There will come soft rains...
What is acid rain?
Acid rain was discovered in Manchester in 1852 (at the height of the industrial revolution), by Robert Angus Smith. Acid rain is any type of precipitation (including mist, snow, hail and rain) that has a lower pH than that found in normal rain. Normal rain has a pH of 5.5 to 5.6.
But I thought rain was pH neutral! Shouldn't it be 7?
It's true that pH neutral is 7, but rain is not pH neutral. Since normal rain has some carbon dioxide mixed into it, the water bonds with the carbon dioxide to form carbonic acid, which is a weak acid.
How is acid rain formed?
Acid rain is formed from sulfur and nitrogen emissions, which bond with oxygen to form sulfur dioxide and nitrogen dioxide by the following equations:
S(s) + O2(g) --> SO2(g) (Sulfur, a solid, joins with Oxygen, to form Sulfur Dioxide)
N(g) + O2(g) --> NO2(g) (Nitrogen, a gas, joins with Oxygen, to form Nitrogen Dioxide)
SO3(g) + H2O(l) --> H2SO4(aq)(Sulfur Trioxide, a gas, joins with Water, a liquid, to form Sulfuric Acid)
That's the last of the equations, I promise. Suffice it to say that these equations result in a rain with a much lower pH level than that found in normal rain, down to 4.5 instead of 5.5.
But that's only one pH! What's the big deal?
The pH scale only works from zero to fourteen, with 7 being neutral. However, the pH scale is not linear. Every whole number (number without a decimal point and a number after it) along the pH scale represents a tenfold increase over the previous number, which means that pH 4 is ten times more acidic than pH 5. In other words, if our rain has a pH of 4.5 instead of 5.5, it's not just a little bit more acidic, it's ten times more acidic.
How does it affect me?
Acid rain affects you in several ways, but it affects the environment far more severely. Since acid rain is rain, it filters down into rivers and streams, although the amount that reaches any given river or stream is affected by how porous the ground is, the geological composition of the ground, and several other factors. However, it is in rivers and streams that we see the greatest impact. Aquatic plants usually grow best at a pH of 7 or higher (in other words, in a slightly basic environment). As the pH decreases, aquatic plants die, depriving water birds of their food. Freshwater shrimp die when the pH reaches 6, and at pH 5.5, bacterial decomposers that live on the floor of the stream or river die. As a result, dead matter (plants, fish, etc.) no longer get removed, depriving freshwater plankton of food. Since the freshwater plankton form the base of a normal aquatic food chain, this means that other animals further up the chain die. At a pH of 4.5, all fish die. The fishermen out there should know that trout die at a pH of 5.0, and bass at 5.5.
It doesn't stop there, though. As the amount of undecomposed matter at the bottom of the stream increases, toxic metals such as aluminum, mercury and lead are released. Other metals will also flow into the river from the surrounding soils. These metals are in very small amounts, but this is where the food chain proves itself to be such a wonderful concentrator. Small animals eat up the heavy metals with their food, they get eaten by bigger animals, and the amount of metal in that animal increases with each smaller animal it eats. The smaller animal gets digested, but the body doesn't know what to do with the metals, and they stay in the system of that animal. Then that animal gets eaten by a bigger animal, and so on and so forth up the food chain, with the metal quantity rising higher almost exponentially at each step. By the time one reaches the top of the food chain, the metals have reached lethal levels, which is why pollution affects most obviously the animals at the top of the food chain. Guess where humans are on the food chain?
If that wasn't enough, consider the damage to crops. Not only does the metal in the food chain problem work just as well (or badly) with plants, since plants are just another step on the food chain, but the plants themselves have trouble growing in soil with a low pH, and many plants are destroyed directly by contact with acid rain. Acid rain eats away at the coating on leaves, destroying their ability to 'breathe'. Acid rain is an acid, which means it has an abundance of hydrogen ions, and these often interact with minerals in the soil, which in turn deprives plants and trees of nutrients. Usually these nutrients are dissolved and washed away into streams, along with heavy metals.
While technically not an acid rain problem, emissions from factories are the cause of poor visibility problems, otherwise known as smog. Look at the sky above your city from a distance, and you'll recognise it - the low hung cloud smothering the place where you live and work, breathing every minute of every day.
And if you're still not worried, consider this chilling aspect: acid rain has been proven to damage the paintwork on cars.
My car! But hang on - I'm an American/European/Australian/Martian. Acid rain doesn't affect me.
Acid rain affects everybody (except maybe Martians - shouldn't you be fixing our rover or something?). Thought at first to be an exclusively European problem (acid rain was first discovered in Manchester, after all), acid rain has since become a very serious problem in the United States (especially in the North-East, where rain with a pH of 4.3 has been documented), Australia, Japan, China and South-East Asia.
But all the lakes around me seem fine!
Check with your local environment group or university, who often keep tabs on this sort of thing. The odds are that you'll find that the number of animals in the lake or stream has gone down over the past few years, and not just as a result of your fishing expeditions. Acid rain is subtle, and takes a while to have a full effect, but measuring the number of various lower lifeforms, and careful measuring of the pH of the water over a long period can give you a good idea of how bad the problem may be in your area. Look also for blooms of algae on the surface, and a general decline in plant life in and around the water.
That's terrible! Someone really should do something about it!
Controls are in place to limit emissions from factories (usually burning fossil fuels), who are the biggest contributors of sulfur and nitrogen into the atmosphere. Although these controls vary from country to country, one of the most common systems is 'emission trading'. In this system, a license is bought as one of the capital costs of starting a factory. As emissions are reduced due to various improvements in equipment and processing methods, parts of the license can be sold, giving the factory a direct incentive to be as clean as possible. Since these licenses can be bought by public interest groups, who buy them to keep down the number of licenses and thus keep the prices high, factories continue to clean up, resulting in less pollution for everyone. A better question might be to ask "What can I do about it?" (hint hint).
Oh, yeah. What can I do about it?
The burning of fossil fuels is the primary cause of acid rain. You burn fossil fuels in two ways, firstly by driving your car, and secondly, by using power from power plants, many of which burn coal for your power. In both cases, cheaper options are available. Electrical cars or hybrid cars cut down on emissions substantially, as do smaller cars, which are more fuel-efficient - so ditch that SUV unless you actually go off road with it! You can also use less power - turn off lights unless you are in the room, use energy-efficient appliances, insulate your home to improve cooling and cut down on air conditioners and heaters which need so much power. Make sure that you keep your car well maintained, and write to your power company or local politician to get a more environmentally friendly power station. It may not be an investment that pays off in your lifetime, but your children will thank you for it.