Acid rain is a rain or any other form of precipitation that is unusually acidic, meaning that it possesses elevated levels of hydrogen ions (low pH). Acid rain is caused by emissions of sulfur dioxide and nitrogen oxides, which react with the water molecules in the atmosphere to produce acids.
Acid rain has become one of the most ubiquitous impacts of industrial pollution and has served as a rallying point for eco-activists and a catalyst for governmental environmental action, such as the Clean Air Act... In 1971, Bormann discovered that the water in the New Hampshire forest where he was conducting his research was considerably more acidic than normal. Subsequent tests showed that the acidity of water all over the east coast had increased up to 1,000 percent since the 1950s.
Rain is naturally acidic as a result of the reaction of water vapor, carbon dioxide and nitrogen in the atmosphere. The acidity can increase through the introduction of sulfur dioxide and nitrogen oxides into the atmosphere.
Motor vehicles and, to a lesser extent, coal-fired generators, are the major sources of nitrogen oxides. About half the wet sulphate deposition in eastern Canada is estimated to come from the United States, while about ten percent of the deposition in the northeastern United States comes from Canada.
Acid rain can be carried great distances in the atmosphere, not just between countries but also from continent to continent. The acid can also take the form of snow, mists and dry dusts. The dry dust can cause respiratory illnesses in animals and humans such as asthma. The rain sometimes falls many miles from the source of pollution but wherever it falls it can have a serious effect on soil, trees, buildings and water.
The extent to which soils can neutralize acid rain depends on several factors: type of soil, thickness, weather, and water flow patterns. If the ground is frozen, as in the winter, soil process cannot work, the acid is not neutralized. If the soil is mainly quartz, such as sand, it is resistant to weathering and no bases are present to neutralize acid. If the soil has very little base such as limestone, the acid is neutralized only slightly or with the passage of time, not at all.
Acids have a corrosive effect on limestone or marble buildings or sculptures. It is well established that either wet or dry deposition of sulfur dioxide significantly increases the rate of corrosion on limestone, sandstone, and marble.
As an example of the effects of acid rain, we can examine a case which is not obvious - effects on non-aquatic, tree nesting birds. It was observed that the proportion of birds laying defective eggs rose from roughly 10% in 1983-84 to 40% by 1987-88. It was also observed that these effects were limited to areas of acid rain. Since the birds did not appear to be directly affected by the acidity, the food chain was examined... In this case, acid precipitation had affects that passed on up the food chain.
Since the early 1980s, emissions of sulfur dioxide have been reduced in both Europe and North America... This seems to have stopped the acidification of lakes but not yet reversed it. The technology exists to generate electricity from coal with greatly reduced emissions and as this technology comes into use, that aspect of the problem should improve.
Each year in the US alone, the burning of fossil fuels dumps 90-billion tons of sulfur dioxide and nitrogen oxides in to the atmosphere. When these chemicals come in contact with water droplets in clouds, they form the phenomena called acid rain.
“Acid rain” became a household term in the 1980s when unchecked emissions from industry and motor vehicles were blamed for causing environmental deterioration. Scientific evidence has linked acid rain to decreased fish and wildlife populations, degraded lakes and streams, and human health hazards. Although the term has since faded from public consciousness, acid rain is a complex and global problem that still exists today.