Acid Rain

Acid Rain

What is acid rain? Acid rain is not a recent phenomenon. In the 17th century, scientists noted the ill effects that industry and acidic pollution was having on vegetation and people. However, the term acid rain was not coined until two centuries later when Angus Smith published a book called 'Acid Rain' in 1872 ( Zumdahl 173).

Acidic pollutants can be deposited from the atmosphere to the Earth's surface in wet and dry forms. The common term to describe this process is acid deposition. The term acid precipitation is used to specifically describe wet forms of acid pollution that can be found in rain, sleet, snow, fog, and cloud vapor. An acid can be defined as any substance that when dissolved in water dissociates to yield corrosive hydrogen ions. The acidity of a substances dissolved in water is commonly measured in terms of pH. According to this measurement scale solutions with pHs less than 7 are described as being acidic, while a pH greater than 7.0 is considered alkaline. Precipitation normally has a pH between 5.0 to 5.6 because of natural atmospheric reactions involving carbon dioxide. Precipitation is considered to be acidic when its pH falls below 5.6. Some sites in eastern North America have precipitation with pHs as low as 2.3 or about 1000 times more acidic than natural ( Zumdahl 171).

One of the main causes of acid rain is sulphur dioxide. Natural sources, which emit this gas, are volcanoes, sea spray, rotting vegetation and plankton. However, the burning of fossil fuels, such as coal and oil, are largely to be blamed for approximately half of the emissions of this gas in the world. When sulphur dioxide reaches the atmosphere, it oxidizes to first form a sulphate ion. It then becomes sulphuric acid as it joins with hydrogen atoms in the air and falls back down to earth. Oxidation occurs mostly in clouds and especially in heavily polluted air where other compounds such as, ammonia and ozone help to catalyze the reaction, converting more sulphur dioxide to sulphuric acid.

One of the direct effects of acid rain is on lakes and its aquatic ecosystems. There are several routes through which acidic chemicals can enter the lakes. Some chemical substances exist as dry particles in the air while others enter the lakes as wet particles such as rain, snow, sleet, hail, dew or fog. In addition, lakes can almost be thought of as the "sinks" of the earth, where rain that falls on land is drained through the sewage systems eventually makes their way into the lakes. Acid rain that falls onto the earth washes off the nutrients out of the soil and carries toxic metals that have been released from the soil into the lakes.

Another harmful way in which acids can enter the lakes is spring acid shock. When snow melts in spring rapidly due to a sudden temperature change, the acids and chemicals in the snow are released into the soils. The melted snow then runs off to streams and rivers, and gradually makes their way into the lakes. The introduction of these acids and chemicals into the lakes causes a sudden drastic change in the pH of the lakes. The aquatic ecosystem has no time to adjust to the sudden change. In addition, springtime is an especially vulnerable time for many aquatic species since this is the time for reproduction for amphibians, fish and insects. Many of these species lay their eggs in the water to hatch. The sudden pH change is dangerous because the acids can cause serious deformities in their young or even annihilate the whole species since the young of many of such species spend a significant part of their life cycle in the water.

Another serious impact of acid precipitation is on forests and soils. Great damage is done when sulphuric acid falls onto the earth as rain. Nutrients present in the soils are washed away. Aluminum, which is also present in the soil, is freed and the roots of trees can absorb this toxic element. Thus, the trees are starved to death as they are deprived of their vital nutrients such as calcium and magnesium. Not the entire sulphur dioxide is converted to sulphuric acid. In fact, a substantial amount can float into the atmosphere, move over to another area and return to the soils unconverted. As this gas returns back to earth, it clogs up the stomata in the leaves, thus hindering photosynthesis. Research has been made where red spruce seedlings were sprayed with different combinations of sulphuric and nitric acid of pH ranging from 2.5 to 4.5. The needles of these seedlings were observed to develop brown lesions. Eventually, the needles fall off. It was also found that new needles grew more slowly at higher concentrations of acid used. Because the rate at which the needles were falling was greater than the rate at which they were replenished, photosynthesis was greatly affected, The actual way in which