Of the 830,000 metric tons of ammonia (NH3) emitted in the United States in 1980 by anthropogenic sources, emissions from livestock-waste management dominate at 540,000 metric tons/yr (Placet and Streets, 1987); animal excrement is estimated to be the major terrestrial source of ammonia worldwide. Other sources (in metric tons per year) are fertilizer production (110,000), agricultural application of anhydrous NH3 (50,000), NH3 synthesis (40,000), petroleum refineries (40,000), motor vehicles (30,000), stationary fossil fuel combustion (20,000), and coke manufacture (10,000). In polluted urban areas such as Los Angeles and Denver, where nitric acid concentrations are high (greater than 1 ppb), NH3 emissions from feedlots can influence urban airborne particle composition and concentration. Studies made in 1978 in the Denver "brown cloud" showed that about 20% of the fine particle mass (i.e., particles less than 2.5 µm in diameter) was composed of ammonium nitrate (NH4NO3) (Countess et al., 1980); these particles accounted for about 17% of the visibility reduction (Wolff et al., 1981).
Söderlund and Svensson (1976) estimated that wild and domestic animals and humans produce globally about 27–50 million metric tons/yr of NH3, but only a small faction of those emissions (2–7 million metric tons/yr) is attributable to wild animal wastes. Therefore, NH3 emissions in national parks and wilderness areas should be very low.
It is possible, but probably unlikely, that visibility in the vicinity of national parks and wilderness areas could be impaired by NH3 emissions from feedlots and other nearby anthropogenic sources. In such cases, control of the emissions might be desired but would be difficult because the openness of most feedlots prevents capture of NH3 by the usual air treatment procedures employed in industrial operations (Bond, 1972). Chemical additives to the animal waste can lower NH 3 emissions; for example, added natural zeolites can lower NH3 emissions by 50% (Miner, 1984).