To me, acid rain seems like a highly unpleasant concept. Rainwater, usually one of the most reliably clean sources of water on earth, becoming caustic enough to increase erosion rates of stone and metal. Also, changing the soil of the plants that it waters in such a way that they die off, and few things if any are capable of growing in the ground thereafter. It just seems wrong, like a perversion of nature in a way; one of the most necessary elements in biological systems, and an invaluable delivery system for cycling and delivering water to regions where it doesn't last long on the surface, becoming a vector for killing off otherwise hardy ecosystems. It's a twisted version of the cycle that is so necessary for life.
While rain itself is naturally more acidic than neutral due to naturally occurring weak acids generated from atmospheric gases, it usually falls within a certain range that organisms have evolved to operate within. If the natural acidity is pushed too far, or reduced to a basic nature, organisms typically suffer from various functions becoming inhibited as the proteins and enzymes within the cells of any organisms subjected to the changed pH become inert or inoperable, if they aren't killed outright by possible corrosive effects. Plants that don't suffer from effects such as these are often victim to either toxic compounds being solvated and absorbed, or to atrophy as vital nutrients are dissolved to the point that they flow away before they can be taken up (http://www.epa.gov/acidrain/effects/forests.html). Bodies of water are also subject to such sensitivity, with similar reasons- fish are susceptible to poisoning by aluminum and acidic conditions, while microorganisms suffer from their medium being changed (http://www.epa.gov/acidrain/effects/surface_water.html).
It's not just the environment that is degrading due to acid rain, either. Various man-made structures are subject to damage from acid rain, especially those made from rocks primarily composed of calcium carbonate, and ferric and cupric metals. "Calcium carbonate in certain stones dissolves in dilute sulphuric acid [from rain] to form calcium sulphate." (http://www.air-quality.org.uk/12.php) Calcium sulphate, often known as gypsum, is a much more brittle stone, and is subject to falling off of the afflicted stonework, leaving an exposed area that is subjected to further reaction with acid rain. Besides the effects upon statues, various buildings of note are feeling the effects, including the Taj Mahal, Notre Dame cathedral, and the roman Colosseum.
Acid rain is generated in various ways, some natural. However, natural acidity is usually caused by carbonic acid, a much weaker acid than the sulfuric and nitric acids that typically causes problems. These are often generated by the release of industrial chemicals from smokestacks, or from the exhaust of motor vehicles. Though both can occur naturally, the quantity of these compounds in the air has increased and been generated more frequently, as compared to being generated from less common events such as wildfires and volcanic eruptions, which combined produce roughly 1/7th of the quantity of sulfur dioxide released from human activities per year. And while the phenomenon is typically referred to as acid "rain," it doesn't necessarily need to come down in aqueous conditions- sometimes these acids occur in gaseous or solid forms and reaches the ground long enough to either stick to something or react with something. When this happens, it's usually referred to as dry deposition. However, it's effects can be just as much, if not more noticeable in the long run, since the quantity deposited will most likely be fully unleashed when exposed to water, and when that occurs, the dry material will solvate and add up to a much higher concentration being administered all at once. When this happens, the pH drops even more dramatically, and the organisms subjected to this treatment will most likely not be able to cope. And while humans may not be susceptible to direct health effects from increased acidification of the environment, it is the gaseous and dry particulate that can cause various severe health issues, as these will dissolve in the mucous lining of the respiratory system, and cause heart and lung problems.
Unlike climate change, this matter has received political attention and study, though not without it's own share of political turmoil. In 1980, a program was put together to try and assess the extent and impact of acid deposition on the environment. This program was called the National Acidic Precipitation Assessment Program(NAPAP), and was supposed to perform a long-term study into the matter of acid precipitation and it's effects on the environment, and come up with a "credible evaluation of the consequences of various policy options" While it seemed to serve it's purpose by the end of it's term, although "advocates from all sides attempted to use the program to support their own policy marketing efforts, or to disparage those of their opponents." (http://www.epa.gov/wed/pages/staff/lackey/pubs/ACID-RAIN-SCIENCE-POLICY-LACKEY-BLAIR-JOURNAL-REPRINT-1997.pdf). Another study, by the DENR, found in 1991 that large bodies of freshwater are becoming more acidic, enough so that some species of fish can no longer survive within them. Meanwhile, the Clean Air Act was passed in 1989, which introduced the concept of "cap and trade," a system where permits are issued for how much of a pollutant gas can be released into the atmosphere by a company per year, and if they don't expect to reach that limit, they can trade those permits to another company for something. This allows for companies with the means to make the large scale changes to their operating facilities to clean their emissions can do so, trading to the companies that don't yet have the capacity to do the same until they do have the capability. It seems to have been a success for this program, and apparently at a lower cost than was initially expected. However, the success may be attributed to certain changes in the industries that were actually subject to exuding the compounds responsible for the acidity in the air. A similar system is being proposed to try and handle greenhouse gases, but part of the reason they are so hard to handle is that they aren't as reactive as sulfur dioxide, and can be harder to contain. There is also the matter of the time that such a plan needs to be effective, as gradual changes come about; climate change is already at an advanced stage of progression, and if this program even had a chance to work here as well, is it too late to help? But that is a different matter.
While sulfur dioxide emissions by human activity haven't been reduced to zero, they have been reduced significantly, perhaps even to the point where our impact on the environment through that vector is minimal. Hopefully our impact will be less than those of natural causes one day, but in the meantime, it's good to be able to celebrate our victories while we can... as long as we don't backslide in the process.
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