Decreasing atmospheric nitrous oxide concentrations

Nitrous oxide in the atmosphere is currently increasing at a rate of around 0.2-0.3% per year for reasons which are still poorly understood. The increase in 1992 (0.16%) was similar in magnitude to annual growth rates between 1977 and 1985, but lower than those observed more recently between 1985 and mid-1991 (0.3%/year). Nitrous oxide is generated by processes of nitrification and denitrification of the soil and from fossil fuel combustion.

Soils appear to be an important but highly variable source of nitrous oxide. Major factors affecting such emissions include soil humidity and temperature, and the types of fertilizers used. Fluxes can vary from slightly negative (small sinks), usually during dry conditions, to large emissions (sources) of up to 60 æg/sq m/hr during wet periods following a prolonged dry spell. The latter may be due to accumulation of nitrates in the soil during the fallow period. Matching of fertilizers with soil moisture conditions to inhibit nitrification can help reduce emissions by up to 50%. Ammonium nitrate, for example, produces significantly less emissions than urea under dry and warm summer conditions, but higher emissions in cool wet springs. Ammonium sulphates appear to generate lowest emissions.

Confusingly nitrogen oxides (NOx) released into the atmosphere at low levels creates ozone and contributes to global warming, but when released into the stratosphere depletes ozone and thus contributes to global warming. Nitrous oxide is 250 times carbon dioxide's (CO) capacity to trap heat in the atmosphere. Its pre-industrial concentration in the atmosphere was 280 parts per 1,000 million and the 1992 level is 310 parts per 1,000 million. Emissions of nitrous oxide in Canada in 1990 were estimated at 92 kilotonnes. Fossil fuel combustion accounted for about 52% of the total, followed by industrial processes (36%), and fertilizer use (12%).

UK nitrous oxide emissions fell by 15 per cent between 1990 and 1995 (from 215 to 183 thousand tonnes), but increased by 3 per cent between 1995 and 1996 (to 189 thousand tonnes), mainly due to increased industrial production of adipic acid. Non-combustion processes accounted for 37 per cent of total nitrous oxide emissions.Studies in Central America show high nitrous oxide emissions occur during the first decade following conversion of forests to pastureland, but declining emissions to levels below those from the original forest ecosystem thereafter. Draining or drying out of northern peatlands may also enhance nitrous oxide releases from nutrient rich areas, although such increases are unlikely to cause a major increase in total global emissions.

Increased use of catalytic converters in cars to control smog is a secondary but growing source of nitrous oxide. Related emissions, currently estimated at 200 kt/year globally, could more than double if all cars were so equipped (Laboratory studies show that catalyst-equipped cars can reach 20 to 30 times the emission level than for other cars).

Coal-fired power stations account for less than 7% of national nitrous oxide emissions of several European countries (equivalent to less than 1% of the CO2 emitted by these power plants), hence do not appear to be the significant source of nitrous dioxide once believed. These emissions vary with fuel type and combustion system, with hard coal producing more than lignite, and fluidized bed combustion systems much more than conventional systems.