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      Atmosphere

The atmosphere is the envelope of gases surrounding the earth, and it is subdivided into regions depending on the altitude. The constituents of the atmosphere are primarily nitrogen (N₂, 78.08% v/v), oxygen (O₂, 20.95% w/w), and water vapor (0 to 0.25% w/w), although the concentration of water vapor (H₂O) is highly variable, especially near the surface, where volume fractions can be as high as 4% in the tropics. There are many minor constituents or trace gases such as (alphabetically rather than by abundance, which is variable) argon (Ar), carbon dioxide (CO₂), helium (He), hydrogen (H₂), krypton (Kr), methane (CH₄). (neon, Ne), nitrous oxide (N₂O), ozone (O₃), and water vapor (H₂O) (Table A-26).

Table A-26 Approximate Composition of the Atmosphere.

In addition to the gaseous constituents, the atmosphere also contains suspended solid and liquid particles. Aerosols are particulate matter usually less than 1 micron in diameter (also called 1 µm which is equivalent to 1 m x 10^-6 in diameter, i.e., one millionth of a meter or one thousandth of a millimeter, 0.001 mm, or approximately 0.000039 of an inch) that are created by gas-to-particle reactions and are lifted from the surface by the winds. A portion of these aerosols can become centers of condensation or deposition in the growth of water and ice clouds. Cloud droplets and ice crystals are made primarily of water with some trace amounts of particles and dissolved gases. Their diameters range up to 100 µm. Water or ice particles larger than approximately 100 microns begin to fall because of gravity and may result in precipitation at the surface.

      Ozone is found in trace quantities throughout the atmosphere, the largest concentrations being location in a layer in the lower stratosphere between the altitudes of 9 and 18 mi (15 and 30 km). This ozone results from the dissociation by solar ultraviolet radiation of molecular oxygen in the upper atmosphere and nitrogen dioxide in the lower atmosphere. Ozone also plays an important role in the formation of photochemical smog and in the purging of trace species from the lower atmosphere.

      The chemistry of ozone formation can be explained in relatively simple terms, although the reactions are believed to be much more complex. Thus, above approximately 19 mi (30 km), oxygen is dissociated during the daytime by energy (hv) from ultraviolet light:

      The oxygen atoms produced then form ozone:

      In this equation, M is an arbitrary molecule required to conserve energy and momentum in the reaction that produces ozone. Although present in only trace quantities (Table A-26), atmospheric ozone plays a critical role for the biosphere by absorbing the ultraviolet radiation with a wavelength from 240 to 320 nm (nm, 1 nm = 1 meter x 10⁹, which would otherwise be transmitted to the surface of the Earth.

      The atmospheric ozone should not be confused with the ozone layer which acts is a region of stratosphere of the Earth that absorbs most of the ultraviolet radiation from the Sun. The ozone layer is mainly found in the lower portion of the stratosphere, from approximately 9 to 22 mi above the Earth, although the thickness of the layer varies seasonally and geographically. This layer is so-named because it contains a high concentration of ozone (O₃) in relation to other parts of the atmosphere, although still small in relation to other gases in the stratosphere. The ozone layer contains up to 10 parts per million of ozone, while the average ozone concentration in the atmosphere of the Earth as a whole is on the order of 0.3 parts per million.

      The ultraviolet radiation is lethal to simple unicellular organisms (algae, bacteria, protozoa) and to the surface cells of higher plants and animals. It also damages the genetic material of cells (deoxyribonucleic acid, DNA) and is responsible for sunburn in human skin. In addition, the incidence of skin cancer has been statistically correlated with the observed surface intensities of the ultraviolet wavelengths from 290 to 320 nm, which are not totally absorbed by the ozone layer.

      Atmospheric pressure decreases as an approximately exponential function of altitude, which largely determines the characteristics of the atmosphere. Thus:

      In this equation, P_h is the pressure at any given height, P_o is the pressure at zero altitude (sea level); m is the average gram molecular mass of air (28.97 g/mole in the troposphere); g is the acceleration of gravity (981 cm x sec^-1 at sea level); h is the altitude (in cm or meters or kilometers), and R is the gas constant (8.314 x 107 erg x deg^-1 x mole^-1), and T is the absolute temperature. Furthermore:

      At sea level where the pressure is 1 atm:

The characteristics of the atmosphere vary widely with altitude, time (season), location (latitude), and even solar activity. At a high altitude, normally reactive species, such as atomic oxygen, persist for long periods of time. At such altitudes, the pressure is low and the distance traveled by a reactive species before it collides with a potential reactant (the mean free path) is high.

      An important effect noted as a result of the changes in the constituents of the atmosphere

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