Environmental Engineering Reference
In-Depth Information
collection of carbon-containing compounds that are important for tropospheric
photochemistry and the generation of ozone and oxidizing compounds. Non-
methane hydrocarbons undergo oxidation via many mechanisms. The complete
oxidation results in one mole of carbon dioxide for each carbon atom in the original
molecule [
17
]. The lifetime of non-methane hydrocarbons vary considerably, but
are usually less than the lifetime of methane. They are important for air quality,
since they are precursors to ozone, and may themselves be toxic, but are not
sufficiently abundant to have radiative impacts [
17
].
Carbon is also a component of atmospheric aerosols, but at much lower
concentrations than the gases described above. These aerosols can, however, be
important for radiation and chemistry, because they exist as a solid in a gas phase.
Sources of carbonaceous aerosols include by-products from combustion
processes, compounds emitted from vegetation, and in situ product from the
condensation of non-methane hydrocarbons.
Nitrogen Cycle
The dominant form of nitrogen in the atmosphere is molecular nitrogen (N
2
), which
represents over 70% of the air. However, this form is chemically inert and not
biologically available. Thus, the nitrogen cycle usually refers to the cycle of
reactive nitrogen compounds. The dominant form of reactive nitrogen in the
atmosphere is nitrous oxide, a long-lived gas (
70 years), which is destroyed
predominately in the stratosphere by photolysis. The sources of nitrous oxide are
in the soils, as by-products of nitrogen fixation and denitrification processes, which
convert molecular nitrogen to reactive nitrogen and back. Humans have increased
nitrous oxide from 270 to 319 ppb, largely due to an increase in nitrogen fertilizer
application [
4
]. Emissions of nitrous oxide are estimated to be 11 Tg/year. Nitrous
oxide is a potent green house, and represents the third largest radiative forcing gas
(after carbon dioxide and methane) [
4
].
Potentially even more important than nitrous oxide are the nitrogen oxides (NO
and NO
2
), which currently have emissions of more than 140 Tg/year, while
preindustrial emissions were thought to be closer to 30 Tg/year [
18
]. Nitrogen
oxides are emitted during combustion process, with smaller contributions from soils
and lightning. Nitrogen oxides have a short lifetime in the atmosphere (a few days),
and are important for atmospheric photochemistry and air quality because of their
role in producing ozone [
17
]. In addition, nitrogen oxides deposit onto the earth's
surface as they are removed from the atmosphere, and thus move nitrogen from one
region to another in large quantities (e.g., [
19
]).
Since preindustrial times, emissions and concentrations of ammonia have
increased with current emissions of 52 Tg/year compared with an estimated
22 Tg/year in preindustrial times [
18
]. Ammonia is emitted predominately from
natural soils and agriculture, with small contributions from oceans and energy use.
Ammonia has a short lifetime in the atmosphere (a few days) and is removed
through wet or dry deposition onto the surface.
>
