Acid RainWhat Causes Acid Rain? One of the main causes of acid rain is sulfur dioxide. Natural sources, which emit this gas, are Volcanoes, sea spray, rottingvegetation and plankton. However, the burning of fossil fuels, such as Coal andoil, are largely to be blamed for approximately half of the emissions of thisgas in the world. When sulfur dioxide reaches the atmosphere, it oxidizes tofirst form a sulfate ion.
It then Becomes sulfuric acid as it joins withhydrogen atoms in the air and falls back down to earth. Oxidation occurs themost in clouds and especially in heavily polluted air where other compounds suchas ammonia and ozone help to catalyze the reaction, changing more sulfur dioxideto sulfuric acid. However, not all of the sulphur dioxide is changed to sulfuricacid. In fact, a substantial amount can float up into the atmosphere, move overto another area and return to earth unconverted. In the following pages I willshow the effects of acid rain on: ? Effect on Trees and Soils ? Effect onLakes and Aquatic Systems ? Effect on Materials ? Effect on Atmosphere ?Effect on Architecture ? Effect on Humans Effect on Trees and Soils One of themost serious impacts of acid precipitation is on forests and soils.
Great damageis done when sulfuric acid falls onto the earth as rain. Nutrients present inthe soils are washed away. Aluminium also present in the soil is freed and theroots of trees can absorb this toxic element. Thus, the trees are starved todeath as they are deprived of their vital nutrients such as calcium andmagnesium. Not all of the sulphur dioxide is converted to sulfuric acid. Infact, a substantial amount can float into the atmosphere, move over to anotherarea and return to the soils unconverted.
As this gas returns back to earth, itclogs up the stomata in the leaves, thus hindering photosynthesis. Research hasbeen made where red spruce seedlings were sprayed with different combinations ofsulfuric and nitric acid of pH ranging from 2. 5 to 4. 5.
The needles of theseseedlings were observed to develop brown lesions. Eventually, the needles falloff. It was also found that new needles grew more slowly at higherconcentrations of acid used. Because the rate at which the needles were fallingwas greater than the rate at which they were replenished, photosynthesis wasgreatly affected, The actual way in which these needles were killed is still notyet known.
However, studies have shown that calcium and magnesium nutrients arewashed away from their binding sites when sulfuric acid enters the system. Theyare replaced by useless hydrogen atoms and this inhibits photosynthesis. Effecton Lakes and Aquatic Systems One of the direct effects of acid rain is on lakesand its aquatic ecosystems. There are several routes through which acidicchemicals can enter the lakes. Some chemical substances exist as dry particlesin 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 throughthe sewage systems eventually make their way into the lakes.
Acid rain thatfalls onto the earth washes off the nutrients out of the soil and carries toxicmetals that have been released from the soil into the lakes. Another harmful wayin which acids can enter the lakes is spring acid shock. When snow melts inspring rapidly due to a sudden temperature change, the acids and chemicals inthe snow are released into the soils. The melted snow then runs off to streamsand rivers, and gradually make their way into the lakes. The introduction ofthese acids and chemicals into the lakes causes a sudden drastic change in thepH of the lakes – hence the term “spring acid shock”.
The aquaticecosystem has no time to adjust to the sudden change. In addition, springtime isan especially vulnerable time for many aquatic species since this is the timefor reproduction for amphibians, fish and insects. Many of these species laytheir eggs in the water to hatch. The sudden pH change is dangerous because theacids can cause serious deformities in their young or even annihilate the wholespecies since the young of many of such species spend a significant part oftheir life cycle in the water. Subsequently, sulfuric acid in water can affectthe 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 andnutrients needed to stay alive.
For freshwater fish, maintaining osmoregulationis key in their survival. Osmoregulation is the process of maintaining thedelicate balance of salts and minerals in their tissues. Acid molecules in thewater cause mucus to form in their gills and this prevents the fish to absorboxygen as well. If the buildup of mucus increases, the fish would suffocate. Inaddition, a low pH will throw off the balance of salts in the fish tissue.
Saltslevels such as the calcium (Ca+2) levels of some fish cannot be maintained dueto pH change. This results in poor reproduction – their eggs produced would bedamaged; they are either too brittle or too weak. Decreased Ca+2 levels alsoresult in weak spines and deformities. For example, crayfish need Ca+2 tomaintain 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 stimulatesthe growth of algae, which logically would mean and increase in oxygenproduction, thus benefitting the fish. However, because of increased deaths inthe fish population due to acid rain, the decomposition process uses up a lot ofthe oxygen, which leaves less for the surviving fish to take in. Indirectly,sulfuric acid releases heavy metals present in soils to be dissociated andreleased. For example, Aluminium (Al+2) is harmless as part of a compound, butbecause acid rain causes Al+2 to be released into the soils and gradually intothe lakes, it becomes lethal to the health of the fish in the lakes. Al+2 burnsthe 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 acidlevel is high enough to release Al+2 from the soils to kill the fish. Thiseffect is further augmented by spring acid shock. The effect of acid rain can bedynamically illustrated in a study done on Lake 223, which started in 1976.
Scientists monitored the pH and aquatic ecosystem of Lake 223. They observedthat as the pH of the Lake Decrease over the years, a number of crustaceans diedout because of problems in reproduction due to the acidity of the lake caused byacid precipitation. At a pH of 5. 6, algae growth in the lake was hindered andsome types of small died out. Eventually, it was followed by larger fish dyingout with the same problem in reproduction; there were more adult fish in thelake than there were young fish.
Finally, in 1983, the lake reached a pH of 5and the surviving fish in the lake was thin and deformed and unable toreproduce. This case study obviously illustrates the significant effect of acidrain on lakes and its aquatic ecosystem. Effect on Materials Acid rain alsodamages materials such as fabrics. For example, flags that are put up are being”eaten away” by the acidic chemicals in the precipitation. Books andage-old art that are centuries old are also being affected; the ventilationsystems of the libraries and museums that hold them do not prevent the acidicparticles from entering the buildings and so, they get in and circulate withinthe building, affecting and deteriorating the materials. Effect on AtmosphereSome of the constituents of acid pollution are sulphates, nitrates, hydrocarbonsand ozone.
These exists as dry particles in the air and contribute to haze,affecting visibility. This makes navigation especially hard for air pilots. Acidhaze also interferes with the flow of sunlight from the sun to the earth andback. In the Arctic, this affects the growth of lichens which in turn, affectthe caribou and reindeer which feed on it.
Effect on Architecture Acid particlesare also deposited on to buildings and statues, causing corrosion. For example,the Capitol building in Ottawa has been disintegrating because of excess sulphurdioxide in the atmosphere. Limestone and marble turn to a crumbling substancecalled gypsum upon contact with the acid, which explains the corrosion ofbuildings and statues. In addition, bridges are corroding at a faster rate, andthe railway industry as well as the airplane industry have to expend more moneyin repairing the Corrosive damage done by acid rain. Not only is this aneconomically taxing problem caused by acid rain, but also a safety hazard to theGeneral public; as an illustration, in 1967, the bridge over the Ohio Rivercollapsed killing 46 people – the reason? Corrosion due to acid rain.
Effects OnHumans Among one of the serious side effects of acid pollution on humans isrespiratory problems. The SO2 and NO2 emmisions give rise to respiratoryproblems such as asthma, dry coughs, headaches, eye, nose and throatirritations. An indirect effect of acid precipitation on humans is that thetoxic metals dissolved in the water are absorbed in fruits, vegetables and inthe tissues of animals. Although these toxic metals do not directly affect theanimals, they have serious effects on humans when they are being consumed. Forexample, mercury that accumulate in the organs and tissues of the animals hasbeen linked with brain damage in children as well as nerve disorders, braindamage and death. Similarly, another metal, Aluminium, present in the organs ofthe animals, has been associated with kidney problems and recently, wassuspected to be related to Alzheimer’s disease.
BibliographyElliott, Thomas C. , and Robert G. Schwieger (Editors). The Acid RainSourcebook. New York: McGraw-Hill, Inc.
, 1984. Bown, William. “Europe’sforests fall to acid rain”. New Scientist. Vol. 127.
August 11, 1990. p. 17Calvert, Jack G. (Editor) “SO2, NO and NO2 Oxidation Mechanisms: AtmosphericConsiderations” Acid Rain Precipitation Series, Volume 3. Toronto:Butterworth Publishers, 1984.
