The Chemistry of Earths Atmosphere
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.
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 NASAs 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.
ESAs 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 sensors wide spectral sensitivity makes it useful for cloud and aerosol research, its ability to view both nadir and the Earths horizon makes it useful for determining the content and distribution of 16 different trace gases in the atmosphere.
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