The Chemistry of Earth’s Atmosphere
Some satellite sensors allow scientists to determine the chemical content of the Earth’s upper atmosphere using a technique called “solar occultation,” in which a sensor is pointed toward the horizon at sunrise and sunset to measure the profile of the stratosphere and mesosphere about 30 times per day. In this way sensors, such as the Stratospheric Aerosol and Gas Experiment (SAGE), can determine the presence and abundance of gases and particulates by measuring precisely the visible and ultraviolet wavelengths that are absorbed within the upper atmosphere. Since the spectra of ozone, nitrogen dioxide, sulfur dioxide, and certain aerosols are well known, scientists can directly correlate SAGE’s readings with the presence of these substances within the stratosphere. The solar occultation technique is particularly effective because the sensor is self-calibrating—each occultation event looks directly at the unattenuated sun outside the Earth’s atmosphere just prior to sunset or just following sunrise. These observations are then compared to observations of the sun obtained by looking through the atmosphere. The direct sun observations establish an ongoing baseline of the sensor’s performance. Adapted from the Stratospheric Aerosol Mission (SAM II) that flew aboard Nimbus 7, the SAGE sensor is essentially a modified sunphotometer. This kind of sensor first flew in 1979 aboard NASA’s Applications Explorer Mission-2 (AEM-2). A subsequent version of SAGE (SAGE II) was launched aboard ERBS in 1984 and performed well throughout 2001, thus giving scientists a 17-year continuous dataset of upper atmosphere profile measurements. In December 2001, an improved SAGE III was launched to extend the dataset on stratospheric aerosols and ozone that began in 1978.

In 1991, NASA launched the Upper Atmosphere Research Satellite (UARS) with a payload of 10 sensors for measuring a wide array of chemical and physical phenomena in the stratosphere and mesosphere (the layer of atmosphere from approximately 10 to 90 km in altitude). Not only did UARS extend scientists' ability to monitor stratospheric ozone concentrations into the 1990s, but it also provided the first comprehensive picture of the photochemical processes involved in ozone destruction. The UARS Microwave Limb Sounder (MLS) demonstrated that there is a direct link between the presence of chlorine, the formation of chlorine monoxide during winter in the Southern Hemisphere, and the destruction of ozone.

This image shows a comparison of chlorine monoxide (left) and ozone concentration (right) derived by MLS at approximately 18 km altitude on August 30, 1996. The high chlorine monoxide within the Antarctic polar vortex in the left-hand figure (green, yellow, and red) is directly associated with, and leads to, a reduced ozone concentration shown in the right-hand figure (purple).

UARS carries the first two spaceborne remote wind sounders ever launched, called the High Resolution Doppler Imager (HRDI) and Wind Imaging Interferometer (WINDII). These sensors measured winds in the mesosphere through detection of shifts in airglow emission lines. Additionally, HRDI can also detect daytime stratospheric winds by observing Doppler shifts in oxygen absorption lines. WINDII and HRDI gave scientists the first complete global picture of mesospheric circulation. Together with the Halogen Occultation Experiment (HALOE) and MLS aboard UARS, the sensors enabled scientists to track the upward transport of water vapor in the tropical stratosphere. Data from these sensors showed that the tropical tropopause (the gateway from the troposphere to the stratosphere) air rises into the stratosphere through towering thunderheads along the Intertropical Convergence Zone (ITCZ) running roughly parallel to the equator. Once in the stratosphere, this air moves slowly upward and outward toward the mid-latitudes. Ozone begins to form as incoming ultraviolet radiation breaks oxygen molecules (O₂) into free oxygen atoms (O) that quickly bond with other oxygen molecules to form ozone (O₃). Because ozone strongly absorbs certain wavelengths of ultraviolet radiation, the air begins to warm, helping to perpetuate the upward movement of the air mass as well as helping to create temperature gradients for stratospheric winds. UARS data showed that it takes about 2 years for water vapor anomalies to move from the tropopause (about 17 km) up to the mid-stratosphere (about 30 km).

A Canadian instrument launched in 1999 aboard NASA’s Terra satellite uses gas correlation spectroscopy to determine the abundance of methane and carbon monoxide in the troposphere. The Measurements Of Pollution In The Troposphere (MOPITT) sensor measures emitted and reflected radiance from the Earth in three spectral bands. As this light enters the sensor, it passes along two different paths through onboard containers of carbon monoxide and methane. The different paths absorb different amounts of energy, leading to small differences in the resulting signals that directly correlate with the presence of these gases in the atmosphere. Both methane and carbon monoxide are byproducts of burning vegetation as well as fossil fuels. Over the last two decades levels of methane in the atmosphere have risen at an average rate of about 1 percent per year. This is concerning because methane (CH₄) is a greenhouse gas about 30 times more efficient than carbon dioxide at trapping heat near the surface. Scientific interest in carbon monoxide (CO) is twofold: the gas controls atmospheric concentrations of oxidants, thus affecting the ability of the atmosphere to clean itself from the ongoing generation of harmful tropospheric ozone from biomass burning and urban smog. Also, through chemical reactions within the lower atmosphere, carbon monoxide contributes to the production of harmful ozone. MOPITT is helping scientists identify the main sources of these gases as well as improve four-dimensional models of their transport through the atmosphere.

The MOPITT sensor aboard NASA’s Terra satellite has assembled the first ever view of the carbon monoxide spreading through the Earth’s atmosphere. The false colors in this animation represent levels of carbon monoxide in the lower atmosphere, ranging from about 390 parts per billion (dark brown pixels), to 220 parts per billion (red pixels), to 50 parts per billion (blue pixels). (Animation courtesy NASA GSFC Scientific Visualization Studio, based on data from the MOPITT Team) Click here for an animation (will open in a new window).

ESA’s Envisat will carry the Scanning Imaging Absorption Spectrometer for Atmospheric Cartography (SCIAMACHY), which is an advanced version of the GOME sensor flying aboard ERS-2. In addition to the same four spectral channels contained on GOME (from visible to ultraviolet wavelengths; 240-800 nm), SCIAMACHY has an additional four channels in the near-infrared region of the spectrum (800-2,400 nm). While the sensor’s wide spectral sensitivity makes it useful for cloud and aerosol research, its ability to view both nadir and the Earth’s horizon makes it useful for determining the content and distribution of 16 different trace gases in the atmosphere.

next: Where Storm Clouds Gather
back: Serendipity and Stratospheric Ozone

Remote Sensing
Introduction
Balancing Earth’s Radiant Energy Budget
Dust in the Wind
Abstract Art or Arbiters of Energy?
Serendipity and Stratospheric Ozone
The Chemistry of Earth’s Atmosphere
Where Storm Clouds Gather
Conclusion

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Aerosols and Climate Change
Clouds and Radiation
Why isn’t Earth Hot as an Oven?

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TOMS Ozone
Precipitation
Cloud Fraction