ABSTRACT
The Earth’s atmosphere is a mixture of gases and particulate-phase substances. The most abundant of these, nitrogen (N
) and oxygen (O
), 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, (O
) are reported in Table 1.1. These include gases present in essentially constant concentrations: N
, O
, argon (Ar), neon (Ne), helium (He), krypton (Kr), hydrogen (H
), and xenon (Xe). Others vary temporally and spatially. These include water vapor (H
O), carbon dioxide (CO
), carbon monoxide (CO), methane (CH
), O
, the nitrogen oxides (nitrous oxide (N
O), nitric oxide (NO), and nitrogen dioxide (NO
)), ammonia (NH
), formaldehyde (HCHO), sulfur dioxide (SO
), a number of reduced sulfur compounds (dimethyl sulfide ((CH
)
S), carbon disulfide (CS
), carbonyl sulfide (COS), and hydrogen sulfide (H
S)), and odd hydrogen species (hydroxyl radical (OH·), hydroperoxyl radical (HO
), and hydrogen peroxide (H
O
)). In addition to these gas-phase substances, the atmosphere contains trace quantities of particulate nitrate (NO
), ammonium (NH
), and sulfate (SO
). Though N
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 NO
, 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 N
to NO
occurs as a
result of atmospheric and symbiotic biological processes. Nitrogen reacts with O
to produce nitrogen oxides (NO
) that include N
O, NO, NO
, gas-phase nitric acid (HNO
), and short-lived substances such as dinitrogen pentoxide (N
O
) and nitrate radical (NO
·). Concentrations of these compounds or substances, unlike their precursors (N
and O
), vary significantly in time and space. Nitrous oxide, a relatively inert gas, was, until several decades ago, thought to be
present in the atmosphere at constant levels. It is one of a number of substances whose concentrations are increasing as a result of human activities.
