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Moving in and out of the AtmosphereHow does pollution get cycled out of the atmosphere? Some types of emissions, such as methane, remain in the atmosphere for years; others, such as carbon dioxide, stay for centuries. In contrast, sulfates can be thought of as sprinters: they stay in the atmosphere only a few days to weeks before falling or raining out. Typically, long-lived pollutants have global impacts, whereas shorter-lived pollutants have time only to cause regional impacts before cycling out of the atmosphere. Different pollutants have different lifespans based on the chemical reactivity of their molecules. The atmosphere also contains cleansing agents that break up pollution molecules. Among the most important of these agents is the hydroxyl radical (often abbreviated OH). It prevents toxic buildups by oxidizing (essentially erasing) pollution and many naturally occurring chemicals in the air. Scientists at NCAR and the Georgia Institute of Technology have found exceptionally high concentrations of hydroxyl radicals over Antarctica, but it is not clear what the source of these radicals is or what the atmospheric impact might be.
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Pollutants sometimes return to the atmosphere after being deposited
on Earth, which complicates antipollution efforts. An example of
this is mercury, a long-lived pollutant that is
highly toxic. Gaseous elemental mercury, which comes from both
natural and human sources, travels the globe for about a year before
being deposited on land or water. It is then re-emitted through
such events as wildfires, which release mercury stored in foliage
and ground litter. NCAR researchers have measured the amount of 
