Nitrous Oxide: No Laughing Matter

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Science  02 Oct 2009:
Vol. 326, Issue 5949, pp. 56-57
DOI: 10.1126/science.1179571

Despite its long-recognized importance, nitrous oxide (N2O, also commonly referred to as laughing gas) sometimes seems like the forgotten atmospheric gas. Concerns about the stratospheric ozone layer have largely focused on reactions of ozone with chlorine and bromine atoms released from the atmospheric dissociation of chlorofluorocarbons and other anthropogenic halocarbons. Meanwhile, concerns about human-induced effects on Earth's climate have concentrated on carbon dioxide (CO2) and methane (CH4) emissions from fossil fuels and other sources. However, future changes in climate and in the distribution of stratospheric ozone depend on the emissions and changing atmospheric concentration of N2O. The report by Ravishankara et al. on page 123 of this issue (1) not only adds to the scientific understanding of this important gas, but is also a strong reminder that nitrous oxide deserves much more attention and consideration for policy action to control future human-related emissions.

Since 1800, the atmospheric mixing ratio (that is, the concentration of N2O relative to the concentration of air) has increased by almost 20%, from about 270 ppb (parts per billion molecules of air) to more than 322 ppb (2, 3). In recent decades, its atmospheric concentration has been increasing at roughly 0.25% (range of 0.2 to 0.3%) per year, and this trend is set to continue. It is mainly removed from the atmosphere through photolysis and reaction with excited oxygen atoms in the middle to upper stratosphere, resulting in a long atmospheric lifetime—roughly 120 years for an e-folding (i.e., removal of 63.2% of the initial emission).

The total emissions of nitrous oxide are about 17.7 teragrams of nitrogen (Tg N) annually, but there are large uncertainty ranges on each of the individual sources. About 70% of its atmospheric emission is natural, mostly from bacterial breakdown of nitrogen in soils and in the oceans. Globally, soils in areas of natural vegetation, especially in the tropics, account for N2O emissions of about 6.6 Tg N annually (2). Oceans account for about another 3.8 Tg N per year (2).

Human activities are responsible for the remaining 30% of N2O emissions, or about 6.7 Tg N per year (2). The largest humanrelated source comes from agricultural practices and activities, including the use of synthetic and organic fertilizers, production of nitrogen-fixing crops, and application of livestock manure to croplands and pasture. These result in increased nitrogen in soils and waterways, causing N2O emissions of about 4.5 Tg N per year (2). Nitrous oxide can also be produced during fossil fuel combustion, but the amount varies with fuel type and technology (for example, catalytic converters can produce N2O). Fossil fuel combustion and industrial processes are responsible for N2O emissions of around 0.7 Tg N per year (2). Other important sources include human sewage and burning of biomass and biofuels.

The vast majority of the reactive nitrogen oxides in the stratosphere result from the dissociation of nitrous oxide (through its reaction with electronically excited oxygen atoms). As a result of the high reactivity of nitrogen oxides with ozone, N2O levels in the preindustrial atmosphere (before 1800) were sufficient to account for the majority of the natural destruction of ozone in the stratosphere. With increasing atmospheric concentration of nitrous oxide, the concentrations of nitrogen oxides in the stratosphere are also rising. Ravishankara et al. point out that nitrogen oxides—and, as a result, N2O—destroy more ozone in the current stratosphere than does any other reactive chemical family.

The execution of the Montreal Protocol effectively controls the emission of some major ozone-depletion gases, particularly chlorine- and bromine-containing gases. If one assumes that only halocarbons from human activities affect ozone and that there is full global compliance with the Montreal Protocol, then the ozone layer outside the polar regions is largely expected to recover from existing human effects on the stratosphere by the middle of the 21st century (3). The Antarctic ozone “hole” is expected to take longer, until roughly 2065 (4). However, this assumes that ozone is not affected by other factors, including other human-related emissions. This assumption is false.

The increasing concentration of CO2 in Earth's atmosphere not only warms the troposphere but also cools the stratosphere, with a tendency to increase the amount of stratospheric ozone. As a result, there is the possibility of a “super”-recovery, where the total amount of atmospheric ozone exceeds that found before 1980, before the major ozone losses due to halocarbons occurred. Furthermore, changes in climate are changing the strength of circulation patterns in the stratosphere (5), thus affecting the distribution of ozone. Potentially even more important is the continuing increase in atmospheric concentrations of nitrous oxide and methane. Methane affects the amount of hydrogen oxides in both the troposphere and stratosphere, which in turn affects the chemistry of ozone (6). Whereas increasing nitrous oxide will tend to destroy more ozone, increasing methane would tend to produce ozone, but each has its largest effects at different locations in the stratosphere (7, 8).

The forgotten gas.

Nitrous oxide has important effects both on the climate system and on stratospheric ozone.

As a result of these effects, the distribution of ozone in the stratosphere is unlikely to resemble its pre-1980 levels at any time this century (or perhaps for several centuries to come). The total ozone column could increase but not really recover as originally expected, and it could even decrease over the course of this century, depending on what happens with these various factors. In any case, the distribution of ozone with altitude and latitude will likely remain different from the pre-1980 levels.

In addition to its effects on ozone, nitrous oxide is also the third most important gas directly affecting climate as a result of human activities. Although the increases in concentrations of CO2 and CH4 have been larger, N2O is also a greenhouse gas, and its changing concentrations are important to climate change. The concerns about its effects on ozone and on climate have implications on future policies directed at nitrous oxide. However, even if the combustion-related sources prove to be relatively easy to control, the agricultural sources may present a large challenge. Greater demand for food may affect the ability to reduce emissions from livestock and the use of fertilizers, although new approaches for increased agricultural efficiency and potential mitigation pathways continue to be developed (9).


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