The advent of aviation provided both a means and a strong incentive to investigate the atmosphere.

Characterizing physical phenomena, such as winds, turbulence, atmospheric electricity, and the behavior of water (especially when it forms clouds or ice), was of great practical concern to early aviators.

 

Subsequently, understanding atmospheric chemistry became important, as the adverse impacts of anthropogenic chemical releases to the atmosphere became more evident.

As in surface waters and porous media, chemistry and physics in the atmosphere are closely intertwined; to understand atmospheric chemistry, a knowledge of atmospheric physics is essential.

For the price of an airplane ticket, the modern student of the atmosphere can visit firsthand 75% (by mass) of Earth’s atmosphere and directly observe processes that earlier generations of scientists had to infer from theory or indirect observation.

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The initial part of an airplane flight is in the troposphere, the lowest layer of the atmosphere (Fig. 4.1).

The word “troposphere” is based on the Greek word “tropos,” meaning change; the ever-changing weather is primarily a tropospheric phenomenon.

Immediately after takeoff, the turbulent nature of tropospheric airflow manifests itself; the ride is often bumpy as the plane passes through large eddies, sometimes called air pockets.

These eddies may be due to wind flowing past irregularities on Earth’s surface or due to convective instability (see Section 4.2).

 

Diffusive transport resulting from these eddies dominates the vertical mixing of atmospheric chemicals.

Chemicals also are transported by advection by the wind.

Recall that eddy diffusion and advection are dominant transport processes in flowing surface waters as well.

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Figure 4.1. Vertical structure of the atmosphere.

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Weather phenomena are confined almost entirely to the troposphere, as are the many air pollutants that are removed by various processes before they can mix upward into the stratosphere.

Certain pollutants, however, such as chlorofluorocarbons (CFCs), are long-lived enough to mix into the stratosphere. Other pollutants can be transported to stratospheric altitudes by occasional energetic events such as strong volcanic eruptions and massive thunderstorms.

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Note that more than one term may refer to a given layer of the atmosphere.

Adapted from Cole (1970). Copyright © 1970, John Wiley & Sons, Inc. Reprinted by permission of John Wiley & Sons, Inc.

Clouds occur frequently in the troposphere, and from an airborne vantage point it is evident that clouds are not randomly scattered in the vertical direction. Usually, clouds have bases at relatively well-defined altitudes, because rising air containing water vapor becomes cooler with height, eventually reaching its dew point, the temperature at which water vapor condenses (see Section 4.2.2). After the plane climbs through the gray murk of a deep cloud layer, it emerges above into brilliant sunlight, which appears all the brighter because of reflection of light (visible albedo) from the clouds below. Albedo, presented as a percentage or a ratio, refers to the portion of all incident solar radiation wavelengths that is reflected. Clouds both reflect and absorb some portion of incoming solar radiation, thereby reducing the amount of solar radiation received at Earth’s surface. Some clouds may reflect as much as 90% of incoming solar radiation, while others may absorb as much as 40% of incoming solar radiation (Rosenberg et al., 1983). Clouds also radiate longwave infrared energy (heat), both out toward space and toward Earth’s surface. The presence of clouds and light in the atmosphere greatly influences the fate of chemicals in the atmosphere. Water in both liquid and solid phases in the clouds facilitates a host of chemical reactions, and the energy of light instigates many reactions.

As the plane continues to climb, the ride often becomes smooth. Coffee served in open cups does not spill, providing testimony that in the tropopause (at approximately 10,000 m (33,000 ft)) and the still higher stratosphere (Fig. 4.1), the size and energy of atmospheric eddies tend to decrease. The stratosphere is similar to the thermocline of a stratified lake in that turbulent diffusion is suppressed and vertical Fickian transport is slowed. Chemicals released into the air near Earth’s surface may mix throughout the troposphere in a few weeks, but take years to move into the stratosphere.

As the plane enters the stratosphere, the pilot’s outside air temperature gauge may read as low as − 55 °C (− 67 °F). The sky is cloudless because most water vapor has been condensed and removed as precipitation far below; the blue sky is evidence that atmospheric gases scatter sunlight, and do so more strongly at shorter wavelengths. To provide a smoother ride, your pilot may deviate to avoid turbulence produced by an intense thunderstorm, one of the few agents, along with strong volcanic eruptions, massive wildfires, and nuclear explosions, capable of carrying substances by vertical advection into the stratosphere. (Exhausts from high-altitude aircraft and spacecraft release chemicals into the stratosphere as well.)

The passenger airplane reaches its maximum altitude capability in the stratosphere; special aircraft, high-altitude balloons, or rockets are needed to go higher. Stratospheric temperature is fairly uniform up to approximately 30,500 m (100,000 ft), at which height the temperature begins to increase with altitude, due to the absorption of ultraviolet solar radiation, primarily by ozone molecules. This upper portion of the stratosphere is considered part of the chemosphere because much of the absorbed energy initiates chemical reactions. For example, a diffuse protective layer of ozone (O3) is produced by ultraviolet solar radiation interacting with oxygen; some of the ozone is destroyed by reaction with anthropogenic chemicals such as chlorofluorocarbons (CFCs). Nuclear chemical reactions occur in the stratosphere as well. For example, highly energetic cosmic rays collide with nitrogen molecules to produce radionuclides such as carbon-14 (14C), which is used by scientists to date natural substances and cultural artifacts as far back in time as 10-20 millennia (Section 1.6.1, Isotopes and Section 2.2.6, The Sedimentary Record).

The stratosphere transitions into the stratopause at approximately 49,000 m (160,000 ft); above lies the mesosphere, within which temperature drops with increasing altitude until reaching the mesopause, at about 75,000 m (250,000 ft). Above the mesopause occurs the thermosphere; temperature again rises with altitude in this rarified layer of the atmosphere. The upper mesosphere and the beginning of the thermosphere also correspond to the altitude at which the chemosphere transitions into the ionosphere. The ionosphere is so named because absorption of solar radiation by atmospheric gases creates ionized layers, such as the E, F1, and F2 layers at approximately 120,000, 220,000, and 380,000 m, respectively; these ionized layers enable global radio communications to be carried out by refracting radio waves back to Earth, as shown in Fig. 4.2. (Now, much commercial radio communication is relayed by satellites.) The ionosphere is also influenced by the solar wind, the constant stream of charged particles, mainly protons and electrons, released by the Sun. Earth’s magnetic field deflects most of the solar wind heading towards Earth; the solar particles follow Earth’s magnetic field lines toward the magnetic poles. As the solar particles pass through the ionosphere, they interact with the sparse air molecules above approximately 150,000 m to cause the spectacular light displays called the aurora borealis in the Northern Hemisphere and the aurora australis in the Southern Hemisphere. The deflection of the solar wind also inhibits gradual stripping of Earth’s atmosphere into space. Above the thermosphere, at approximately 500,000-600,000 m, is the exosphere, in which air molecules can escape Earth’s gravitational field into space.

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Figure 4.2. A simplified view of the ionospheric layers that enable long-range radio transmission. The degree of radio wave refraction is dependent on frequency as well as the angle at which the waves enter the ionosphere. Waves originating in Denver and following path 1 reach Chicago, whereas waves following path 2 skip Chicago, but are received in Washington. Radio waves following path 3 enter the ionosphere at an angle too close to vertical to be returned to Earth’s surface.

Adapted from Howard W. Sams & Company, Inc. (1977).