Nitrous Oxide

Nitrous oxide (N2O) is an intermediate product of the denitrification pathway and is also a greenhouse gas.

From: Wetzel's Limnology (Fourth Edition), 2024

Nitrous Oxide

S.C. Gad, in Encyclopedia of Toxicology (Third Edition), 2014

Acute and Short-Term Toxicity (or Exposure)

Animal

The primary toxicological action of nitrous oxide is vitamin B12 depletion in mammals, in which it is an essential cofactor. It is an asphyxiate and narcotic at higher concentrations. The inhalation LC50 is 160 mg m−3 in rats.

Human

Nitrous oxide can cause dizziness, drowsiness, and headache. Loss of consciousness can occur at concentrations of 400 000–800 000 ppm. Anesthesia with nitrous oxide as the sole anesthetic in normal humans for periods of 2–4 h has induced tachypnea, tachycardia, increased systemic blood pressure, atrioventricular junctional rhythm, acute cardiovascular failure, mydriasis, diaphoresis, and occasional clonus and opisthotonus.

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Marine Biogeochemistry

David M. Karl, Anthony F. Michaels, in Encyclopedia of Ocean Sciences (Third Edition), 2019

Nitrous Oxide Production

Nitrous oxide (N2O) is a potent greenhouse gas that has also been implicated in stratospheric ozone depletion (Capone, 1991). The atmospheric inventory of N2O is presently increasing, so there is a renewed interest in the marine ecosystem as a potential source of N2O. Nitrous oxide is a trace gas in seawater with typical concentrations ranging from 5 to 50 nmol L 1. Concentrations of N2O in oceanic surface waters are generally in slight excess of air saturation, implying both a local source and a sustained ocean-to-atmosphere flux. Typically there is a mid-water (500–1000 m) peak in N2O concentration that coincides with the dissolved oxygen minimum. At these intermediate water depths, N2O can exceed 300% saturation relative to atmospheric equilibrium. The two most probable sources of N2O in the ocean are nitrification and denitrification, although to date it has been difficult to quantify the relative contribution of each pathway for most habitats. Isotopic measurements of nitrogen and oxygen could prove invaluable in this regard. Because the various nitrogen cycle reactions are interconnected, changes in the rate of any one process will likely have an impact on the others. For example, selection for N2-fixing organisms as a consequence of dust deposition or deliberate iron fertilization would increase the local NH4+ inventory and lead to accelerated rates of nitrification and, hence, could lead to enhanced N2O production in the surface ocean and flux to the atmosphere.

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Petroleum Industry Activities and Climate Change

Michael Adetunji Ahove, Sewanu Isaac Bankole, in The Political Ecology of Oil and Gas Activities in the Nigerian Aquatic Ecosystem, 2018

18.4.5 Ozone Layer Depletion

Nitrous oxide is the new and most important culprit damaging the ozone layer (Fig. 18.5). It is the largest cause of ozone layer depletion. This is because CFCs and many other gases that damage the ozone layer were banned by the Montreal Protocol (MP), and currently their atmospheric concentrations have reduced substantially. Nitrous oxide is not restricted by the MP, so while the level of other ozone layer depleting substances (ODS) are declining, nitrous oxide levels are increasing. These impacts are expected to become more severe, unless concerted efforts are made to reduce emissions.

Figure 18.5. The earth showing the stratospheric ozone around its surface.

From BBC Bitesize. (2017). National 4 chemistry: Fuels revision 1 Retrieved 18:01, March 3, 2017, from www.bbc.co,uk/education/guides/z8yj6sg/revision.
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Nitrogen in Soils/Cycle☆

Hanna Poffenbarger, ... Wilbur W. Frye, in Reference Module in Earth Systems and Environmental Sciences, 2018

Climate Change and Ozone Destruction

Nitrous oxide and NO are trace gases formed during nitrification and denitrification transformations of N in soil. Molecule for molecule, N2O is 298 times more effective at adsorbing thermal radiation than is CO2, the major greenhouse gas. Nitric oxide also plays a well-known role in destroying stratospheric ozone as illustrated in the following reactions:

(17)O3+hvO2+O·
(18)N2O+O·2NO
(19)NO+O3NO2+O2
(20)NO2+O·NO+O2

Ozone is essential in protecting terrestrial life from ultraviolet radiation (Eq. 17).

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Spaceship Earth

Daniel A. Vallero Ph.D., in Paradigms Lost, 2006

Nitrous Oxide

Nitrous oxide (N2O) makes up approximately 5% of U.S. GWP-weighted greenhouse gas emissions. Emissions estimates for N2O are more uncertain than those for either carbon dioxide or methane. Estimated nitrous oxide emissions have been roughly constant in the 1990s, without an obvious trend. The revised estimates of nitrous oxide emissions include one large class of sources and two small classes (see Figure 9.5). Agriculture is the principal source, dominated by emissions from nitrogen fertilization of agricultural soils. Secondary N2O emissions from nitrogen in agricultural runoff into streams and rivers have been incorporated. Motor vehicles equipped with catalytic converters also emit significant amounts of N2O.9 Chemical processes, fuel combustion, and wastewater treatment plants are comparably small emitters of N2O.

FIGURE 9.5. Nitrous oxide emissions in the United States by type of source.

Source: Department of Energy, 1998. Environmental Information Agency.
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Global gases

Wendy H. Yang, ... Gavin McNicol, in Principles and Applications of Soil Microbiology (Third Edition), 2021

Nitrous oxide and nitric oxide budgets

Balancing nitrous oxide sources and sinks to develop a global nitrous oxide budget  (Table 20.2) has been a major challenge for researchers. Soils from natural and agricultural ecosystems account for more than half of global nitrous oxide emissions. The remaining nitrous oxide is mostly produced in the oceans, by fossil fuel combustion and industrial processes, and from biomass burning (e.g., forest fires). Nitrous oxide is removed from the atmosphere mostly by photochemical reactions in stratosphere, which is a slow process: the average residence time of an N2O molecule is more than 100 years. Soil microbes can remove small amounts of atmospheric nitrous oxide under some circumstances, but this is usually counterbalanced by much greater rates of nitrous oxide production.

A relatively consistent annual increase in atmospheric N2O accumulation (~4 Tg year–1 of N2O-N) has occurred over the last several decades. This observed increase agrees with the difference between our global estimates of sources (~17 Tg N year–1) and sinks of nitrous oxide (13 Tg N year–1; Table 20.2). Thus, at the global scale, our “bottom-up” estimates of nitrous oxide emissions from multiple sources and removal pathways approximately match “top down” observations of atmospheric nitrous oxide. However, at smaller scales these estimates can disagree considerably. One widely used approach for estimating bottom-up emissions involves emissions factors, for which a given fraction of nitrogen inputs (e.g., 1% of synthetic fertilizer) or ecosystem nitrogen pools (e.g., 1% crop residues) is predicted to be released from soil as nitrous oxide. The United States Corn Belt is a region of intensive agriculture that receives large inputs of reactive nitrogen from fertilizer and biological nitrogen fixation. Top-down measurements in this region show much greater atmospheric nitrous oxide concentrations than can be accounted for by soil nitrous oxide emissions estimated from emissions factors. Small streams that drain from agricultural watersheds can produce very high nitrous oxide emissions from denitrification and could thus account for at least part of this missing source (Griffis et al., 2017). However, long-term measurements have also shown that high nitrous oxide emissions from agricultural soils also contribute to regional underestimates by default emissions factors (Gillette et al., 2018). Improving our capacity to estimate nitrous oxide emissions at the site and regional scales remains an active area of research that is critical to inform policy and management for decreasing the emissions of this potent greenhouse gas.

In contrast to nitrous oxide, nitric oxide is a very short-lived compound in the atmosphere (hours–weeks), and its budget is even harder to estimate than for nitrous oxide. Based on extrapolation from field measurements of soil nitric oxide emissions, global nitric oxide emissions from soil average 21 Tg NO-N year–1 but with large uncertainty spanning 4−10 Tg NO-N year–1 (Davidson and Kingerlee, 1997).

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Trace gas measurements using cavity ring-down spectroscopy

Shui-Ming Hu, in Advances in Spectroscopic Monitoring of the Atmosphere, 2021

8.3.1.4 N2O

Nitrous oxide (N2O), commonly known as laughing gas, is the highest nitrogen-containing compound in the lower atmosphere. Its potential for warming per unit molecule is about 300 times that of carbon dioxide. It is also one of the greenhouse gases limited by the Kyoto Protocol. The natural source of nitrous oxide is mainly the release of marine and tropical forests. Anthropogenic sources are mainly agricultural production processes, industrial production, and livestock emissions, accounting for more than 1/3 of the total emissions. Nitrous oxide can only be eliminated by slow photolysis in the stratosphere, and therefore has a long lifetime of about 120 year in the atmosphere. The concentration of nitrous oxide has been maintained at around 270 ppbv for more than 1000 years before the industrial revolution but increased at a rate of 0.2%–0.3% per year after that. By 2008, the global average concentration of nitrous oxide has reached 321.8 ppbv.

Because the N2O concentration in the atmosphere is relatively low and its absorption bands in the near-infrared are relatively weak, optical detection of N2O mainly uses its absorption lines in the midinfrared, including those lines at 4.5 μm (Fleisher et al., 2017), 5.2 μm (Banik et al., 2018), and 7.7 μm (Wojtas et al., 2013). QCLs were applied in these studies, and the detection limit of N2O can reach 2 ppbv.

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Ocean Interfaces & Human Impacts

M.I. Scranton, M.A. de Angelis, in Encyclopedia of Ocean Sciences (Third Edition), 2001

Nitrous Oxide (N2O)

Nitrous oxide is another important greenhouse gas that is present in elevated concentrations in estuarine environments. At present, N2O is responsible for about 5–6% of the anthropogenic greenhouse effect and is increasing in the atmosphere at a rate of about 0.25% per year. However, the role of estuaries in the global budget of the gas has only been addressed recently.

Nitrous oxide is produced primarily as an intermediate during both nitrification (the oxidation of ammonium to nitrate) and denitrification (the reduction of nitrate, via nitrite and N2O, to nitrogen gas), although production by dissimilatory nitrate reduction to ammonium is also possible. In estuaries, nitrification and denitrification are both thought to be important sources. Factors such as the oxygen level in the estuary and the nitrate and ammonium concentrations of the water can influence which pathway is dominant, with denitrification dominating at very low, but non-zero, oxygen concentrations. Nitrous oxide concentrations are typically highest in the portions of the estuary closest to the rivers, and decrease with distance downstream. A number of workers have reported nitrous oxide maxima in estuarine waters at low salinities (<5–10 on the PSU scale), but this is not always the case. The turbidity maximum has been reported to be the site of maximum nitrification (presumably because of increased residence time for bacteria attached to suspended particulate matter, combined with elevated substrate (oxygen and ammonium)).

Table 2 presents a summary of the data published for degree of saturation and air–estuary flux of nitrous oxide from a variety of estuaries, all of which are located in Europe and North America. Concentrations are commonly above that predicted from air–sea equilibrium, and estimates of fluxes range from 0.01 μmol m−2 h−1 to 5 μmol m−2 h−1. Ebullition is not important for nitrous oxide because it is much more soluble than methane. Researchers have estimated the size of the global estuarine source for N2O based on fluxes from individual estuaries multiplied by the global area occupied by estuaries to range from 0.22 Tg N2O y−1 to 5.7 Tg y−1 depending on the characteristics of the rivers studied. Independent estimates based on budgets of nitrogen input to rivers, assumptions about the fraction of inorganic nitrogen species removed by nitrification or denitrification, and the fractional ‘yield’ of nitrous oxide production during these processes indicate that nitrous oxide fluxes to the atmosphere from estuaries is about 0.06–0.34 Tg N2O y−1.

Table 2. Nitrous oxide saturation values (R) and estimated fluxes to the atmosphere for US and European estuaries

EstuaryRFluxa N2O (μmol m−2 h−1)
Europe
 Gironde River1.1–1.6n.a.
 Gironde River≈1.0–3.2n.a.
 Oder River0.9–3.10.014–0.165
 Elbe2.0–16n.a.
 Scheldt≈1.0–311.27–4.77
 Scheldt≈1.2–303.56
UK
 Colne0.9–13.61.3
 Tamar1–3.30.41
 Humber2–401.8
 Tweed0.96–1.1≈0
Mediterranean
 Amvraikos Gulf0.9–1.10.043 ± 0.0468
North-west USA
 Yaquina Bay1.0–4.00.165–0.699
 Alsea River0.9–2.40.047–0.72
East coast USA
 Chesapeake Bay0.9–1.4n.a.
 Merrimack1.2–4.5n.a.
a
All fluxes given are for diffusive flux to the atmosphere. n.a. indicates that insufficient data were given to permit calculation of flux.
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