ATMOSPHERE

The earth's atmosphere is simple in some respects, and complex in others. It is relatively uniform in composition with respect to its major mass components (oxygen and nitrogen), yet extremely variable in some minor components, such as water vapor and ozone (O₃), which play major roles in its heat and radiation fluctuations. The atmosphere has a complex structure based on temperature gradients. This structure governs its mixing characteristics and the buildup of contaminants, yet is usually invisible, except when light-scattering particles suspended in the air make it visible. The structure of the atmosphere is of major importance to the dilution and dispersion of contaminants. It is governed by the lapse rate, which is the rate of change of air temperature with height above the ground.

The lowest of the atmospheric layers is the troposphere, which contains about 75 percent of the mass of the atmosphere, and almost all of its moisture. It extends to a height that varies from about 9 kilometers at the poles to about 15 kilometers at the equator, and it has an average lapse rate of about −6.5°C/km. The boundary between the troposphere and the next layer, the stratosphere, is known as the tropopause. The stratosphere contains essentially all of the remainder of the mass of the atmosphere; it is nearly isothermal (the temperature does not change with altitude) in the lower regions and shows a temperature increase with height in the upper regions. There is very little air exchange between the well-mixed and turbulent troposphere and the nearly stagnant stratosphere.

The major constituents of dry air at ground level are nitrogen (N₂) at 78.1 percent by volume, oxygen (O₂) at 21.0 percent, and argon (Ar) at 0.9 percent. Carbon dioxide (CO₂) is present at about 330 ppm by volume and methane (CH₄) at about1.5 ppm by volume. About 3 percent of the total mass of the lower atmosphere is water vapor (H₂O), but the concentration is extremely variable in both space and time. In general, the warmer portions of the atmosphere contain more water vapor. The water vapor content becomes lower with increasing altitude and with increasing latitude. Water vapor plays a critical role in governing the earth's heat exchange and the motion of the atmosphere, due to its high heat capacity, absorption of infrared radiation, and heat of vaporization. Further effects attributable to atmospheric water result when air motion creates clouds (aerosols of water droplets), in which the energy received as sunshine in one place is liberated as the latent heat of vaporization in another.

Of the incoming radiant energy, about 30 to 50 percent is scattered back toward space, reflected primarily by clouds and, to some extent, by solid particles or by the earth's surface. About 20 percent of the incident radiant energy is absorbed as it passes through the atmosphere. Stratospheric O₃ absorbs about 1 to 3 percent, primarily in the short-wave ultraviolet (UV) portion of the spectrum; this effectively limits further penetration to those wavelengths greater than 0.3 microns. In the troposphere, 17 to 19 percent of the incoming radiation is absorbed, due primarily to water vapor and secondarily to CO₂.

The average radiation into space essentially equals that absorbed from the sun, and a substantial amount of energy must flow from the tropics toward the poles within the oceans and the troposphere. This flow of energy is accomplished primarily by systems of warm air and ocean currents that flow toward the poles and cool currents that flow toward the tropics.

The dispersion of contaminants within the atmosphere is generally referred to as diffusion. For practical purposes, the dispersion of contaminants by molecular diffusion is negligible because the extent of movements are generally infinitesimal compared to the movements of the air volumes containing them by the turbulent motions of the air (turbulent diffusion).

Atmospheric turbulence is a complicated phenomenon that has defied mathematical description. When considering contaminant dispersion, contaminant sources can be divided into three different categories: (1) point sources, such as tall industrial smokestacks; (2) line sources, such as highways; and (3) area sources, such as whole urban regions. The simplest is an elevated point source. The light-scattering properties of the aerosol in the plume from such a stack, consisting of fly ash and condensed water, enable us to observe plume dispersion with the unaided eye.

The vertical mixing of air is dependent upon the temperature profile of the atmosphere (the lapse rate). The immediate ground level concentrations of air contaminants may be reduced by vertical mixing, since dispersal into higher regions dilutes the contaminants. Poor vertical mixing may allow concentrations released at low altitudes to remain there in relatively concentrated form. An extreme case of atmospheric stability occurs when the atmospheric lapse rate is negative (when the temperature increases with altitude). This condition is known as a temperature inversion. There is virtually no vertical air movement within inversion layers and contaminants accumulate within them.