ACID DEPOSITION

• the wet or dry deposition of acid



• "Acid rain"
• Acid rain refers to all types of precipitation--rain, snow, sleet, hail, fog--that is acidic in nature



• precipitation that is polluted by sulphur dioxide (SO₂) and nitrogen oxides (NOX)



• Acidity is measured in pH units
• the scale of pH units goes from 0 to 14, with 7 being neutral



• The lower the pH, the more acidic the substance



• Every point on the pH scale represents a tenfold increase over the previous number
• pH 4 is 10 times more acidic than pH 5 and 100 times more so than pH 6



• Similarly, pH 9 is 1O times more basic than pH 8 and 100 times more basic than pH 7



• "pure" rain's acidity is pH 5.6-5.7



• How atmospheric acids form
• S + O₂-->SO₂ and N + O₂-->NO₂



• In the atmosphere, sulphur dioxide reacts with oxygen to produce sulphur trioxide gas: 2SO₂ + O₂-->2SO₃



• Some of this sulphur trioxide dissolves into water droplets in the atmosphere to produce tiny beads of sulphuric acid
• SO₃+ H₂O-->H₂SO₄



• NO3 + H20 -> H₂NO₄



• What causes the problem
• Public power generation. These are, in general, large plants burning fossil fuel to generate electricity.



• Commercial, Institutional, Residential Combustion Plant



• Industrial Combustion Plant and Processes with Combustion - pulp mills, metal smelters (when metal sulfide ores such as lead sulfide are roasted or smelted to convert the metal ore to free metal), refineries, heating boilers.



• Extraction and Distribution of Fossil Fuels



• Road Transport



• Other Transport



• Natural Sources - volcanic eruptions, oxidation of sulfur-containing by-products of decomposition of organic matter
• Other hazards of Sulphur pollution: Sulphurous smogs
• Produced by high output of SO₂, converted to acids on contact with atmospheric moisture. Very prevalent in Europe and North American cities during first part of 20th Century



• London. 4-10th December 1952: Cold, high-pressure conditions trapped coal smoke in foggy air. Output of smoke increased by cold. Sulphuric acid droplets resulted in pH estimated as 1.4 to 1.9: as acidic as car battery acid. Visibility reduced to 5m at times. Smog lasted for 5 days, eventually extending over 50km radius. Approx. 4,000 people died as result of inhaling pollution, mainly old and sick and those with chest problems.



• Donora, USA: October 1948: industrial town of 14,000, with zinc smelter, steel mill, and sulphuric acid plant, all of which introduced sulphuric acid into atmosphere. Anticyclonic conditions trapped emissions, which continued to be produced. Air became highly acidic, with sickening smell of sulphur. Over five day period, over half of population suffered ill effects, and 22 died



• Effects on Ecosphere
• Terrestrial Ecosystems
• Acid rain damages the protective waxy coating of leaves and allows acids to diffuse into them



• interrupts the evaporation of water and gas exchange so that the plant no longer can breathe



• This stops the plant's conversion of nutrients and water into a form useful for plant growth and affects crop yields.
• acid precipitation destroys, overall, $13,000 million annually in the eastern part of the nation



• $1,750 million yearly in forest damage, $8,300 million in crop damage in the Ohio River basin alone by about the year 2000



• $40 million in health costs in the State of Minnesota



• Soils
• Nutrient leaching occurs when acid rain adds hydrogen ions to the soil which interact chemically with existing minerals. This displaces calcium, magnesium and potassium from soil particles and deprives trees of nutrition.



• Toxic metals such as lead, zinc, copper, chromium and aluminum are deposited in the forest from the atmosphere. The acid rain releases these metals and they stunt the growth of trees and other plants and also that of mosses, algae, nitrogen-fixing bacteria and fungi needed for forest growth.
• pasted onto groundwater, rivers, streams and lakes, fish, people, etc..

• Aquatic Ecosystems
• Aquatic plants grow best between pH 7.0 and 9.2



• As acidity increases submerged aquatic plants decrease and deprive waterfowl of their basic food source.



• At pH 6, freshwater shrimp cannot survive.



• At pH 5.5, bottom-dwelling bacterial decomposers begin to die and leave undecomposed leaf litter and other organic debris to collect on the bottom...water becomes malnourished



• This deprives plankton--tiny creatures that form the base of the aquatic food chain--of food, so that they too disappear



• Below a pH of about 4.5, all fish die.



• Most of the frogs and insects also die when the water reaches pH 4.5.



• Acid Rain Damage to Buildings and Cultural Monuments
• In urban areas, acid deposition is rapidly deteriorating buildings, sculptures, paintings, metal, glass, paper, leather, textiles and rubber



• European officials say that "ancient buildings and sculptures in a number of cities have weathered more during the last 20 years than in the preceding 2,000"



• Sandstone, limestone and marble are the easiest victims of sulphur dioxide because they contain calcium, and when the two react they form gypsum which washes away easily with rainfall.



• Paper and textiles are also damaged by SO₂ and NO_x pollution because paper absorbs these gases and become more and more brittle as they absorb more pollutant. "About 5% of the British Library's collections are thought to be seriously damaged by sulphur-contaminated air" (Green Issues, pg. 16)



• Bridges are corroding at a faster rate (in 1967,the bridge over the Ohio River collapsed killing 46 people-the reason was corrosion due to acid rain)



• Where is this a problem
• More than two million square kilometers of North America now receive rain with a pH of 4.6 or lower, that is 10 or more times as acidic as "clean rain"



• The technology does exist to substantially reduce emissions of SO₂ and NOx now.