Nitrous oxide (N2O) is one of the greenhouse gases in the atmosphere whose atmospheric concentration has been increasing since pre-industrial times. Despite many existing uncertainties, several sources of N2O have been identified. This paper gives a brief overview of the current knowledge on atmospheric concentrations and emissions of N2O and discusses some aspects of future research.
Although N2O occurs in the atmosphere in minute quantities compared to CO2 and water vapor, its contribution to the greenhouse effect is considerable. This is caused by its long residence time in combination with the relatively large energy absorption capacity per molecule. Per unit mass the global warming potential of N2O is about 310 times greater than that of CO2. The global annual atmospheric CO2 increase is about 3000 million ton CO2-C, primarily from fossil fuels. Although the anual increase of the mass of N2O in the atmosphere of 4-5 million ton N2O-N is close to three orders of magnitude smaller than this amount, its contribution to global warming is of the same order of magnitude as that of CO2.
Nitrous oxide is a long-lived gas because it is inert in the troposphere. However, in the stratosphere N2O is removed by photolysis and reaction with excited oxygen atoms, O(1D). The oxidation of N2O according to the latter reaction yields NO, providing the major input of NOx to the stratosphere, thus in part regulating stratospheric ozone and influencing the NOx balance in the upper troposphere (Crutzen, 1970).
Analysis of Antarctic ice core samples indicates that the atmospheric N2O concentration has risen from about 275 ppbv in pre-industrial times to about 293 ppbv in the beginning of the 20th century and to 311 ppbv now. Records from longer periods indicate that the atmospheric N2O concentration was at least 30% lower during the last Glacial Maximum than during the Holocene epoch, and that present-day N2O concentrations are unprecedented in the past 45,000 years (Leuenberger and Siegenthaler, 1992). The atmospheric N2O concentration started to increase rapidly during this century, but an accelerated increase may have started only after 1940. The observed increase of atmospheric N2O during the 1980s was 0.25% or 0.8 ppbv per year. However, the trends over the last decade are extremely variable, ranging from 0.5 to 1.2 ppbv per year.
The annual increase of atmospheric N2O during the 1980s of 0.25% was caused by an imbalance of sources over sinks of 4-5 million ton N2O-N per year, accounting for 25% or more of the total annual source of 16 million ton N2O-N. The stratospheric destruction removes 12 million ton N2O-N per year (Minschwaner et al., 1993). The atmospheric lifetime of N2O based on the destruction rates and the atmospheric burden amounts to 120 years.
Most of the N2O in the Earth's atmosphere stems from microbiological processes. In soils and aquatic systems the major sources of N2O are generally accepted to be denitrification and nitrification. In subsurface environments denitrification is the major source of N2O. Under reducing conditions with no other available source of N, N2O may be consumed in soils. Uptake of N2O by the ocean surface has also been observed. At present the knowledge on the conditions at which soils and aquatic systems act as sinks for N2O, and the parameters affecting the influx when they do so, is too limited to evaluate their importance at the global scale.
Although in recent years considerable progress has been made in the identification of source candidates, the uncertainty in the various source estimates has not been reduced. The N2O emissions from fossil fuel combustion and biomass burning had long been considered the major cause of the atmospheric increase. This view changed in 1988 when it was discovered that in mixtures of flue gases (such as in power plant effluent), that are stored in stainless steel canisters even during short periods, reactions occur in the presence of SO2, NOx, and H2O that can produce substantial amounts of N2O. All past work on N2O from coal combustion had relied on stored samples and became suspect. Now it is accepted that direct N2O emission from stationary fossil fuel combustion contributes less than 1% of the global source. However, as yet unknown amounts of N2O may be formed in smoke plumes resulting from biomass burning, in exhaust gases from other combustion processes, and during catalytic reduction of NOx.
The recent history of global "budgets" of emissions of N2O illustrates the change in views before and after the discovery of the sampling artifact (see table). A great number of source candidates have been identified, including fertilized agricultural soils, livestock production systems, soils under natural vegetation, aquatic sources, biomass burning, land use changes, fossil fuel combustion, automobiles, industrial, and other sources.
The major sources in most N2O budgets are formed by soils under natural vegetation, followed by oceans. Despite the uncertainty in the global N2O budget, the most recent IPCC assessment indicates that agricultural activities are the most important anthropogenic source of N2O (IPCC, 1995). The increase in the use of catalytic converters in cars, which cause much higher N2O emissions than cars not equipped with catalysts, may lead to an important increase in emissions in the future. Atmospheric oxidation of ammonia (NH3) to N2O by hydroxyl radicals (OH) may be an important and increasing source.
There are a great number of poorly known, minor sources of N2O, such as lightning and corona processes around high-voltage electrical transmission lines. Recently, global warming has also been mentioned as a potential N2O source (see table). Other minor sources not listed in the table include fresh water and coastal marine waters, effects of N deposition on soil N2O emission, the production and use of explosives, medical and industrial use of N2O, and use of N2O as an aerosol propellant.
Current global estimates of emissions and atmospheric removal do not account for other possible removal processes, such as uptake of N2O by soils and aquatic ecosystems, and the potential role of the oceans as a reservoir of N2O. If sinks of N2O turn out to be important, the source estimates need to be revised as well to obtain the correct increase in N2O concentration.
It is difficult to quantitatively determine biogenic fluxes of N2O. This is caused by the extreme temporal and spatial variability of the processes of N2O formation and exchange in all biogenic sources. In addition, in early studies a few representative measurements were used to extrapolate to the global flux. Nowadays, more attention is paid to techniques of scaling. For example, terrestrial ecosystems should be stratified by delineation of functional types on the basis of soil, vegetation, and terrain characteristics. Remote-sensing observations are increasingly used for delineating. Similarly, functional groupings can be made in oceans on the occurrence of upwelling, temperature, concentrations of nitrous oxide, nitrate, oxygen, and organic matter. For biomass burning the type of fuel and fire intensity can be used as a basis for scaling.
Functional types can form the basis for measurement schemes, so that the variability within delineations is reduced compared to that of the whole system. Micrometeorological (e.g., eddy correlation and eddy accumulation techniques) and remote sensing techniques are used to determine fluxes for larger areas. The traditional enclosure methods, whereby the fluxes are determined from the concentration change within a flux chamber, are still needed to study the processes and their regulating factors and to develop process models to simulate fluxes. With appropriate techniques for integrating knowledge acquired at a detailed scale towards larger scale levels, and validation of the results against measurements at higher scale levels, fluxes can be extrapolated. Hence, scaling not only involves bottom-up methods, but also top-down approaches. At the global and regional scale a promising technique is inverse modeling, whereby atmospheric concentrations are used to calculate backwards where the source regions are, and to determine the fluxes from these regions. Inverse modeling of N2O is hampered by the small number of long-term monitoring stations. At present N2O records are available from 10 monitoring stations of the Atmospheric Lifetime Experiment-Global Atmospheric Gases Experiment (ALE-GAGE) and NOAA Climate Monitoring and Diagnostics Laboratory (CMDL) networks. Most of these stations are located in remote places away from major source regions, where air is thoroughly mixed, to establish global trends in concentrations. For atmospheric modeling the global coverage may not be adequate, because signals from continental sources are not recorded.
Isotopic ratios of stable isotopes are a very promising tool in top-down scaling (see, e.g., Kim and Craig, 1993). The 18O/16O isotopic ratios of N2O can be sensitively measured, and d18O values for N2O derived from nitrification are lower than those for N2O from denitrification. Assuming that these isotopic differences are uniform among different systems, the process of formation of N2O may be determined. Nitrous oxide in soil and groundwater may be significantly depleted in 15N and 18O relative to tropospheric N2O. In surface ocean waters down to 600-m depth, N2O is depleted in both heavy isotopes, but at greater depth N2O is enriched in 15N and 18O. Coal plant and engine exhaust have been shown to be enriched in 18O relative to N2O in the troposphere and in soil and groundwater. The N2O from the stratospheric backflux to the troposphere may be heavier than tropospheric N2O. However, more determinations of the isotopic ratios of N2O in the atmosphere are needed to identify and quantify N2O sources.
There are several options to reduce emissions of N2O. If current trends continue, global emissions may increase by 4-13 million ton N2O-N per year during the next century. Some of this increase can be avoided. Most importantly, emissions related to industry and stationary combustion can be reduced. In agriculture a reduction of N2O emission per hectare seems feasible in several world regions. A fast increase in food production and increasing importance of animal production is expected in the coming decades, caused by fast population increase and economic growth in large parts of the world. In addition, an increase in the number of cars equipped with catalytic converters is envisaged. These developments may lead to significant increases in N2O emissions. Hence, although there are regions where N2O emissions can be reduced considerably, it is questionable whether these reductions can avoid a further increase in worldwide emissions.
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