Information Sheet for Activity 2 – Understanding the Acid Rain Problem WHAT IS ACID RAIN? Robert Angus Smith, an English chemist, first used the phrase "acid rain" in 1852 when he noted the connection between London's polluted skies and the acidity of its rainfall. "Acid rain" is a general name for a whole range of atmospheric acidic pollution effects associated with precipitation including acid fog, acid mist and acid snow. Although we associate the acid threat with rainy days, acid deposition occurs all the time, even on sunny days. Indeed, it is not necessary to have wet conditions to have acidic pollution effects. Some pollutants can be transported directly to the surface as gases or small particles without being absorbed by moisture, a phenomenon known as dry deposition. WHAT CIRCUMSTANCES GIVE RISE TO ACIDIC POLLUTION? Acidic pollution can arise from both natural processes and human activities. Volcanic activity and animal and plant decomposition are well known examples of the former. However, in many urban and industrial areas the dominant source is from human activities. There are four major pollutants that cause acid air pollution: sulphur dioxide, nitrogen oxides, hydrocarbons and ozone. Sulphur dioxide Sulphur occurs widely in nature, in all animal and plants, and in the fossil fuels - coal, oil and natural gas - which were formed from ancient plant and animal remains. Oil and natural gas generally have relatively low sulphur contents, usually less than 1 percent. However, some coals can have as much as 14 percent sulphur. When fossil fuels are burned to generate energy the sulphur combines with the oxygen in the atmosphere to form sulphur dioxide. Globally, about 50 percent of the SO2 levels in the atmosphere are estimated to be of natural origin. However, in many urban and industrial areas human activity may account for around 90 percent of the SO2 in the air. Nitrogen oxides A number of different kinds of nitrogen oxides exist in the atmosphere. Collectively they are given the name NOx. Like SO2, about 50 percent of the NOx in the air on a global basis is estimated to be from natural sources. However, in industrial areas only about 10 percent of NOx are of 'natural' origin. The vast majority are produced when fossil fuels are burned. The nitrogen present in the fuel and in the atmosphere combines with oxygen to produce several different types of NOx. Coal-burning power stations and transport are the major sources. Hydrocarbons and ozone Unburnt hydrocarbons emitted as a result of improper combustion of fossil fuels constitute a major air pollutant in many urban and industrial areas. These hydrocarbons can react with NOx in the presence of sunlight to produce ozone which can reach levels 10 times higher than the natural background level which is generally low. This ozone then reacts with other pollutants to form acids. WHAT ARE THE ACID PROCESSES TAKING PLACE IN THE ATMOSPHERE? The common reactions in acid rain formation: Sulphur dioxide + water Sulphuric Acid Nitrogen oxides + water Nitric Acid The SO2 and NOX are transformed into acids in the atmosphere either by dry deposition or by wet deposition. During dry deposition, which usually occurs close to the source of the pollution, the pollutants are deposited on to surfaces where they then turn into acids. With wet deposition, which can occur over 1,000 km from the source, the gases become acids after coming into contact with water droplets in the air. Generally the longer that the pollutant stays in the air the more of it will be converted into acid. The diagram summarizes these processes. ACIDITY OF WATER Acidity is measured on the pH scale which runs from pH 0, the most acid, to pH 14, the most alkaline. The midpoint of the scale, pH 7, is termed neutral. The scale is logarithmic so that each point is ten times greater or less than the point above or below it respectively. For example a pH of 5 is ten times more acidic than pH 6, and is 100 times more acidic than pH 7. The scale below shows the acidity of some everyday objects on the pH scale. Normally, rainwater from an unpolluted atmosphere will have a pH of about 5.6 or 5.7, that is, it is naturally acidic, being about 25 times more acidic than neutral distilled water which has a pH of 7. This is because the carbon dioxide in the air reacts with water to form a weak carbonic acid. In addition, during thunderstorms, some nitrogen oxides are formed, which when dissolved in water produces dilute nitric acid. However, when acidic pollutants are present the additional sulphuric and nitric acids formed as a result of the 'acid' processes in the atmosphere greatly reduce the pH value. Badly polluted rainwater can have a pH as low as 2.4, about the same as lemon juice. In severe situations pH may be as low as 1.5, about the same as the acid in a car battery, and some 10,000 times more acidic than unpolluted rainwater. Websites http://www.meteorology.org.hk/acid_guide.htm http://www.epa.gov/airmarkets/acidrain/#what http://www.msc-smc.ec.gc.ca/cd/factsheets/acidrain/index_e.cfm http://www.ns.ec.gc.ca/msc/as/acidfaq.html http://www.ktca.org/newtons/9/acidrain.html http://school.discovery.com/sciencefaircentral/jakesattic/lab/acidrain/p1whatsacidrain.html References William P. Cunningham, Understanding Our Environment – An Introduction, Wm. C. Brown Publishers, 1994. Daniel D. Chiras, Environmental Science – Action for a Sustainable Future, The Benjamin/Cummings Publishing Company, Inc.,1994. Eldon D. Enger and Bradley F. Smith, Environmental Science – A Study of Interrelationships, McGraw Hill, 2000.