Environmental Engineering Reference
In-Depth Information
NO and NO
2
in the lower stratosphere to form stable compounds such as ClONO
2
that do not react with ozone [
10
,
17
].
Today the international treaties to limit the production of CFCs have shown
substantial effects, but at the same time this brings back N
2
O as a major sink for
ozone that will play a key role in the foreseeable future [
16
]. Nitrous oxide is among
the greenhouse gases considered in the second phase of the Kyoto Protocol to the
United Nations Framework Convention on Climate Change, together with CO
2
,
CH
4
,SF
6
, and the CFCs. This treaty covers the commitment period 2013-2020, but
remains to be ratified by most countries, with a decision by the European Parliament
projected for 2015 [
18
].
2.3 Abiotic and Biotic Sources
Nitrous oxide is primarily produced in the course of a variety of biological
processes, in particular bacterial denitrification (see Section
2.4
), but in recent
times a number of abiotic routes for the generation of substantial quantities of the
gas were found. The nitrate-dependent oxidation of Fe(II) was recognized as a
central geochemical process, but while for most minerals the reduced nitrogen
species generated here are nitrite or ammonium, the mineral siderite (FeCO
3
) was
described to yield N
2
O as a main product [
19
]. More recently, a study investigating
a brine pond in Antarctica revealed that several Fe(II) components of the mineral
dolerite could also generate N
2
O in large quantities [
20
].
On a global scale, such processes nevertheless are dwarfed by the biological
generation of nitrous oxide through the pathways of nitrification and denitrifi-
cation in the nitrogen cycle [
6
,
21
]. Approximately 40 % of the global nitrous
oxide emissions originate directly or indirectly from human activities
(US Environmental Protection Agency,
http://epa.gov/climatechange/ghgemissions/
gases/n2o.html
)
. Herein, 70 % are related to agricultural soil management, with a
minor fraction of approximately 5 % being due to the breakdown of nitrogen
compounds in the manure and urine of livestock, while the bulk is due to the use of
nitrogen fertilizers in industrial crop production (see Section
2.4
). About 10 % of
anthropogenic N
2
O release are attributed to combustion engines in industry and
transportation, and another 10 % to industrial and chemical production processes.
A major factor hereby is the release of N
2
O as a side product in the synthesis of
adipic acid, one of the two components required for the production of nylon [
22
].
2.4 Bacterial Denitrification
In the metabolic pathway of denitrification, N
2
O is generated through the reductive
coupling of two molecules of nitric oxide (NO) by the membrane-integral nitric
oxide reductase. It is released to the periplasmic space in Gram-negative bacteria,
Search WWH ::
Custom Search