r and nitrogen combine with atmospheric moisture to yield sulfuric and nitric acids, which may then be carried long distances from a source before they are deposited by rain, is called acid rain This pollution may also take the form of snow, fog, or a dry form of precipitation. Acid rain is currently a subject of great controversy because of widespread environmental damage for which it has been blamed, including eroding structures, injuring crops and forests, and threatening or depleting life in freshwater lakes.
One of the main causes of acid rain is sulfur 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 sulfur dioxide reaches the atmosphere, it oxidizes to first form a sulfate ion. It then becomes sulfuric acid as it joins with hydrogen atoms in the air and falls back down to earth. Oxidation occurs the most in clouds and especially in heavily polluted air where other compounds such as ammonia and ozone help to catalyze the reaction, converting more sulfur dioxide to sulfuric acid. However, not all of the sulfur dioxide is converted to sulfuric acid. In fact, a substantial amount can float up into the atmosphere, move over to another area and return to earth unconverted. The following are the stoichiometric equations for the formation of sulfuric acid: S (in coal) + O2? SO2
2 SO2 + O2? 2 SO3
SO3 + H2O? H2SO4
Nitric oxide and nitric dioxide are also components of acid rain. Its sources are mainly from power stations and exhaust fumes. Like sulfur dioxide, these nitrogen oxides rise into the atmosphere and are oxidized in clouds to form nitric acid. These reactions are also catalyzed in heavily polluted clouds where iron, manganese, ammonia and hydrogen peroxide are present.
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 make 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 make 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 – hence the term “spring acid shock”. 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.
Subsequently, sulfuric acid in water can affect the fish in the lakes in two ways: directly and indirectly. Sulfuric acid (H2SO4) directly interferes with the fish’s ability to take in oxygen, salt and nutrients needed to stay alive. For freshwater fish, maintaining osmoregulation is key in their survival. Osmoregulation is the process of maintaining the delicate balance of salts and minerals in their tissues. Acid molecules in the water cause mucus to form in their gills and this prevents the fish to absorb oxygen as well. If the buildup of mucus increases, the fish would suffocate. In addition, a low pH will throw off the balance of salts in the fish tissue. Salts levels such as the calcium (Ca+2) levels of some fish cannot be maintained due to pH change. This results in poor reproduction – their eggs produced would be damaged; they are either too brittle or too weak. Decreased Ca+2 levels also result in weak spines and deformities. For example, crayfish need Ca+2 to maintain a healthy exoskeleton; low Ca+2 levels would mean a weak exoskeleton. Another type of salt N+ also influences the well being of the fish. As nitrogen-containing fertilizers are washed off into the lakes, the nitrogen stimulates the growth of algae, which logically would mean an increase in oxygen production, thus benefiting the fish. However, because of increased deaths in the fish population due to acid rain, the decomposition process uses up a lot of the oxygen, which leaves less for the surviving fish to take in.
Indirectly, sulfuric acid releases heavy metals present in soils to be dissociated and released. For example, aluminum (Al+2) is harmless as part of a compound, but because acid rain causes Al+2 to be released into the soils and gradually into the lakes, it becomes lethal to the health of the fish in the lakes. Al+2 burns the gills of the fish and accumulates in their organs, causing much damage. So, although many fish may be able to tolerate a pH of approximately 5.9, this acid level is high enough to release Al+2 from the soils to kill the fish. This effect is further augmented by spring acid shock. The effect of acid rain can be dynamically illustrated in a study done on Lake 223, which started in 1976. Scientists monitored the pH and aquatic ecosystem of Lake 223. They observed that as the pH of the lake decrease over the years, a number of crustaceans died out because of problems in reproduction due to the acidity of the lake caused by acid precipitation. At a pH of 5.6, algae growth in the lake was hindered and some types of small died out. Eventually, it was followed by larger fish dying out with the same problem in reproduction; there were more adult fish in the lake than there were young fish. Finally, in 1983, the lake reached a pH of 5 and the surviving fish in the lake were thin and deformed and unable to reproduce. This case study obviously illustrates the significant effect of acid rain on lakes and its aquatic ecosystem.
One of the most serious impacts of acid precipitation is on forests and soils. Great damage is done when sulfuric acid falls onto the earth as rain. Nutrients present in the soils are washed away. Aluminum also present in the soil is freed and this toxic element can be absorbed by the roots of trees. Thus, the trees are starved to death as they are deprived of their vital nutrients such as calcium and magnesium. Not all of the sulfur dioxide is converted to sulfuric 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 sulfuric 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 these needles were killed is still not yet known. However, studies have shown that calcium and magnesium nutrients are washed away from their binding sites when sulfuric acid enters the system. They are replaced by useless hydrogen atoms and this inhibits photosynthesis.
In addition, severe frosts may also further aggravate this situation. With sulfur dioxide, ammonia and ozone present in the air, the frost-hardiness of trees are reduced. Ammonia oxidizes with sulfur dioxide to form ammonium sulfate. This product forms on the surface of the trees. When ammonium sulfate reaches the soils, it reacts to form both sulfuric and nitric acid. Such conditions also stimulate the growth of fungi and pests like the ambrosia beetle. When trees are under such stress, they release chemicals such as terpenes which attract the ambrosia beetle.
Nitrogen oxide and nitric oxide, also components of acid rain, can force trees to grow even though they do not have the necessary nutrients. As well, the trees are sometimes forced to grow well into late autumn when it is actually time for them to prepare for severe frosts in the winter.
Acid rain also damages materials such as fabrics. For example, flags that are put up are being “eaten away” by the acidic chemicals in the precipitation. Books and age-old art that are centuries old are also being affected; the ventilation systems of the libraries and museums that hold them do not prevent the acidic particles from entering the buildings and so, they get in and circulate within the building, affecting and deteriorating the materials.
Among one of the serious side effects of acid pollution on humans is respiratory problems. The SO2 and NO2 emissions give rise to respiratory problems such as asthma, dry coughs, headaches, eye, nose and throat irritations. An indirect effect of acid precipitation on humans is that the toxic metals dissolved in the water are absorbed in fruits, vegetables and in the tissues of animals. Although these toxic metals do not directly affect the animals, they have serious effects on humans when they are being consumed. For example, mercury that accumulates in the organs and tissues of the animals has been linked with brain damage in children as well as nerve disorders, brain damage and death. Similarly, another metal, Aluminum, present in the organs of the animals, has been associated with kidney problems and recently, was suspected to be related to Alzheimer’s disease.