空气(air)
组成地球大气的各种气体的混合物。空气中有一些近於浓度不变的气体和一些浓度随空间和时间变化的气体。浓度不变的气体中最重要的是占78%的氮（N2）和占21%的氧（O2）。其他占少量的有氩（Ar）、氖（Ne）、氦、甲烷（CH4）、氪（Kr）、氢（H2）、一氧化二氮（N2O）、氙（Xe），其浓度比例几乎维持不变。其他气体发生在有浓度变化的空气中，以水汽（H2O）、臭氧（O3）、二氧化碳（CO2）、二氧化硫（SO2）和二氧化氮（NO2）最重要。这些浓度易变的空气成分，虽然量较少，但对地球上生命的维持却非常重要。水汽是各种降水的来源，也是红外辐射的重要吸收体和放射体。二氧化碳除了参与光合作用（Photosynthesis）外，也是红外辐射的重要吸收体和放射体。臭氧（ozone）主要存在於同温层（stratosphere，参阅ozone layer），是太阳紫外辐射（ultraviolet radiation）的有效吸收体，但在地表却会是具侵略蚀性的污染物及主要的烟雾构成物。
English version：
air
Mixture of gases constituting the earth's atmosphere. Some gases occur in steady concentrations. The most important are molecular nitrogen (N2), 78% by volume, and molecular oxygen (O2), 21%. Small amounts of argon (Ar; 1.9%), neon (Ne), helium (He), methane (CH4), krypton (Kr), hydrogen (H2), nitrous oxide (N2O), and xenon (Xe) are also present in almost constant proportions. Other gases occur in variable concentrations: water vapor (H2O), ozone (O3), carbon dioxide (CO2), sulfur dioxide (SO2), and nitrogen dioxide (NO2). Air also contains trace amounts of ammonia and hydrogen sulfide. The variable constituents are important for maintaining life. Water vapor is the source for all forms of precipitation and is an important absorber and emitter of infrared radiation. Carbon dioxide is necessary for photosynthesis and is also an important absorber and emitter of infrared radiation. Ozone in the stratosphere (see ozone layer) is an effective absorber of ultraviolet radiation from the sun, but at ground-level is a corrosive pollutant and a major constituent of smog.
