Air is not a chemical compound but a mechanical mixture of gases that is kept
mixed by solar heating in the lower atmosphere. There are many gases in the
atmosphere, but only a few main ones. Nitrogen is the dominant gas of the atmos-
phere, comprising about 78 percent of air without water vapor. Breathable oxygen
is next at almost 21 percent and is the byproduct of photosynthesis in green plants
(seeBiogeochemical cycles). The third gaseous element is argon with somewhat
over 0.9 percent of the atmospheric volume. Argon is seldom heard of because
unlike the more plentiful gases, it is chemically inert. After argon, come gases
such as neon, helium, methane, krypton, hydrogen and others. They are present
in such small amounts that none of them makes up more than a couple of thou-
sandths of a percent of the atmosphere. The atmospheric amounts of these gases
have remained quite steady over time, and they will change meaningfully only
over time spans of millions of years.
Another set of gases are significantly variable over times that can range from
minutes to years. They are water vapor, carbon dioxide, carbon monoxide, ozone
(O3), sulfur dioxide, and nitrogen dioxide. Water vapor is, by far, the most variable
over time and place, and it can range from nearly 0 percent of air’s volume over
the Antarctic continent in winter to almost 4 percent in the first few meters over
a tropical ocean. Water vapor can be extremely variable by location and, for in-
stance, its presence can increase or decrease by several orders of magnitude with
frontal passages in the middle latitudes.
Air has always been impure and carries with it many small solids and liquids
corporately called particulates. These can be salts from the breaking of ocean
waves, dust from the deserts, smoke from burning vegetation, and volcanic ash
among other substances. The particulates have tiny masses and can be buoyant
in the winds and air currents if they are small enough. Pollutants are human addi-
tions of mass to the atmosphere. They include the same particulates and gases pro-
vided to the atmosphere by natural means and also many chemical substances not
found in nature. Pollutant concentrations tend to be highest in cities where human
activities are at a maximum. There are potential interferences with biogeochemi-
cal cycles as humanpopulationincreases along with the variety of its activities.
Atmospheric carbon dioxide and ozone have already been affected by the pres-
ence of humans. Since the middle of the 1880s, carbon dioxide has increased by a
third. Carbon dioxide receives much publicity but is now and always destined to
be a rather small part of the atmosphere. It is approaching 400 parts per million
due to human activities; however, its small relative volume belies its ability to
absorb and reradiate thermal infrared energy as part of the greenhouse effect.
Ozone is the substance that absorbs most solar ultraviolet radiation before it
reaches Earth’s surface. In that sense, it keeps us alive, and it been shown to be
Atmosphere 19