ACID RAIN Natural rain is slightly acidic, with a pH of 5.6. Acid rain is rain with a pH lessthan 5.6. In some parts of the world, pH values have been reported as low as 2.6,which is about as acidic as lemon juice or vinegar. It forms when airborne pollutants,sulfur dioxide and nitrogen oxides, combine with water vapor in the atmosphere toproduce acids. Although natural sources such as volcanoes and forest fires release SO2, theprimary sources of acid rain today are from the burning of fossil fuels in automobilesand coal in industrial plants. When coal and oil are burned, the sulfur impurities combine with oxygen in the air to produce SO2 and SO3. The reaction of SO3 with water forms sulfuric acid. S + O2 SO2 SO2 + O2 SO3 SO3 + H2O H2SO4 Nitrogen oxide forms at high temperatures in the engines of automobiles as air containing nitrogen and oxygen gases is burned. As NO is emitted into air, it combines with more oxygen toform nitrogen dioxide, then dissolves in water to form nitric acid. N2 + O2 NO NO + O2 NO2 NO2 + H2O HNO3 The acids in acid rain have detrimental effects on marble and limestonestructures, lakes, and forests. Throughout the world, monuments made of marble aredeteriorating as acid rain dissolves the marble. Acid rain is changing the pH of manylakes and streams. When the pH of a lake falls below 4.5, most fish and plant lifecannot survive. Trees and forests are susceptible to acid rain too. Acid rain breaksdown the protective waxy coating on leaves and interferes with photosynthesis. In an effort to decrease the formation of acid rain, legislation has required areduction in SO2 emissions. Coal-burning plants have installed equipment calledscrubbers that absorb SO2 before it is emitted. In a smokestack, scrubbing removes95% of the SO2 .