BOX 5.1

Atmospheric Structure

The atmosphere is often divided into layers to differentiate regions with different characteristics. The lowest layer is called the troposphere, from Greek words indicating a region of overturning. In this region the temperature generally decreases with altitude to a height called the tropopause. Above this altitude lies the stratosphere, where the temperature remains constant or increases with altitude up to the stratopause, at about 50 km altitude. The temperature again decreases in the overlying mesosphere up to the mesopause (~80 km), above which it rises in the thermosphere under the influence of solar radiation. The temperature structure in the stratosphere tends to suppress vertical motions, leading to more horizontal winds.

cal relatives (nitrogen dioxide, bromide, carbon monoxide, formaldehyde).

UNDERSTANDING AND REMOVING THE THREAT OF STRATOSPHERIC OZONE DEPLETION

Until about 1964 it was thought that the Chapman (1930) scheme, based only on forms of oxygen, could explain the stratospheric abundance of ozone (Wayne 1985). Subsequently, improved laboratory measurements of reaction rate coefficients showed that this approach overestimated the amount of stratospheric ozone by a factor of 2. Further laboratory measurements showed that reactions involving compounds of hydrogen, chlorine, and bromine from natural sources could enter into catalytic cycles that would speed up the rate of ozone destruction, decreasing estimates of ozone amounts and bringing them into better agreement with ground-based observations.

A very alarming fact was that two of these gases had large and potentially rapidly increasing anthropogenic sources. A projected fleet of 500 commercial supersonic airplanes flying many hours each day was expected to inject large amounts of nitrogen oxides into the lower stratosphere, with deleterious effects (Crutzen 1970, Johnston 1971). Ultimately this fleet did not materialize. However, the techniques and models developed to address the former problem were ready to be applied to the next threat to the ozone layer: chlorine, which was being released in significant amounts by the photolysis of CFCs in the stratosphere. The chlorine released from CFCs was also predicted to cause a serious reduction in ozone (Cicerone et al. 1974, Molina and Rowland 1974).

FIGURE 5.1 Schematic of biogeochemical cycling with human contributions included, illustrating the major gas-phase constituents in the lower atmosphere that are measured from space (shaded in gray). NO2 and NO are encircled to represent equilibrium; their sum is referred to as NOx. NOTE: BrO = bromine monoxide; CH4 = methane; CO = carbon monoxide; h = Planck’s constant; H2O = water; HCHO = formaldehyde; HO2 = hydroperoxyl; v = photon frequency; NO = nitric oxide; NO2 = nitrogen dioxide; O(1D) = electronically excited oxygen atoms; O2 = molecular oxygen; O3 = ozone; OH = hydroxide; RH = hydrocarbon species; SO2 = sulfur dioxide. SOURCE: Drawing by A.M. Thompson and K.M. Dougherty, Pennsylvania State University.

These gases would reduce the amounts of stratospheric ozone below the natural background level, letting more UV radiation reach the surface, causing increased incidence of human skin cancer as well as damage to other biological processes. Subsequent studies showed that bromine, which has natural and anthropogenic sources, could also cause significant ozone depletion (Wofsy et al. 1975, Yung et al. 1980). Because of the dire nature of these predictions, it was crucial to develop a better understanding of this region of the atmosphere as quickly as possible.

OBSERVING STRATOSPHERIC DYNAMICS

To predict the ozone distribution and its changes in the stratosphere, it is also necessary to understand atmospheric motions. These are closely linked to radiative heating and cooling, which depends on the atmospheric composition, notably the ozone distribution (Craig 1965). Understanding this interacting system of chemistry dynamics, and radiation requires global observations, unavailable from ground-based measurements, as well as the synergistic use of models to incorporate this information and allow accurate and trustworthy predictions to be made.

The stratosphere was first identified in 1899, when balloonborne measurements showed that the atmospheric



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