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 sulfide ((CH3)2S), carbon disulfide (CS2), carbonyl sulfide (COS), and hydrogen sulfide (H2S)), and odd hydrogen species (hydroxyl radical (OH·), hydroperoxyl radical (HO2·), and hydrogen peroxide (H2O2)). In addition to these gasphase substances, the atmosphere contains trace quantities of particulate nitrate (NO3−), ammonium (NH4+), and sulfate (SO4-2).
