The Variable Atmosphere
Unlike the few "permanent" gases, categorized as such because their proportions are nearly constant near the earth's surface, the concentrations of the numerous other substances found in the earth's atmosphere are variable. With the exception of water vapor, each one of these variable substances exists in the atmosphere in concentrations far less than one percent by volume. Because these constituents exist in such small amounts, their proportions are often recorded in parts per million (ppm) and parts per billion (ppb) by volume.
The figures in the table below are representative of the average concentrations of many of the variable substances found in our atmosphere. Note that in contrast with the permanent gases, the variable substances include gases as well as particulate matter. The air we breathe is not simply a composite of a few isolated molecules, but rather, a complex mixture of gaseous, liquid, and solid substances.
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