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Introduction
The earth's atmosphere, as discussed in unit 1-2, is mostly
nitrogen and oxygen. These two constituents alone account for 99.03% of
all atmospheric "dry air" gas molecules below 100 km (water vapor accounts for 0-4% of the total of
all gas molecules
depending on latitude, altitude and weather conditions). Concentrations of
trace gases are given in Figure 1. Water vapor, carbon dioxide, and ozone
are, to different degrees, variable in concentration, but all others are
relatively stable.