; thermal control via radiators and louvres to space as well as heaters; thermal operating range is 10-35ºC.
Table 3: MLS instrument frequency bands
Figure 31: Line drawing of the MLS instrument (image credit: NASA)
Figure 32: Signal flow block diagram of the MLS instrument (image credit: NASA)
Figure 33: The MLS GHz module antenna concept, showing Cassegrain configuration, edge tapers, and surface tolerances of the reflectors (image credit: NASA)
Figure 34: The MLS THz module optical scheme (image credit: NASA)
OMI (Ozone Monitoring Instrument):
The OMI instrument is a contribution of NIVR (Netherlands Institute for Air and Space Development) of Delft in collaboration with FMI (Finnish Meteorological Institute), Helsinki, Finland, to the EOS Aura mission. The PI is Pieternel F. Levelt of KNMI; co-PIs are: Gilbert W. Leppelmeier of FMI and Ernest Hilsenrath of NASA/GSFC. OMI was manufactured by Dutch Space/TNO-TPD in The Netherlands, in cooperation with Finnish subcontractors VTT and Patria Finavitec. 65) 66) 67) 68) 69) 70) 71) 72) 73) 74)
OMI is a nadir-viewing UV/VIS imaging spectrograph which measures the solar radiation backscattered by the Earth's atmosphere and surface over the entire wavelength range from 270 to 500 nm, with a spectral resolution of about 0.5 nm. The design is of GOME heritage, flown on ERS-2, as well as of SCIAMACHY and GOMOS heritage, flown on Envisat. The overall objective is to monitor ozone and other trace gases (continuation of the TOMS measurement series) and to monitor tropospheric pollutants worldwide. The OMI measurements are highly synergistic with the HIRDLS and MLS instruments on the Aura platform. The OMI observations provide the following capabilities and features:
• Mapping of ozone columns at 13 km x 24 km and profiles at 36 km x 48 km (continuation of TOMS and GOME ozone column data records and the ozone profile records of SBUV and GOME)
• Measurement of key air quality components: NO2, SO2, BrO, OClO, and aerosol (continuation of GOME measurements)
• Distinguish between aerosol types, such as smoke, dust, and sulfates
• Measurement of cloud pressure and coverage
• Mapping of the global distribution and trends in UV-B radiation
• A combination of processing algorithms is employed including TOMS version 7, DOAS (Differential Optical Absorption Spectroscopy), Hyperspectral BUV retrievals and forward modeling to extract the various OMI data products
• Near real-time production of ozone and other trace gases.
Figure 35: The OMI instrument (image credit: KNMI)
OMI is the first of a new generation of UV-Visible spaceborne spectrometers that use two-dimensional detectors (CCD arrays). These detectors enable OMI to daily observe the entire Earth with small ground pixel size (13x24 km2 at nadir), which makes this instrument suitable for tropospheric composition research and detection of air pollution at urban scales. OMI is a wide-angle, non-scanning and nadir-viewing instrument measuring the solar backscattered irradiance in a swath of 2600 km. The telescope has a FOV of 114º. The instrument is designed as a compact UV/VIS imaging spectrograph, using a two-dimensional CCD array for simultaneous spatial and spectral registration (hyperspectral imaging in frame-transfer mode). The instrument has two channels measuring in the spectral range of 270-500 nm.
The Earth is viewed in 1500 bands in the along-track direction providing daily global coverage. OMI employs a polarization scrambler to depolarize the incoming radiance. The radiation is then focussed by the secondary telescope mirror. A dichroic element separates the radiation into a UV and a VIS channel. The UV channel is split again into two subchannels UV1 (270-314 nm) and UV2 (306-380 nm). In the UV1 subchannel, the spatial sampling distance per pixel is a factor two larger than in the UV2 subchannel. The idea is to increase the ratio between the useful signal and the dark current signal, hence, to increase SNR in UV1. The resulting IFOV values of a pixel in the cross-track direction are 6 km for UV1 and 3 km for UV2 and VIS. The corresponding spatial resolution is twice as good as the sampling distances. Groups of 4 or 8 CCD detector pixels are binned in the cross-track direction. The basic detector exposure time is 0.4 s, corresponding to an along-track distance of 2.7 km. In OMI, five subsequent CCD images are electronically co-added, resulting in a FOV of 13 km in the along-track direction. In addition, one column (wavelength) of each CCD data is downlinked without co-adding (monitoring of clouds, ground albedo). The pixel binning and image co-adding techniques are used to increase SNR and to reduce the data rate.
Figure 36: Conceptual design of the OMI instrument (image credit: KNMI)
Table 4: OMI instrument parameters
The CCD detector arrays are of the back-illuminated and frame-transfer type, each with 576 (rows) x 780 (columns) pixels in the image section and the same amount in the storage or readout section. The frame transfer layout allows simultaneous exposure and readout of the previous exposure. This in turn permits fair pixel readout rates (130 kHz) and good data integrity. There are two zoom modes, besides the global observation mode, for regional studies with a spatial resolution of 13 km x 13 km. In one zoom mode, the swath width is reduced to 725 km; in the other zoom mode, the spectrum is reduced to 306 - 432 nm. Cloud coverage information is retrieved with a high spatial resolution, independent of the operational mode. 75)
Instrument calibration: Daily measurements of the sun are taken with a set of reflective quartz volume diffusers (QVD) for absolute radiometric calibration. Relative radiometric calibration is performed using a WLS (White Light Source) and two LEDs per spectral (sub-) channel. The two LEDs fairly uniformly illuminate the CCD's. Spectral calibration is being performed using Fraunhofer features in the sun and nadir spectra. This is supported by a dedicated spectral correction algorithm in the level 0-1 b software. Dark signal calibration is performed at the dark side of the orbit using long-exposure time dark measurements. Straylight is always monitored at dedicated rows at the side of the images; there are also covered CCD pixels measuring dark current and, at the top and bottom of the image, exposure smear. 76) 77) 78) 79)
Table 5: Characteristics of the main observation modes of OMI
Figure 37: The optical bench of OMI (image credit: KNMI)
Figure 38: Schematic layout of the OMI optical bench (image credit: SRON, KNMI)
Table 6: Performance parameters of OMI
Figure 39: Schematic of measurement principle of the OMI instrument (image credit: Dutch Space)
Figure 40: Photo of the OMI instrument (image credit: SRON, Ref. 79)
TES (Tropospheric Emission Spectrometer):
The TES instrument is of ATMOS (ATLAS), and AES (Airborne Emission Spectrometer) heritage (PI: Reinhard Beer, NASA/JPL). TES has been developed for NASA by JPL. TES is a high-resolution infrared imaging Connes-type FTS (Fourier Transform Spectrometer), with spectral coverage from 3.2 - 15.4 µm (spectral resolution of 0.03 cm-1). TES has the capability to make both limb and nadir observations. Limb mode: height resolution = 2.3 km, height coverage = 0 - 34 km. In the nadir modes, TES has a spatial resolution of 0.53 km x 5.3 km with a swath of 5.3 km x 8.5 km. TES is a pointable instrument; it can access any target within 45º of the local vertical, or produce regional transects up to 885 km in length without any gaps in coverage. TES employs both, the natural thermal emission of the surface and atmosphere, and reflected sunlight, thereby providing day and night coverage anywhere on the globe. 80) 81) 82)
Figure 41: Schematic layout of the TES optics (image credit: NASA/JPL)
Observations from TES will further understanding of long-term variations in the quantity, distribution, and mixing of minor gases in the troposphere, including sources, sinks, troposphere-stratosphere exchange, and the resulting effects on climate and the biosphere. TES will provide 3-D global maps of tropospheric ozone (primary objective) and its photochemical precursors (chemical species involved in ozone formation and destruction). Other objectives: 83) 84)
- Simultaneous measurements of NOy, CO, O3, and H2O, determination of the global distribution of OH.
- Measurements of SO2 and NOy as precursors to the strong acids H2SO4 and HNO3
- Measurements of gradients of many tropospheric species
- Determination of long-term trends in radiatively active minor constituents in the lower atmosphere.
The following key features are part of the TES instrument design:
• Back-to-back cube corner reflectors to provide the change in optical path difference
• Use of KBr (potassium bromide) material for the beam splitter-recombiner and the compensator
• Only one of two input ports for actual atmospheric measurements. The other input views an internal, grooved, clod reference target
• A diode-pumped solid-state Nd:YAG laser for interferogram sampling control
• Cassegrain telescopes for condensing and collimating wherever possible to minimize the number of transmissive elements in the system.
• A passive space-viewing radiator to maintain the interferometer and optics at 180 K.
• A two-axis gimbaled pointing mirror operating at ambient temperature to permit observation of the full field of regard (a 45º cone about nadir plus the trailing limb).
• Two independent focal plane assemblies maintained at 65 K with active pulse-tube coolers. 85)
The TES instrument operates in a step-and stare configuration when in downlooking mode. At the limb the instrument points to a constant tangent height. Thus, the footprint is smeared along the line-of-sight by about 110 km during the 16 s limb scan (this is comparable to the effective size of the footprint itself). Hence, atmospheric inhomogeneity in the atmosphere becomes an issue; it must be dealt with in data processing (usually through a simplified form of tomography).
The routine operating procedure for TES is to make continual sets of nadir and limb observations (plus calibrations) on a one-day on, one-day off cycle. During the off-days, extensive calibrations and spectral product observations are made.
Table 7: TES performance characteristics
Figure 42: Observation geometry of the TES instrument (image credit: NASA/JPL)
TES status: After launch, TES went through a lengthy outgassing procedure to minimize the ice buildup on the detectors. After seven month of operation, the translator mechanism (which moves the reflecting surfaces of the spectrometer) began to show signs of bearing wear. The TES instrument team commanded the instrument to skip the limb sounding modes in May 2005. TES is now operating only in the nadir mode. This will increase the bearing life of the translator and the life of the instrument.
Figure 43: Schematic view of TES on the Aura spacecraft (image credit: NASA/JPL)