Water vapor, tropospheric residence time

Upward diffusion of water vapor through the cold temperatures of the tropopause is very inefficient in fact, the upper limit of cloud formation often occurs at the tropopause. Thus the stratosphere is so dry as to prevent rain formation, and particles and gases have very much longer residence times there than in the troposphere. Stratospheric removal requires diffusion back through the tropopause, which then may be followed by precipitation scavenging. 

The turnover time of water vapor in the atmosphere obviously is a function of latitude and altitude. In the equatorial regions, its turnover time in the atmosphere is a few days, while water in the stratosphere has a turnover time of one year or more. Table 7-1 Qunge, 1963) provides an estimate of the average residence time for water vapor for various latitude ranges in the troposphere. Given this simple picture of vertical structure, motion, transport, and diffusion, we can proceed to examine the behavior of... 

The distribution of water vapor in the troposphere is controlled by general climatic conditions rather than by atmospheric photochemistry. On the basis of rainfall measurements, Junge (128) estimated a 10-day average residence time for water vapor. 

Vapor-phase solvents can dissolve into water vapor, and be subject to hydrolysis reactions and ultimately, precipitation (wet deposition), depending on the solubility of the given solvent. The solvents may also be adsorbed by particulate matter, and be subject to dry deposition. Lyman asserted that atmospheric residence time cannot be directly measured that it must be estimated using simple models of the atmosphere. Howard et al. calculated ranges in half-lives for various organic compounds in the troposphere, and considered reaction rates with hydroxyl radicals, ozone, and by direct photolysis. 

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