ure Oxides of sulfur emitted from fossil fuel power plants and other industries, and oxides of nitrogen from automobile exhaust, are converted in the moist atmosphere into acids that become dispersed over wide areas. Unlike unpolluted rain (average pH 5.6), the ph of acid rain has been measured at 4.2 and even lower. Plants, such as these trees photographed in the Black Forest of germany, may be damaged when the resulting increase in soil acidity causes certain minerals, particularly calcium ions, to leach out of the soil. (Hans Reinhard/Bruce Coleman)Figure 13-10