20 



MISCELLANEOUS PUBLICATION NO. 1065, U.S. DEPARTMENT OF AGRICULTURE 



substances including sulfur oxides, fluorine, and 

 hydrocarbons. 



Space heating, refuse burning, and forest fires 

 contribute to the mucky atmosphere by emitting 

 10 to 12 million tons of stuff largely appearing as 

 smoke. 



The concentration, transportation, and disper- 

 sion of air pollutants from the various sources are 

 determined by topography and meteorological 

 conditions. Wind is the transporting and dispers- 

 ing agent. In the typical American city, the air 

 is three times as polluted when the wind speeds 

 are 6 miles per hour or less than on days when the 

 winds are 18 miles per hour or more. Concentra- 

 tion of pollutant decreases with distance from the 

 polluting source except as modified by topography 

 and temperature inversion layers. Hills or moun- 

 tains surrounding an area can provide a pocket 

 for concentrating air pollutants. Such a situation 

 exists in the Los Ano-eles Basin. Although the 

 western part of this basin is not blocked topo- 

 graphically, the prevailing winds are from the 

 west. 



Under normal atmospheric conditions, air tem- 

 perature decreases with altitude. Expansion of the 

 air under decreasing pressure with altitude in- 

 volves adiabatic cooling. In the presence of an 

 inversion layer, temperature increases with alti- 

 tude. The situation is well illustrated by a diagram 

 presented by Wanta (J SI). Cool air that displaces 

 warm air, such as air that blows from a cool ocean 

 onto warm land, can cause a pronounced inversion. 

 Similarly, warm air may flow over a cold surface 

 layer, especially one trapped in a valley, and cause 

 an inversion. 



Inversions effectively suppress vertical air 

 movement and cause an atmospheric stagnation 

 in which smoke and other volatile contaminants 

 cannot rise from the earth's surface. Persistent 

 inversions occur in Los Angeles, New York, and 

 other industrial and metropolitan areas. Under 

 such conditions, potentially lethal layers of toxi- 

 cants including sulfur dioxide, carbon monoxide, 

 ozone, and nitrogen oxides can become stati- 

 cally entrapped for days. Forest trees, horticul- 

 tural plants, field crops, and domestic animals 

 living in such an area can be sorely afflicted. In 

 fact, vegetation damage from air contamination 

 has been recorded in over half of the States of 



the Nation and no doubt occurs to some degree in 

 all States. 



Different air contaminants have specific effects 

 on agricultural crops and forest trees. 



Sulfur Dioxide 



One of the really classic examples of the drastic 

 effects of sulfur dioxide (S0 2 ) contamination still 

 prevails at Copper Basin in the Ducktown- 

 Copperhill, Tenn., area. Over a century ago, this 

 basin in the southern Appalachian Mountains was 

 covered with hardwoods and some conifers. Min- 

 ing was started in the basin about 1850. Open- 

 hearth wood-burning furnaces were later installed 

 directly at the mine locations, with their greatest 

 activity being in the years 1890-95. Soon after 

 1900, the sulfur dioxide fumes combined with 

 timber overcutting to fire the furnaces and wild 

 fires had killed most of the vegetation in the basin. 

 Even today, some 7,000 acres to the leeward of the 

 smelters are nude of vegetation. Erosion has been 

 intense. Most all of the soil and subsoil are gone. 

 Another 17,000 acres surrounding the nude area 

 are devoid of trees and produce only broomsedge 

 and some plantings of kudzu. White pines 30 miles 

 from the smelters were injured. One cannot visit 

 Ducktown without standing in awe at the tre- 

 mendous damages that airborne sulfur dioxide 

 can inflict. Most smelters have now installed ab- 

 sorptive equipment to prevent pouring S0 2 into 

 the atmosphere. Smelters in the Ducktown- 

 Copperhill area for many years have converted 

 the S0 2 into a profitable byproduct, sulfuric acid, 

 but the scars remain on land still highly acid. 



Sulfur dioxide still threatens the vegetation 

 around cities. Philadelphia daily releases about 

 850 tons of S0 2 into the atmosphere. New York 

 City air most of the time contains S0 2 near the 

 threshold level of injury to sensitive plants. Injury 

 to vegetation depends on concentration in the at- 

 mosphere, length of exposure, and tolerance of the 

 species. Alfalfa, cotton, wheat, and conifers are 

 relatively sensitive to S0 2 , whereas potatoes, corn, 

 and maples are more tolerant. If the SO L » content of 

 air is 0.3 to 0.5 part per million for several days, 

 sensitive vegetation will be injured. The threshold 

 value for alfalfa is one hour's exposure to 1.0 

 p. p.m. of S0 2 . 



Since industry and metropolitan areas in the 

 United States are pouring 30 million tons of sulfur 

 oxides into the atmosphere each year, appreciable 



