is logarithmic, a change in pH from 6 to 5 represents a ten- fold increase in 

 acidity. 



Nitrogen and sulfur oxides enter the atmosphere from various sources. These 

 oxides are converted to strong acids in the atmosphere and transported to the 

 ground by rain and snow. Prior to the burning of massive amounts of fossil 

 fuels, the pH of precipitation in the United States was consistently above 

 5.0. The first recorded measurement of the pH of precipitation in the United 

 States was made in Maine in 1939; it was 5.9 (Likens et al. 1979). 



Over the last few decades, the pH of precipitation over the Eastern United 

 States has slowly decreased to a point between 4 and 4.5. This represents at 

 least a ten-fold increase in acidity, an increase that is correlated with 

 increased concentrations of sulfate and nitrate oxides emitted into the 

 atmosphere over the past 20 years (Likens et al. 1979). Changes in acidity 

 already have caused environmental degradation, particularly in sensitive 

 freshwater systems (Davis et al. 1979; Johnson 1979; Hendrey et al. 1976; 

 Schofield 1976; and Gorham 1975). Since oxides may remain in the atmosphere 

 for a long period of time, local pollution may be transported many miles 

 before it is washed from the atmosphere by rain. 



The vulnerability of natural ecosystems to atmospheric pollutants, 

 predominantly acid rains, is influenced by climatic, geographical, 

 topographical, morphometrical , biotic, and human inputs (Gorham and McFee 

 1978). Chief among the climatic factors are the amount and type of 

 precipitation, wind speed and direction, and seasonality. Rain and to a 

 lesser extent snow introduce acids and metals into the ground. Since Maine 

 receives about 40 inches (102 cm) of precipitation per year, ample opportunity 

 exists for atmospheric deposition. Although rain introduces acids relatively 

 quickly, snow cover tends to store acids, so that spring thaws can become 

 sudden pulsations of pollution into aquatic systems (Gorham and McFee 1978; 

 and Hornbeck et al. 1975). 



Seasons and prevailing winds determine types of precipitation. Acidity of 

 precipitation at Hubbard Brook, New Hampshire, was found to be maximal in the 

 summer months, intermediate in the spring and fall and minimal in winter 

 (Hornbeck et al. 1975). The high summer levels are caused by the prevailing 

 winds that transport atmospheric pollution from the industrialized 

 northeastern and central States into northern New England. In cooler months, 

 northwestern prevailing winds bring air from less industrialized regions, 

 probably reducing the relative influence of acidity. 



In many cases, geological factors influence the buffering capacity of natural 

 systems. Areas with granite and metamorphic silicaceous bedrock and thin 

 soils are most sensitive to acid rains because of their poor buffering 

 capacities. Soil texture, thickness, and structure can greatly influence the 

 effects of acids and heavy metals. The soils in coastal Maine are generally 

 thin and of granitic and metamorphic origin. 



Biotic factors are important in altering acidity and in the uptake of heavy 

 metals and nutrients received from atmospheric deposition. Canopy of hardwood 

 forests can lower the acidity of rain effectively (Eaton et al. 1973). 

 However, the acidity of rain actually may increase as it passes through 

 spruce-fir forests. 



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