2.2 Composition of the Atmosphere

The composition of the dry atmosphere is shown in Table 2. Earth’s atmosphere differs dramatically from the other terrestrial planets in large part due to biological processes and a balance between production and loss. The dominant atmospheric gases are [latex]N_{2}[/latex] and [latex]O_{2}[/latex] and these account for more than 99 percent of atmospheric pressure. The composition listed in Table 2 is representative of the troposphere—from sea level to about 12,000 m—where mixing processes are strong. The primary noble gas in the atmosphere is [latex]Ar[/latex], constituting just under one percent of the total pressure. Gases used for age dating groundwater are present in trace amounts, and their respective partial pressure does not significantly affect the total pressure. Also shown in Table 2 are the major isotopes of each gas with their relative abundances. The composition of major and noble gases in the atmosphere can be considered stable over the time scale of the age‑dating tracers that are discussed in this book.