Earth's Atmosphere

The atmosphere is the envelope of gases that extends from the earth's surface outward until it merges with interplanetary space. The atmosphere is a laboratory of varied and complex chemical interactions occuring under conditions that are often unfamiliar to bench chemists.  Complex changes in temperature, pressure, and the chemical composition of the gases occurs as a function of distance from the earth's surface.  Because of the important role of temperature, pressure and radiant energy from the sun on chemical reactions, when studying atmospheric chemistry it is useful to classify various layers of the atmosphere based on both the temperature/ pressure profile in each distinct layer and the chemical composition of atmospheric layers.

Gases are compressible. As a result, the pressure of the atmosphere decreases smoothly as function of distance from the earth's surface with a more rapid decrease in pressure at lower altitudes.  There is no distinct atmospheric boundary but at an altitude of  around 10,000 km the properties of the atmosphere are essentially identical with those in outerspace.  About 99% of the atmosphere's mass lies within 30 km of the surface and about 75% can be found within the 11 km of atmosphere that are closest to the earth. Regions of the atmosphere have been assigned names that define pressure and temperature layers.
 

Major Features of Temperature-defined Regions of the Atmosphere

Region	Pressure (Torr)	Temperature    (K)	Characteristics
Troposphere	760-150	300 to 218	The temperature drops about 7oCper km to -55oC.  It includes the region from the surface to about 11 km up.  All of earth's weather occurs in the troposphere and most aircraft fly in this region.
Stratosphere	from 150 to 2	rises from 218 to about 280	The stratosphere reaches to about 50 km above the earth's surface.  The number of gas molecules is greatly reduced while the amount of energy from the sun is increased.
Mesosphere	from 2 to about 1x10-3	Temperature drops smoothly to about -93oC	Reaches about to about 80 km above the earth's surface.
Thermosphere	<1 x 10-3	Temperature rises to between 700 and 2000 depending upon the intensity of solar radiation.	Reaches to an altitude of about 500 km.

As pressure decreases rapidly with altitude, the density of gas particles in the atmosphere also decreases. The sparse distribution of matter in this region means that the number of collisions between these gas particles is greatly reduced. At the same time, more and more solar energy is able to penetrate. When collision frequency is very low, a thermometer (or, for that matter, any other objects) would experience very little transfer of the kinetic energy. Temperature is defined as: the average kinetic energy of the particles passed from object to object as the result of collisions. While the kinetic energy of individual gas particles would be very high as a result of solar radiation, the small number of collisions that would result in the rarified upper atmosphere would provide few opportunities to transfer that energy to other objects. The temperature of the gas is very high, but the heat that would be transferred to an object in the region is very low.

Free radicals (atoms or groups of atoms with unpaired electrons) react almost immediately at ground level. In the upper atmosphere however, the half-life of free radicals is greatly extended. The greater distances between gas particles provide fewer collision opportunities so that free radicals produced by the interaction of some molecules with energy from the sun remain have a longer residence time in the upper atmosphere. Because of their extreme reactivity and the natural formation of free radicals, free radicals are key actors in several air pollution problems.