ABSTRACT
The Earth’s atmosphere is a mixture of gases and particulate-phase substances. The most abundant of these, nitrogen (N2) and oxygen (O2), comprise approximately 78% and 21%, respectively, of atmospheric mass and volume. A number of trace gases make up the remaining 1%. Average concentrations (with the exception of stratospheric ozone; O3) are reported in Table 1.1. These include gases present in essentially constant concentrations: N2, O2, argon (Ar), neon (Ne), helium (He), krypton (Kr), hydrogen (H2), and xenon (Xe). Others vary temporally and spatially. These include water vapor (H2O), carbon dioxide (CO2), carbon monoxide (CO), methane (CH4), O3, the nitrogen oxides (nitrous oxide [N2O], nitric oxide [NO], and nitrogen dioxide [NO2]), ammonia (NH3), formaldehyde (HCHO), sulfur dioxide (SO2), a number of reduced sulfur compounds (dimethyl sulde [[CH3]2S], carbon disulde [CS2], carbonyl sulde [COS], and hydrogen sulde [H2S]), and odd hydrogen species (hydroxyl radical [OH], hydroperoxyl radical [HO2], and hydrogen peroxide [H2O2]). In addition to these gas-phase substances, the atmosphere contains trace quantities of particulate nitrate NO3
−( ), ammonium NH4+( ), and sulfate SO4
2−( ). Although N2 is the most abundant constituent of the atmosphere, it has a
relatively limited direct role in atmospheric and life processes. It serves as a precursor molecule for the formation of NO3
−, from which plant processes synthesize amino acids, proteins, chlorophyll, and nucleic acids (organic molecules that are directly or indirectly essential to all living things). The conversion of N2 to NO3
− occurs as a result of atmospheric and symbiotic biological processes.
