What is air? Fundamentally, it is a mixture of gases. This mixture of gases also contains relatively low levels of vapors and aerosol
particles, many of which we consider pollutants of the air. A "gas" may be defined as a formless state of matter which at normal room
temperatures and pressures has low density and viscosity and readily and uniformly distributes itself throughout its container. In
contrast, a "vapor" may be defined as the gaseous form of a substance which – at the same time – coexists as a solid or liquid at
normal temperatures and pressures. We consider gaseous forms of water, gasoline, or nail polish to be vapors whereas oxygen,
nitrogen, and carbon dioxide are considered to be gases. Dry air – air without any water vapor in it – is composed of about 78% nitrogen, 21%
oxygen, and 1% the noble gas argon. Carbon dioxide comprises about 0.04% of the atmosphere, or 400 parts per million, and its
concentration is rising due to emissions from human activities. No other gas is present in uncontaminated dry air at a concentration
greater than 100 parts per million, or 0.01%. The composition of dry air is fairly uniform throughout Earth's atmosphere. Water vapor is not
a part of dry air. However, it is, of course, a prominent part of our atmosphere. Our human experience tells us that the water content of air
is highly variable. During the winter, many of us experience dry, cracked skin due to the relative absence of water vapor. In the summer, on the
other hand, we experience high humidity that decreases our comfort level, makes it difficult for our sweat to evaporate, and contributes to
heat-related illnesses. The amount of water vapor in air can be so low as to be almost negligible, or it can be so high that it is the third
most prevalent component of air in a given location, after only nitrogen and oxygen. Let's take a look at some examples of the amount of
water vapor in the air on a percentage basis in some different atmospheric conditions. Our first set of conditions is a temperature of -30
degrees Celsius (-22 degrees Fahrenheit) at 70% relative humidity. Relative humidity is the amount of water vapor present in air expressed
as a percent of the amount that would saturate the air – the point at which dew would begin to form – at the same temperature. So, at -30
degrees Celsius and 70% relative humidity, water vapor would make up less than 0.03% of the air. This is less than the amount of carbon
dioxide that would be present. This situation occurs regularly in northern Minnesota, for example, during the middle of winter. At a
temperature of 30 degrees Celsius (86 degrees Fahrenheit) and 70% relative humidity, water vapor would be about 3% of the atmosphere,
higher than argon. This condition is common in Minnesota and many other places during the summer. In an industrial drying process, you
may have very high temperatures with significant amounts of water vapor present. For example, at 90 degrees Celsius (194 degrees
Fahrenheit) and 20% relative humidity, water vapor would comprise 16% of the air, nearly as much as oxygen. Air comprises Earth's
atmosphere. The structure of the atmosphere is complex, separated into layers across which vertical mixing is reduced. These layers are
largely defined by shifts in temperature gradients through the atmosphere as shown in the figure by the red line, which is a plot of
average atmospheric temperature on the horizontal axis against elevation in kilometers or miles on the vertical axes. Air pressure
decreases as one travels upward through the atmosphere because the force due to gravity of the
atmosphere that is above is reduced. The lowest layer of the atmosphere is the troposphere, extending only about 10 to 15
kilometers, or about 6 to 9 miles, from the surface of the Earth. This is the zone in which people live. In the troposphere, the reduction in
air pressure with elevation is reflected in a general decrease of air temperature with elevation, as shown in the figure. The proximity
of the lowest part of the troposphere to the heat-absorbing surface of the earth also contributes to the temperature profile. Relative to other
parts of the atmosphere, air in the troposphere experiences rapid vertical mixing. Above the troposphere is the stratosphere, which contains
the ozone layer that protects life on Earth by absorbing harmful ultraviolet radiation produced by the sun. The stratosphere extends to about
50 kilometers above Earth's surface. Because the electromagnetic energy absorbed by the ozone layer is converted to heat, air
temperature generally increases with elevation in the stratosphere even as pressure continues to decrease. The mesosphere is above the
stratosphere. Here air temperatures decrease again with elevation to the coldest observed in the atmosphere. Next is the thermosphere, in
which nitrogen and oxygen absorb short wavelength radiation, producing heat that leads to increasing temperatures with elevation. The
thermosphere contains the ionosphere where photoionization produces free ions. This can lead to the auroras. There is even a layer not
shown in the image, the exosphere, that extends about 500 kilometers above Earth's surface. At the top of the exosphere, air
molecules with sufficient energy can escape from Earth's gravitational pull. To summarize, air is a mixture of gases also containing vapors
and aerosol particles. Dry air is composed of about 78% nitrogen, 21% oxygen, and 1% argon. Water vapor content in air can vary
greatly, from almost 0% to more than 10% in certain high-temperature industrial operations. Lastly, the structure of the atmosphere is
complex. The troposphere is the lowest layer, where people live. It is only about 10 to 15 kilometers, or 6 to 9 miles, thick.