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Gas Concentrations
Concentrations of gases expected to occur in the earth's atmosphere
can be calculated by use of some concepts of thermal and chemical
equilibrium from freshman chemistry. However, the results of such
calculations, shown in Figure 8
, give equilibrium concentrations of constituents that are nowhere near
the observed present fractional concentrations. For example, the
equilibrium calculation suggests nitrogen should have a concentration
of 10-10, whereas, in fact, it makes up 78% (0.78). Equilibrium
calculations suggest there would be no oxygen, but the present value is
almost 22%. Methane would be expected to comprise only 10-35 rather
than the observed amount of 1.7 parts per million. Likewise, nitrous
oxide, ammonia, and hydrogen calculated by these methods give values
far below present levels. We can only conclude that our assumption of
equilibrium is wrong, and that, in fact, the atmosphere must be in a
constant state of chemical reaction with inputs and outputs. Knowledge
of the equilibrium concentrations and rates of the inputs and outputs
allows for an estimate of the residence time for each molecule. As
shown, in the table, these range from 10 days to 107 years.
NEXT: Distribution of Temperature