What we measure
ACD uses many instruments to analyze the composition of the atmosphere. Only
ultraprecise sensors can fully capture the ebb and flow of the atmosphere's
constituents. ACD has developed some of the world's most sensitive instruments for
monitoring air chemistry and its changes over time.
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 item to be measured 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.
Tools we useWhole air samplers:
ACD scientists use whole-air samplers to bring small samples back to the laboratory for analysis. ACD'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. ACD has collaborated with other labs and universities on two major instruments aboard NASA satellites:
- Launched in 1999, MOPITT (Measurements of Pollution in the Troposphere), senses infrared radiation, from which the distribution of carbon monoxide in the troposphere, 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.
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 ACD'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.