In addition to the greenhouse gases, important reactive gas species are produced by the cycles of

sulfur, nitrogen and chlorine. These play key roles in acid precipitation and in ozone destruction. Sources of these species are as follows:

• Nitrogen species. The reactive species of nitrogen are nitric oxide (NO) and nitrogen dioxide (NO2). NOx refers to these and other odd nitrogen species with oxygen. Their primary significance is as a catalyst for tropospheric ozone formation. Fossil fuel combustion (approximately 40 percent for transportation and 60 percent for other energy uses) is the primary source of NOx (mainly NO) accounting for ~25 × 109kg N/year. Biomass burning and lightning activity are other important sources. NOx emissions increased by some 200 percent between 1940 and 1980. The total source of NOx is about 40 × 109kg N/year. About 25 percent of this enters the stratosphere, where it undergoes photochemical dissociation. It is also removed as nitric acid (HNO3) in snowfall. Odd nitrogen is also released as NHx by ammonia oxidation in fertilizers and by domestic animals (6-10 × 109kg N/year).