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.

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