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Aura science objectives
Posted: July 7, 2004


The stratospheric ozone layer shields life on Earth from harmful solar ultraviolet (UV) radiation. Research has clearly shown that excess exposure to UV radiation is harmful to agriculture and causes skin cancer and eye problems. Excess UV radiation may suppress the human immune system.

Ozone is formed naturally in the stratosphere through break-up of oxygen molecules (O2) by solar UV radiation. Individual oxygen atoms can combine with O2 molecules to form ozone molecules (O3). Ozone is destroyed when an ozone molecule combines with an oxygen atom to form two oxygen molecules, or through catalytic cycles involving hydrogen, nitrogen, chlorine or bromine containing species. The atmosphere maintains a natural balance between ozone formation and destruction.

The natural balance of chemicals in the stratosphere has changed, particularly due to the presence of man-made chlorofluorocarbons (CFCs). CFCs are non-reactive and accumulate in the lower atmosphere. They are destroyed in the high stratosphere where they are no longer shielded from UV radiation by the ozone layer.

Destruction of CFCs yields atomic chlorine, an efficient catalyst for ozone destruction. Other man-made gases such as nitrous oxide (N2O) and bromine compounds are broken down in the stratosphere and also participate in ozone destruction.

Satellite observations of the ozone layer began in the 1970s when the possibility of ozone depletion was just becoming an environmental concern. NASA's Total Ozone Mapping Spectrometer (TOMS) and Stratospheric Aerosol and Gas Experiment (SAGE) have provided long-term records of ozone. In 1985, the British Antarctic Survey reported unexpectedly deep ozone depletion over Antarctica. The annual occurrence of this depletion, popularly known as the ozone hole, alarmed scientists.

Specially equipped high-altitude NASA aircraft established that the ozone hole was due to man-made chlorine. Data from the TOMS and SAGE satellites showed the synoptic (continental) scale of this depletion and also showed smaller but significant ozone losses outside the Antarctic region. In 1987 an international agreement known as the Montreal Protocol restricted CFC production. In 1992, the Copenhagen amendments to the Montreal Protocol set a schedule to eliminate all production of CFCs.

Severe ozone depletion occurs in winter and spring over both polar regions. The polar stratosphere becomes very cold in winter because of the absence of sunlight and because strong winds isolate the polar air. Stratospheric temperatures fall below -88 deg C (-126.4 deg F). Polar stratospheric clouds (PSCs) form at these low temperatures. The reservoir gases HCl and ClONO2 react on the surfaces of cloud particles and release chlorine.

Ground-based data have shown that CFC amounts in the troposphere are leveling off, while data from the Halogen Occultation Experiment (HALOE) on the Upper Atmosphere Research Satellite (UARS) have shown that amounts of HCl, a chlorine reservoir that is produced when CFCs are broken apart, are leveling off as well. Recent studies have shown that the rate of ozone depletion is also decreasing.

Recovery of the ozone layer may not be as simple as eliminating the manufacture of CFCs. Climate change will alter ozone recovery because greenhouse gas increases will cause the stratosphere to cool. This cooling may temporarily slow the recovery of the ozone layer in the polar regions, but will accelerate ozone recovery at low and middle latitudes.

What will Aura do?
Aura's instruments will observe the important sources, radicals, and reservoir gases active in ozone chemistry. Aura data will improve our capability to predict ozone change. Aura data will also help untangle the roles of transport and chemistry in determining ozone trends.


Agriculture and industrial activities have grown dramatically along with the human population. Consequently, in parts of the world, increased emissions of pollutants have significantly degraded air quality. Respiratory problems and even premature death due to air pollution occur in urban and some rural areas of both the industrialized and developing countries. Wide spread burning for agricultural purposes (biomass burning) and forest fires also contribute to poor air quality, particularly in the tropics.

The list of culprits in the degradation of air quality includes tropospheric ozone, a toxic gas, and the chemicals that form ozone. These ozone precursors are nitrogen oxides, carbon monoxide, methane, and other hydrocarbons. Human activities such as biomass burning, inefficient coal combustion, other industrial activities, and vehicular traffic all produce ozone precursors.

The U.S. Environmental Protection Agency (EPA) has identified six criteria pollutants: carbon monoxide, nitrogen dioxide, sulfur dioxide, ozone, lead, and particulates (aerosols). Of these six pollutants, ozone has proved the most difficult to control. Ozone chemistry is complex, making it difficult to quantify the contributions to poor local air quality. Pollutant emission inventories needed for predicting air quality are uncertain by as much as 50%. Also uncertain is the amount of ozone that enters the troposphere from the stratosphere.

Long Range Pollution Transport
For local governments struggling to meet national air quality standards, knowing more about the sources and transport of air pollutants has become an important issue. Most pollution sources are local but satellite observations show that winds can carry pollutants for great distances, for example from the western and mid-western states to the East Coast of the United States, and even from one continent to another.

Satellite measurements by EOS Terra's MOPITT instrument have shown carbon monoxide streams extending almost 18,000 km (11,180 miles) from their source. TOMS has tracked dust and smoke events from northern China to the East Coast of the United States.

On July 7, 2002, MODIS on EOS-Terra and TOMS captured smoke from Canadian forest fires as the winds transported it southward. This pollution event was responsible for elevated surface ozone levels along the East Coast. TOMS has high sensitivity to aerosols like smoke and dust when they are elevated above the surface layers. By having this capability on OMI, we will be able to extend the previous TOMS record to enable us to look at longer-term trends, as well as interannual variability through the end of this decade.

Observations and models show that pollutants from Southeast Asia contribute to poor air quality in India. Pollutants crossing from China to Japan reach the West Coast of the United States. Pollutants originating in the United States can affect air quality in Europe.

Precursor gases for as much as 10% of ozone in surface air in the United States may originate outside the country. We have yet to quantify the extent of inter-regional and inter-continental pollution transport.

What will Aura do?
The Aura instruments are designed to study tropospheric chemistry; together Aura's instruments provide global monitoring of air pollution on a daily basis. They measure five of the six EPA criteria pollutants (all except lead). Aura will provide data of suitable accuracy to improve industrial emission inventories, and also to help distinguish between industrial and natural sources. Because of Aura, we will be able to improve air quality forecast models.


Carbon dioxide and other gases trap infrared radiation that would otherwise escape to space. This phenomenon, the greenhouse effect, makes the Earth habitable.

Increased atmospheric emissions from industrial and agricultural activities are causing increases in the greenhouse effect and climate change. Industry and agriculture produce trace gases that trap infrared radiation. Many of these gases have increased and thus have added to the greenhouse effect. Since the turn of the century, the global mean lower tropospheric temperature has increased by more than 0.4 deg Celsius (0.72 deg Fahrenheit). This increase has been greater than any in any other century in the last 1000 years.

Ozone plays multiple roles in climate change, because it absorbs both ultraviolet radiation from the sun and infrared radiation from the Earth's surface. Tropospheric ozone is as important as methane as a greenhouse gas contributor to climate change. An accurate measurement of the vertical distribution of tropospheric ozone will improve climate modeling and climate predictions.

Aerosols are an important but uncertain agent of climate change. Aerosols alter atmospheric temperatures by absorbing and scattering radiation. Aerosols can either warm or cool the troposphere. Therefore, aerosols also modify clouds and affect precipitation. Sulfate aerosols can reduce cloud droplet size, making clouds brighter so that they reflect more solar energy. Black carbon aerosols strongly absorb solar radiation, warming the mid-troposphere and reducing cloud formation. Poor knowledge of the global distribution of aerosols contributes to a large uncertainty in climate prediction.

Ozone absorbs solar radiation, warming the stratosphere. Man-made chlorofluorocarbons have caused ozone depletion, leading to lower temperatures. Low temperatures, in turn, lead to more persistent polar stratospheric clouds and cause further ozone depletion in polar regions.

Increasing carbon dioxide (CO2) also affects the climate of the upper atmosphere. Where the atmosphere is thin, increasing CO2 emits more radiation to space, thus cooling the environment. Observations show that over recent decades, the mid- to upper-stratosphere has cooled by 1 to 6 deg Celsius (2 to 11 deg Fahrenheit) due to increases in CO2. This cooling will produce circulation changes in the stratosphere that will change how trace gases are transported.

Water vapor is an important greenhouse gas. Some measurements suggest that water vapor is increasing in the stratosphere. This increase may be due to changes in the transport of air between the troposphere and the stratosphere caused by climate change, or it could be due to changes in the microphysical processes within tropical clouds. More measurements of upper tropospheric water vapor, trace gases and particles are needed to untangle the cause and effect relationships of these various agents of climate change. We can verify climate models of the atmosphere only with global observations of the atmosphere and its changes over time.

What will Aura do?
Aura will measure greenhouse gases such as methane, water vapor, and ozone in the upper troposphere and lower stratosphere. Aura also will measure both absorbing and reflecting aerosols in the lower stratosphere and lower troposphere, water vapor measurements inside high tropical clouds, and high vertical resolution measurements of some greenhouse gases in a broad swath (down to the clouds) across the tropical upwelling region. All of these measurements contribute key data for climate modeling and prediction.