Table 15.1:Concentrations of Atmospheric GasesGas Symbol Concentration (%)
Nitrogen N 2 78.08
Oxygen O 2 20.95
Argon Ar 0.93
Neon Ne 0.0018
Helium He 0.0005
Hydrogen H 0.00006
Xenon Xe 0.000009
Water vapor H 2 O 0 to 4
Carbon dioxide CO 2 0.038
Methane CH 4 0.00017
Krypton Kr 0.00011
Nitrous oxide N 2 O 0.00005
Ozone O 3 0.000004
Particles (dust, soot) 0.000001
Chlorofluorocarbons(CFCs) 0.00000002(Source:http://upload.wikimedia.org/wikipedia/commons/7/7a/Atmosphere_gas_proportions.
svg,License: GNU-FDL)
In nature, air is never completely dry. Up to 4% of the volume of air can be water vapor.
Humidityis the amount of water vapor in air. The humidity of the air varies from place
to place and season to season. This fact is obvious if you compare a summer day in At-
lanta,Georgia where humidity is very high, with a winter day in Phoenix, Arizona where
humidity is very low. When the air is very humid, it feels heavy or sticky. Your hair might
get really curly or frizzy when it is very humid outside. Most people feel more comfortable
when the air is dry. The percentage of water vapor in our atmosphere is listed with a wide
range of values in the table above because air can be both very humid or dry.
Argon, neon, helium, xenon, and krypton arenoble gases. They are colorless, odorless,
tasteless, and they do not become part of ordinary chemical reactions because they are
chemically inert. The noble gases simply exist in the atmosphere.
Some of what is in the atmosphere is not a gas. Particles of dust, soil, fecal matter, metals,
salt, smoke, ash and other solids make up a small percentage of the atmosphere. This
percentage is variable, as anyone who has spent a windy day in the desert knows (Figure
15.2). Particles are important because they provide starting points (or nuclei) for water
vapor to condense on, which then forms raindrops. Some particles are pollutants, which are
discussed in the chapter on human actions and the atmosphere.