
Watch this 2 minute overview of Acid Rain.
What is acid rain? Acid rain is any form of precipitation with high levels of nitric and sulfuric acids. It can occur in the form of snow, fog, and even dry materials that settle to earth. Most acid rain is caused by human activities.
When people burn fossil fuels, sulfur dioxide and nitrogen oxides are released into the atmosphere. These gases react with water, oxygen, and other substances to form sulfuric and nitric acid. Winds may spread these acidic solutions over hundreds of miles.
After it falls to earth, acid rain enters water systems as runoff and sinks into the ground. This can make water toxic to prey fish, clams, fish, and other aquatic animals. The rest of the food chain, including non-aquatic species such as birds, is often affected as well. Acid rain also harms forests by damaging trees' leaves, robbing the soil of essential nutrients, and making it hard for trees to take up water. By designing cleaner power plants and using fewer fossil fuels, we can reduce the number of pollutants that create acid rain
The media is keen on using simplistic terminology to explain environmental issues, acid rain is one of those terms. What is it that the media has got wrong here? Glad you asked! The simple answer is that "acid rain" is not just rain. It can come in all forms of precipitation that you might encounter — snow, hail, fog, etc. But once again, this doesn't cover all the bases. The meteorology chaps use the term precipitation to indicate water falling to earth in any of its many forms (there is a big difference between 2 inches precipitation as water and 2 inches precipitation as snow). But here is the crux - not all precipitation is wet - sulfate aerosols are precipitation and they are dry.
So, now you know that people who use the term acid precipitation make me much happier than those who use the term acid rain. And students like you, who understand that acid deposition is a combination of all the components below, make me happiest of all (Now all I need to do is get you all jobs in the media).
Components of Acid Deposition
- Rain
- Snow
- Hail
- Fog
- Ice
- Sulfate aerosols
Who is to blame?
Blame is something that we like to assign, especially as it can be passed away from our own consumption patterns, but remember the pollution is generated as a by-product when electricity or fossil fuels are used. We could blame the utilities (but not nuclear or renewable utilities) but they did generate electricity cheaply. Back in 2000, 19 of the 20 cheapest electricity generation utilities were coal-fired. We like cheap electricity. We complain when the prices go up. But we also require clean air and water — but we don't want to pay for it. It is similar to gasoline. Additional removal of the S costs money, it is a cost that is passed on to you, the consumer. The cost is around 7 cents a gallon to meet the latest Clean Air Act amendments-API estimates. Less pollution typically cost additional investment and change that come at a cost ($).
The presence of SO2 and NOX in the atmosphere is what makes the deposition (rain) acidic.
SO2 + 1/2 O2→SO3
SO3 + H2O → H2SO4
NOx + H2O → HNO3
These acids, when dissolved in water, exist in the ionic state so that the hydrogen ions are free to wander around: 2H+ and SO4 2–. The pH scale is essentially a measure of the ions in the solution.

Source of SO2 and NOX emissions
As shown in the charts below, the culprits for the emission of these gasses were vehicles for NOx, and utilities (mostly coal-fired) for SO2.

As you can see in the chart below, emissions of SO2 have been decreasing as we use less coal (changes in our electricity sources), are using cleaner coal, and through the use of SO2 scrubbers on coal-fired utilities. Some of these changes are due to emission reductions in the Clean Air Act. The largest reduction is due to the adoption of natural gas (and renewables). The natural gas switch was due to changing economics rather than policy.

Natural Sources of SO2 and NOX?
There are natural sources as well as anthropologic. Here is some more volcano information concerning gasses.

Listen
Click on the link to listen to this audio file about Natural sources of acid precipitation.
Dr. Mathews: One whole day I had as a young man, we went to the Canary Island, which is a Spanish speaking set of islands off the North African coast; beautiful volcanic islands. And I remember two things very vividly. One was going up Mount Haidi. It was an incredible drive. I was amazed how religious these people were; there were crosses everywhere. And upon getting to the top there was a weather observatory and an astronomical observatory because the air was nice and clean there. And also there was sulfur lying around. There were little holes everywhere where you could actually pick up chunks of sulfur if you wished to. It smelt of rotten eggs, the volcano was dormant but giving off lots of gasses. The other thing that I remember vividly is on the way down the crosses were not there. The crosses were not there just for religious reasons, they were there when someone had gone over the edge. And with a thousand foot drop I was terrified the whole way back done. Anyway, volcanoes give off lots of gasses.
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