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Acid rain is rain with a lower than normal pH, caused by emissions of sulfur dioxide (SO2) and nitric oxides (NOx), typically from factories, though there are natural sources. This is quite bad for aquatic animals, plants, and buildings. Government regulations have had positive results on curbing acid rain. The term "acid rain" is bit misleading as all rain is slightly acidic. Most people think of "GAH, MY FLESH IS MELTING!" when they think of acid rather than toxicity to aquatic life if allowed to build up in higher concentrations. Human health effects are also indirect. You won't actually melt if you stand out in acid rain, nor will you suffer effects from the rain touching your skin; the damage actually comes from breathing in the sulfur and nitric oxides.
Discovery and denialism
Hubbard Brook and the emerging scientific consensus
The term "acid rain" was coined in the late 1800s when it was discovered that some plant life downwind of industrial centers seemed to die off at curiously high rates. Residents of cities with large industrial sectors also noted corrosion of marble and limestone surfaces. It was not considered a serious environmental threat until the 1960s and 1970s. The earliest research was done at the Hubbard Brook Experimental Forest, which determined that the forest was shrinking due to acid rain.
One of the causes of increased acidification of rainfall during the mid-20th century was the construction of higher smokestacks on factories and power plants. To avoid belching smog in people's faces, factory and plant owners had smokestacks built higher, but this had the effect of shooting emissions more directly into regional atmospheric circulation. Ironically, an EPA regulation enacted in the 1970s raising the minimum height of smokestacks on power plants was partially responsible for this. Some scientists joked that the acid rain problem could be solved more quickly if the stacks were lowered and people would have to "breathe their own pollution."
In 1980, Congress passed the National Acidic Precipitation Assessment Program (NAPAP) to conduct further research and monitoring of acid rain. Throughout the 1980s, the National Academies of Science released a number of consensus reports detailing human contribution to acid rain.
OSTP report, Fred Singer, and denialism
The White House Office of Science and Technology Policy (OSTP) was commissioned to produce a report on acid rain by 1984. Reagan administration insiders managed to get general purpose denialist S. Fred Singer onto the OSTP's acid rain peer review panel. The OSTP report outlined a number of the uncertainties in the research (e.g., rate of acid deposition, the area over which acid deposition would spread, etc.). Singer attempted to play up these uncertainties and slip political statements into the report (the report was intended to conclude whether or not policy was necessary, not to recommend a specific policy). The final draft of the report in 1984 found that the problem posed by acid rain was sufficient to warrant policy action and Singer's nonsense had been confined to an appendix at the end of the report. The appendix flogged the uncertainty tactic, essentially saying "it's not that bad, and if it is, it's too expensive to do anything about it anyway." Interestingly, despite his "uncertainty" about the problem of acid rain, Singer was pretty certain that the downsides of a cap on emissions would outweigh the positives (though another point of interest is his mention of a cap-and-trade scheme, which would eventually be adopted). He wrote in his conclusion:
“”In the absence of even order-of-magnitude estimates of economic damage attributable to acid deposition, and with emission control costs certainly in the multibillion dollar range, one must question whether we are attacking a million-dollar problem with a billion-dollar solution.
Singer's appendix was relentlessly quote mined by acid rain deniers to declare that it wasn't a problem so we should do nothing. They cooked up a number of talking points in order to delay action on the issue, e.g. blaming acid rain on volcanoes (in fact, acid rain was also falling in areas nowhere close to volcanoes long after eruptions and prior research had ruled this out already as the only cause) and claiming the problem was hyped as many waterways were not severely acidified (this was due in part to absorption of acidic chemicals into soils such that they didn't run off into water supplies, however, soil acidification was often selectively ignored by deniers). Singer himself also blasted acid rain as a phony crisis in the press.
The Reagan administration blew off the problem of acid rain, dismissing it as nonsense and claiming an emissions cap would be too harmful to the economy. This pissed off the Canadians, as a good deal of the pollution the US was spewing was crossing over the border into their country. Over 70 acid rain bills were defeated in Congress during the Reagan administration. The tide turned under the George H.W. Bush administration, when a cap-and-trade amendment to the Clean Air Act was proposed. The utilities and energy industry lobbies launched a denialist campaign against the bill. One of the big players was none other than the now-defunct Koch Industries front group Citizens for a Sound Economy.
Acid rain today
Acid rain still exists as an environmental problem today, however, the cap-and-trade program that passed has deeply cut emissions since 1990 (and without blowing up the economy, as some flacks claimed would happen). A 2009 EPA report showed a halving of emissions since 1990. Acid rain denialism is still doctrine among anti-environmentalist wingnuts like El Rushbo, though, who has called it "a lie." This tactic is useful as a tool in global warming denial as it allows deniers to pull the "science was wrong before" gambit, even though it wasn't actually wrong in this case.
It's also still pretty common (read: ubiquitous) on Venus, where the rain takes form of the extremely hazardous sulfuric acid, even if the acid evaporates long before it can reach the planet's surface due to its searing hot temperatures.
- Acid rain overview, EPA
- "Acid Deposition", in Keeping Pace with Science and Engineering: Case Studies in Environmental Regulation, a 1993 report by the National Academy of Engineering
- Christine Marro. The Acid Rain Problem. Research/Penn State, Vol. 7, no. 3 (June 1986)
- Acid Rain Overview, National Geographic
- Volcanoes and lightning strikes are a few examples.
- Effects of Acid Rain - Human Health, EPA
- Hubbard Brook acid rain research page
- Robert Gannon. "How Scientists Are Tracking Acid Rain." Popular Science, Aug. 1984, p. 69. 
- National Academies reports: Atmosphere-Biosphere Interactions:Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion (1981); Acid Deposition: Atmospheric Processes in Eastern North America (1983); Acid Deposition: Long-Term Trends (1986)
- OSTP Report of the Acid Rain Peer Review Panel, Jul. 1984, William Nierenberg, chairman.
- Singer's SEPP think tank still maintains this position and has copious references to the "acid rain crisis" in scare quotes on the website, as in this newsletter for example.
- Ronald Reagan on the environment, On the Issues
- The Political History of Cap and Trade, Smithsonian Magazine
- FreedomWorks, a spin-off from the group, still has some of the anti-acid rain material hosted on their site, for example, this brochure entitled Acid Rain: Headline or Hoax
- See also the Think Progress report From Promoting Acid Rain To Climate Denial: Over 20 Years Of David Koch’s Polluter Front Groups
- Acid Rain Program 2009 Report, EPA
- April 18, 2011 Rush Limbaugh Show Transcript
- How it rains on different planets: what the science says