Sources of Nitrous Oxide

We know the sources of nitrous oxide. Natural sources include oceans, tropical soils, wet forests, dry savannas, and extra-tropical forests. Total emissions are about 4-10 x 10¹² grams, or 4-10 Tg. Anthropogenic sources include cultivated soils (including use of nitrogen fertilizers), biomass burning and other combustion processes, and acid production processes. The largest known process for destruction of nitrous oxide is stratospheric photolysis (breakdown by solar energy, principally ultraviolet radiation). From these estimates (Figure 2) we can see that in spite of large uncertainty, human contributions to the nitrous oxide loading of the atmosphere are comparable with natural sources and are likely the cause of the 3-4.5 Tg per year increase in the amount of nitrous oxide in the atmosphere. Because of their long lifetime (stability) in the troposphere, natural removal processes are incapable stemming this increase.

Agricultural use of nitrogen fertilizer is a significant anthropogenic source of nitrous oxide. As shown in Figure 3, China is a big user, followed (according to these data) by the former Soviet Union, the United States, and India. During talks with state officials on my recent trip to Russia, I learned that the present economic difficulties in that country have significantly reduced their availability of fertilizers and pesticides, so this chart may not reflect current conditions.

In a later summary information we will be coming back to the physics and chemistry of the stratosphere and will revisit the problems caused by nitrous oxide in the stratosphere.

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