In an atmosphere that is 99 percent nitrogen and oxygen, it is easy to forget about the minor constituents that comprise the air we breathe.

Air is a collection of gases, aerosols and particles, but the bulk of it is invisible, tasteless and odorless gas.

Of the variable gases, water is the most abundant. In addition to its role in transporting heat and moisture, it is a significant greenhouse gas. It can reach concentrations of 3 percent or more in very humid tropical air near Earth’s surface. It is constantly cycled between surface and atmosphere in the hydrologic cycle.

If all of the water vapor in the atmosphere were condensed, it would cover the entire Earth to a depth of 1 inch. The water in Lake Superior very nearly equals the amount of water vapor in the atmosphere.

Carbon dioxide concentration is about .038 percent (which is more easily expressed at 380 parts per million, or ppm). It has risen from 330 ppm in 1975, an increase of 15 percent. Carbon dioxide cycling is the most complex of all atmospheric gases since it has components in the atmosphere, the oceans, the rock cycle and the biosphere.

Methane is another greenhouse gas that has also increased in the atmosphere with industrialization. It is emitted in the decay of organic material and from animal digestion. There are six sources of methane: wetlands, fossil fuels, landfills, ruminant animals, rice paddies and biomass combustion. Primary emitters are termites and cows. With an increase in world consumption of beef and the number of cattle in domestic production, the amount of methane added to the atmosphere each year has increased dramatically.

Methane gas is relatively short-lived in the atmosphere. It is broken down by hydroxyl radicals that are left over after hydrogen is released by the breakdown of water molecules by ultraviolet radiation.

The ozone layer is at the bottom of the stratosphere, just above the tropopause. Even here the total concentration of ozone is only about 10 ppm. The conversion of atmospheric oxygen to ozone absorbs most of the sun’s ultraviolet light. If not for the ozone layer, it is unlikely that there would be any life above the sea or ground surface.

The noble gases are inert. Argon is the third most abundant gas. Relative amounts and isotopic compositions of argon, helium, neon and radon help us understand the origin and evolution of the atmosphere.

Hydrogen and helium, which comprise 99 percent of the universe, are rare in Earth’s atmosphere. These gases are lightweight (think blimps and party balloons) and tend to rise to the upper atmosphere.

Because of their low molecular mass, the thermal motions of molecules of hydrogen and helium exceed the escape velocity at that height, and they are lost into space.

This leakage would have depleted them completely were it not for replacement. Radioactive decay of heavier elements such as uranium emits helium nuclei in the form of alpha particles, while breakdown of water molecules by sunlight generates hydrogen.

Richard Brill is a professor of science at Honolulu Community College. Email questions and comments to rickb@hcc.hawaii.edu.