Topics : Tools

Atmospheric Chemistry

From its earliest days, NCAR has analyzed the makeup of the atmosphere and the human activities that change it. Only ultraprecise sensors can fully capture the ebb and flow of the atmosphere's constituents. NCAR has developed some of the world's most sensitive instruments for monitoring air chemistry and its changes over time. The center also works closely with other institutions on global projects to assess the validity of measurement techniques.

Most of the chemicals that play large roles in our atmosphere, such as the greenhouse gas carbon dioxide, are called trace gases. They exist in tiny proportions--measured in parts per million, per billion, or even per trillion. Some of these gases form and dissolve in seconds, while others circle the globe for millennia.

Another focus of air-chemistry research is aerosols. These tiny airborne particles, including salt, soot, or other pollutants, remain in the air only a few weeks. Yet due to their strong variations in size, color, and other qualities, they have complex effects on Earth's climate that have yet to be fully sorted out.

NCAR uses a variety of tools to measure trace gases and aerosols.

Whole air samplers
By trapping a small quantity of air and bringing it to NCAR, scientists can carry out the most accurate laboratory analysis. A whole-air sampler might capture air from a tropical rain forest, a polluted city, or along the path of a research flight far above the ocean. NCAR's new cryogenic whole air sampler is designed to retrieve and return larger quantities of air from heights of more than 20 miles (32 km) above sea level.

Satellites

From space, sensors can intercept upward-flowing radiation and use it to obtain a global-scale picture of air chemistry. NCAR has collaborated with other labs and universities on two major instruments aboard NASA satellites:

High levels of carbon monoxide from agricultural fires across Northwest Africa are visible in this image created from a composite of data collected February 1-25, 2004, by the MOPITT instrument aboard NASA's Terra satellite. A pollution plume laden with carbon monoxide can be seen drifting westward from Africa across the Atlantic Ocean and reaching all the way to South America . (Image courtesy NCAR and University of Toronto MOPITT Science Teams.)

Launched in 1999, MOPITT ( Measurements of Pollution in the Troposphere), senses infrared radiation, from which the distribution of carbon monoxide in thetroposphere, the lowest layer of the atmosphere, can be determined. Carbon monoxide persists in the atmosphere for several weeks, making it a good marker for pollution.

Since its launch in 2004, HIRDLS (High Resolution Dynamics Limb Sounder) has been gearing up to gather fine-scale data on aerosols and 10 trace gases, including ozone and water vapor, from altitudes of 5 to 50 miles (8-80 km). Looking at Earth's limb (the edge of Earth from the satellite perspective), HIRDLS gathers information by measuring the infrared radiation emitted by atmospheric gases.

Spectrometers
Each component of the atmosphere absorbs, reflects, and refracts radiation in the light spectrum in a different way. Some of these effects can be measured by how much of the light at various frequencies is depleted after it passes through air. Active spectrometers use a focused light beam to analyze the makeup of the air in a laboratory setting or in the field. One of NCAR's newest spectrometers can distinguish between different isotopes of carbon dioxide, which offers clues about the different processes that produce and/or scavenge this greenhouse gas.

Chromatographs
A frequently used method to measure trace species (chemicals found in small amounts that are nevertheless important to the Earth system) is chromatography. This technique sends air through a column filled with an adsorbing material. Different gases take different amounts of time to emerge from the column. By measuring the amount of material that comes out as a function of time, the various components can be separated and quantified. Chromatographs are especially useful aboard flights and in field projects, when sampled chemicals might otherwise react with each other before they could be brought back to the laboratory. NCAR's newest in-flight chromatograph can quantify the presence of more than 40 chemical compounds about once every five minutes.

Along with community collaborators, NCAR researchers often design special configurations of instruments for capturing the chemistry at work in such locations as an evergreen forest or a snow-covered polar site. Whether in the field or the laboratory, the goal is not only to measure the air as it is, but also to deduce the changes--some rapid, some slow--that keep the atmosphere's chemical makeup in constant flux.

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