Global-Scale Transport of Chemicals

The global presence of radioactive fallout from the nuclear bomb tests of the 1950s and 1960s clearly illustrated the capacity of the atmosphere to distribute chemicals around Earth. Global transport has been demonstrated for a large number of chemicals that do not rapidly degrade or settle out of the atmosphere. Examples are methane (CH4), nitrous oxide (N20), and chloro-fluorocarbons (CFCs), all of which have half-lives in the atmosphere ranging from years to decades. 

Chemicals become well mixed throughout large portions of the troposphere in a matter of weeks. Exchange of atmospheric chemicals across the equator, however, is relatively slow compared with the mixing time within either hemisphere alone. Thus, for some purposes, the troposphere may be thought of as containing two boxes, the Northern and Southern Hemispheres. Within each box, chemicals mix in a few weeks to mix between them, however, requires 1 to 2 years. 

The average residence time of an air parcel in the upper stratosphere (above approximately 16 km) is approximately 2 years. The stratosphere should not be regarded as fully mixed, however. Upwelled layers of air, their exact chemical composition changing with season, have been shown to retain an identity for many months. Another example of stratospheric heterogeneity is the air in polar stratospheric vortices (see Section 4.6.4, Fig. 4-41 later) these vortices may have a trace gas chemistry that differs from that of stratospheric air in the tropics. 

Whether a chemical in the stratosphere is predominantly degraded or is mixed back into the troposphere depends on whether its stratospheric resi- 

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