Regions And Characteristics Of The Atmosphere

Figure 1.1 shows the different regions of the atmosphere. (See also Appendix V for typical pressures and

Pressure (Torr)

10"3 10"2 10"1 1 10 102

Pressure (Torr)

10"3 10"2 10"1 1 10 102

The Atmosphere And Their Characteristics

Temperature (K)

FIGURE 1.1 Typical variation of temperature with altitude at mid-latitudes as a basis for the divisions of the atmosphere into various regions. Also shown is the variation of total pressure (in Torr) with altitude (top scale, base fO logarithms) where f standard atmosphere = 760 Torr.

Temperature (K)

FIGURE 1.1 Typical variation of temperature with altitude at mid-latitudes as a basis for the divisions of the atmosphere into various regions. Also shown is the variation of total pressure (in Torr) with altitude (top scale, base fO logarithms) where f standard atmosphere = 760 Torr.

temperatures as a function of altitude.) In the troposphere, the temperature generally falls with increasing altitude (except in the presence of inversions; see Chapter 2.B.3). This is due to the strong heating effect at the surface from the absorption of radiation. Because hot air rises, this causes strong vertical mixing so that species emitted at the earth's surface can rise to the tropopause, the region separating the troposphere from the stratosphere, in a few days or less, depending on the meteorological conditions. Essentially all of the water vapor, clouds, and precipitation are found in the troposphere, which provides an important mechanism for scavenging pollutants from the atmosphere.

However, at the tropopause the temperature profile changes, increasing with altitude throughout the stratosphere. The reason for this increase is a critical series of photochemical reactions involving ozone and molecular oxygen. The "Chapman cycle," reactions (l)-(4), hypothesized in the 1930's by Sir Sydney Chapman,

is responsible for generating a steady-state concentration of 03 in the stratosphere.

Stratospheric ozone is essential for life on earth as we know it, because it strongly absorbs light of A < 290 nm. As a result, sunlight reaching the troposphere, commonly referred to as actinic radiation, has wavelengths longer than 290 nm. This short-wavelength cutoff sets limits on tropospheric photochemistry; thus only those molecules that absorb radiation at wavelengths longer than 290 nm can undergo photodissociation and other primary photochemical processes.

Ozone absorbs light strongly between approximately 200 and 310 nm and weakly up into the visible. Dissociation to electronically excited 02 ('Ag) and O('D) requires light energetically equivalent to 310 nm. Therefore, the excess energy available after absorption of light up to this threshold value is released as heat; energy is also released from the O + 02 reaction (2). Both give rise to the increase in temperature in the stratosphere. Relatively little vertical mixing occurs in the stratosphere, and no precipitation scavenging occurs in this region. As a result, massive injections of particles, for example, from volcanic eruptions such as the Mt. Pinatubo eruption, often produce layers of particles in the stratosphere that persist for long periods of time (see Chapter 12).

In the mesosphere, from ~ 50 to ~ 85 km, the temperature again falls with altitude and vertical mixing within the region occurs. This temperature trend is due to the decrease in the 03 concentration with altitude. At about 85 km the temperature starts to rise again because of increased absorption of solar radiation of wavelengths < 200 nm by 02 and N2 as well as by atomic species. This region is known as the thermosphere.

The transition zones between the various regions of the atmosphere are known as the tropopause, stratopause, and mesopause, respectively. Their locations, of course, are not fixed, but vary with latitude, season, and year. Thus Fig. 1.1 represents an average profile for mid-latitudes. Specific temperatures, pressures, densities, winds, and the concentrations of some atmospheric constituents as a function of altitude, geographic position, and time are incorporated into a NASA model, the Global .Reference Atmosphere Model (GRAM); information on obtaining this model and data is included in Appendix IV.

We shall see throughout this book that different chemical and physical processes occur in the troposphere and stratosphere, and we shall frequently refer to different regions in Fig. 1.1. However, it is important to put the atmosphere in perspective with respect to the size of the earth itself. The earth's average diameter is 12,742 km, yet the average distance from the earth's surface to the top of the stratosphere is only ~ 50 km, less than 0.4% of the earth's diameter! The space shuttle orbits outside the atmosphere, but at an altitude of only several hundred miles, which is less than the distance from Los Angeles to San Francisco. Clearly, the atmosphere is a very thin, and as we shall see, fragile shield upon which life as we know it on earth depends.

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