Nitrous Oxide

Nitrous oxide (N2O) is a trace gas in Earth’s atmosphere with a mixing ratio in 2005 of 319±0.12 parts per billion (ppb) by volume. Atmospheric N2O is steadily increasing because of human activities. Nitrous oxide absorbs terrestrial radiation (i.e., radiation emitted by the Earth) and consequently is one of the important anthropogenic greenhouse gases (GHGs) targeted for control within the Kyoto Protocol. Nitrous oxide also absorbs solar radiation, which can split the molecule, releasing reactive species that contribute toward stratospheric ozone depletion.

Biological Activities and Human Sources

Various natural and anthropogenic sources add N2O to the atmosphere. The main natural sources are related to biological activity in soils and the upper ocean. The largest anthropogenic source is from the use of nitrogenous fertilizer in the agricultural sector; others include combustion of fossil fuel, biomass, and biofuel, and industrial processes. Nitrous oxide emissions related to biofuel production are an example of reducing emissions of one GHG, carbon dioxide (CO2), at the expense of increasing emissions of another.

Nitrous oxide is relatively inert in the troposphere (the atmosphere’s lowest at 6–9 mi., or 10–15 km). Higher up, in the stratosphere, energetic ultraviolet radiation starts to break the molecule apart. This photochemical destruction in the upper atmosphere removes about 0.9 percent of all N2O every year, determining the average atmospheric residence time of an N2O molecule, which is currently around 114 years. This is long compared to timescales of most atmospheric transport and mixing processes, which typically range from hours to a few years. Nitrous oxide is consequently referred to as a “well-mixed” and “long-lived” gas.

Because nitrous oxide is well mixed, atmospheric measurements at sites sufficiently remote from sources/sinks are representative of the whole atmosphere. For example, data collated by the World Data Centre for Greenhouse Gases shows that N2O levels at Mace Head (Ireland) rose from c.315 ppb in 2000 to c.323 ppb in 2010. Very similar values were recorded at Cape Grim (Australia); whereas slightly lower (by c.1 ppb) absolute values, but with the same increase, were measured at the South Pole. This recent growth in atmospheric N2O of nearly 1 ppb/year is a result of its sources exceeding its sinks.

For times before the era of direct measurements, atmospheric levels of N2O can be estimated using ice cores. Air bubbles found within ice cores represent samples of past atmospheric composition, trapped shortly after snowfall. Ice core data indicate that the preindustrial atmosphere contained 270±7 ppb N2O. Over the preceding 10,000 years, N2O ranged between 250 and 275 ppb. Ice core data going back 100,000 years indicate a range of 180–290 ppb.

The Intergovernmental Panel on Climate Change (IPCC) calculates that the increase in N2O from preindustrial times up to 2005 (+49 ppb) resulted in a radiative (climate) forcing of +0.16±0.02 W/m2. Radiative forcing is a measure of how much the energy balance of the Earth–atmosphere system is changed, for example by the change in concentration of an atmospheric constituent that interacts with radiation. To put the radiative forcing from N2O in perspective, corresponding values for CO2 and methane are +1.66 and +0.48 W/m2, respectively.

Global Warming Potential

Radiative forcing provides a good measure of how changes in individual gases have influenced climate change up to the present day. However, to estimate the contribution of a particular gas’ present-day emissions toward future climate change, the global warming potential (GWP) is a more useful quantity.

The GWP of N2O is the time-integrated radiative forcing following a 1 kg pulse emission of N2O, relative to the same quantity following a 1 kg pulse emission of CO2. The GWP time horizon must also be specified; typically, it is 100 years, but the IPCC also presents GWPs for 20- and 500-year time horizons. The choice of time horizon relates to the context in which the GWP is used. The 100-year GWP of N2O is 298, which means that the warming influence from emitted N2O, integrated over the next 100 years, is 298 times larger than that of the same mass of emitted CO2. The GWP accounts for different gases absorbing radiation with different efficiencies and having different atmospheric lifetimes. The GWP value is essential for comparing the relative merits of emissions-control strategies for different gases, such as within emissions trading frameworks.

Akin to the GWP, the importance of a species as a stratospheric ozone-depleting substance (ODS) is expressed by its ozone depletion potential (ODP). An ODP is the ratio of the amount of ozone destroyed by the emission of a unit mass of a substance at the Earth’s surface, relative to the amount destroyed by the emission of a unit mass of chlorofluorocarbon-11 (CFC-11, CFCl3). Ozone changes

are for a steady state over the entire lifetime of the emitted substance. The ODP of N2O is 0.017 and has only recently been calculated (in 2009). Successful regulation of CFCs following the Montreal Protocol now means that N2O is the dominant emitted ODS and is likely to remain so throughout the 21st century. Despite its importance, N2O remains unregulated by the Montreal Protocol.