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Urban Climate
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What can we do to counteract acid rain?
Each of us can do something to limit the production of acid rain or help neutralise its effects. These actions vary from international agreements, through soil and water liming, to filtering the exhaust gases.
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Two directions
All efforts undertaken to counteract acid rain may be divided into two main groups:
- limiting its causes.
- neutralising the effects;
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1. Causes of acid rain and what we can do to counteract them.
Click the image to see it enlarged (87 KB)!
Author: Anita Bokwa
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Limiting the causes
The image on the left shows the chain of phenomena and processes that cause the production of acid rain. At each step there is something we can do to counteract it. The paragraphs below explain such activities.
Each of us can help!
Each of us can modify everyday life in such a way as to cause lower emissions of SO2 and NOx. First of all, energy saving leads to reductions in burning of fossil fuels, and thus to lower emissions. We can also filter exhaust gases in our cars and lower the emissions using the same amount of energy. Also use of clean, renewable energy resources result in less burning of fossil fuels.
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Better technology
There are many technologies which can be used in industry to reduce the emissions of pollutants to the atmosphere and these can be applied before, during or after combustion.
Examples of pre-combustion sulfur control technology (removing sulfur before burning) include coal scrubbing and oil desulfurization. One of the post-combustion sulfur controls (removing sulfur after burning) is Flue Gas Desulfurization (FGD). This process is rather expensive; the cost of eliminating one kilogram of sulfur is about 3-6 USD (=2.4-5 euro). Have a look at figure 2 to see how dry scrubbing system works.
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2. The Dry Flue Gas Desulfurization process utilizes high reactivity lime as the primary reagent material and requires particulate collection equipment downstream of the spray dryer absorbers.
Click on the image to see it enlarged (81 KB)!
Source: Hamon Research-Cottrell, Inc.
http://www.hamon-researchcottrell.com/Prod_FlueGasDry.asp
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3. Location of catalytic converter in car. The catalytic converter treats the exhaust before it leaves the car and removes a lot of the pollution. Most modern cars are equipped with three-way catalytic converters. See next figure to learn more about it!
Source: HowStuffWorks
http://www.howstuffworks.com/catalytic-converter.htm
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Cars which are equipped with catalytic converters have reduced emissions of oxides of nitrogen. These gases are oxidised in the atmosphere to form acidic rain. Figures 3 and 4 explain how a catalytic converter works.
Unlike sulfur, it is not possible to reduce the nitrogen content of the fuel (used in industry) before combustion by physical cleaning as it is combined within the organic matter of the fuel. Instead, nitrogen oxides can be removed during combustion. Advanced low nitrogen oxides burners can reduce emissions by up to 30%. Such burners can be installed on either new or existing combustion plants.
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Emissions of nitrogen oxides, as with sulfur dioxide, can also be reduced by treating the flue gases. One method involves mixing the flue gases with ammonia, converting the nitrogen oxides to nitrogen and water. This process is suitable for fitting to existing plants and newly built applications, and can achieve an emissions reduction of up to 80 to 90%.
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4. A three-way catalytic converter. "Three-way" refers to the three regulated emissions it helps to reduce -- carbon monoxide, VOCs and NOx molecules. The converter uses two different types of catalysts, a reduction catalyst and an oxidization catalyst. Both types consist of a ceramic (or metal) structure coated with a metal catalyst, usually platinum, rhodium and/or palladium. The idea is to create a structure that exposes the maximum surface area of catalyst to the exhaust stream, while also minimizing the amount of catalyst required (they are very expensive). There are two main types of structures used in catalytic converters -- honeycomb and ceramic beads. Most cars today use a honeycomb structure.
Explanations: A - reduction catalyst; B - oxidization catalyst; C - honeycomb.
Source: HowStuffWorks
http://www.howstuffworks.com/catalytic-converter.htm
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International co-operation
By the late 1970s concern led to international efforts to identify the causes and effects of long-range (transboundary) transport of air pollutants, and thus during the 1980s much research was conducted in Europe and North America. International legislation during the 1980s and 1990s has led to reductions in SO2 emissions in many countries but reductions in emissions of NOx have been much less.
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5. Legislation on long-range and acidifying emissions. See text for explanations.
Author: Anita Bokwa
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In 1979, the United Nations Economic Commission for Europe (UNECE) implemented the Convention on Long-Range Transboundary Pollution (the so-called Geneva Convention). In 1985, in Helsinki, most UNECE members adopted the Protocol on the Reduction of Sulfur Emissions, agreeing to reduce SO2 emissions by 30% (from 1980 levels) by 1993. This was called the 30% club. All of the countries that signed the Protocol achieved this reduction, and many of those that did not sign, have met these reductions.
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The Sofia Protocol for reducing NOx emissions was adopted in 1988. The emissions should be stabilised against 1987 levels. However, many countries are unlikely to meet these targets, due to increases in road traffic, despite European Union legislation requiring cars built after 1993 to be fitted with a catalytic converter.
In June 1994, a number of European countries signed the Second Protocol for sulfur. Its main objective was reducing acidifying emissions to a level at which critical loads are not exceeded, i.e. reduce sulfur emissions by 70-80% by the year 2000 (against 1980 levels). Eastern European countries generally have a lower target of 40-50% (against 1980 levels).
The most recent UNECE Convention on Long Range Transboundary Air Pollution protocol was signed by 27 countries in December 1999. The Gothenburg Protocol, designed to Abate Acidification, Eutrophication and Ground-level Ozone aims to cut emissions of four pollutants: SO2, NOx, VOCs and NH3, by setting country-by-country emission ceilings to be achieved by the year 2010.
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6. Effects of acid rain on natural environment and what we can do to counteract them.
Click to enlarge the image (78 KB)!
Author: Anita Bokwa
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Once the acid rain occurs, all we can do is neutralising its effects. However, as shown in figure 6, our actions can be efficient only at the beginning. When the next stages of the influence of acid rain have already occurred, it is much more difficult to counteract the negative effects. The most commonly used method is liming, described below. |
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Natural and artificial buffers
The resulting acidity of water is greatly influenced by the amount of limestone in the surrounding rock formations. The river beds and lake beds formed from limestone rocks allow the water to dissolve some of it and this neutralizes the acid rain. At present, the main way of artificial reversing acidification in freshwaters is liming the water body or its surrounding catchment, i.e. adding lye (NaOH(aq), sodium hydroxide) or slaked lime (Ca(OH)2(aq), calcium hydroxide). However, it causes aluminum and other metals to come out of solution and fall to the bottom of the lake, improving the conditions for fish life, but causing toxicity problems for organisms living on the lake bed. Acidified lakes in Sweden have been restored in the short term by liming. The same method is used for lakes in USA and Canada. Liming on such a large scale, however, is expensive.
Liming provides only a temporary solution, hence it is far better to attack the source of the problem by reducing emissions of acidifying pollutants, SO2 and NOx.
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About this page:
Author: Anita Bokwa - Jagiellonian University - Cracow / Poland
Scientific reviewer: Tony Dore - Edinburgh Research Station - Midlothian / United Kingdom - 2004-08-16
Educational reviewing: Michael Seesing - University of Duisburg - Duisburg / Germany
last update: 2004-12-17
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