Category: ACID RAIN


Acid Rain: Cause and Effect

Acid rain” is a broad term used to describe several ways that acids fall out of the atmosphere. A more precise term is acid deposition, which has two parts: wet and dry.

Wet deposition refers to acidic rain, fog, and snow. As this acidic water flows over and through the ground, it affects a variety of plants and animals. The strength of the effects depend on many factors, including how acidic the water is, the chemistry and buffering capacity of the soils involved, and the types of fish, trees, and other living things that rely on the water.

Dry deposition refers to acidic gases and particles. About half of the acidity in the atmosphere falls back to earth through dry deposition. The wind blows these acidic particles and gases onto buildings, cars, homes, and trees. Dry deposited gases and particles can also be washed from trees and other surfaces by rainstorms. When that happens, the runoff water adds those acids to the acid rain, making the combination more acidic than the falling rain alone.


Prevailing winds blow the compounds that cause both wet and dry acid deposition across state and national borders, and sometimes over hundreds of miles. Scientists discovered, and have confirmed, that sulfur dioxide (SO2) and nitrogen oxides (NOx) are the primary causes of acid rain. In the US, About 2/3 of all SO2 and 1/4 of all NOx comes from electric power generation that relies on burning fossil fuels like coal.


Acid rain occurs when these gases react in the atmosphere with water, oxygen, and other chemicals to form various acidic compounds. Sunlight increases the rate of most of these reactions. The result is a mild solution of sulfuric acid and nitric acid.


How Do We Measure Acid Rain?


Acid rain is measured using a scale called “pH.” The lower a substance’s pH, the more acidic it is. Pure water has a pH of 7.0. Normal rain is slightly acidic because carbon dioxide dissolves into it, so it has a pH of about 5.5. As of the year 2000, the most acidic rain falling in the US has a pH of about 4.3.


Acid rain’s pH, and the chemicals that cause acid rain, are monitored by two networks, both supported by EPA. The National Atmospheric Deposition Program measures wet deposition, and its Web site features maps of rainfall pH (follow the link to the isopleth maps) and other important precipitation chemistry measurements.

The Clean Air Status and Trends Network (CASTNET) measures dry deposition. Its web site features information about the data it collects, the measuring sites, and the kinds of equipment it uses.


Effects of Acid Rain


Acid rain causes acidification of lakes and streams and contributes to damage of trees at high elevations (for example, red spruce trees above 2,000 feet) and many sensitive forest soils. In addition, acid rain accelerates the decay of building materials and paints, including irreplaceable buildings, statues, and sculptures that are part of our nation’s cultural heritage. Prior to falling to the earth, SO2 and NOx gases and their particulate matter derivatives, sulfates and nitrates, contribute to visibility degradation and harm public health.

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ACID RAIN

Processes involved in acid deposition (note that only SO2 and NOx play a significant role in acid rain).

acid rain again

Acid rain is caused by emissions of sulfur dioxide and nitrogen oxides.  Natural sources of these gases do exist; however, more than 90% of the sulfur and 95% of the nitrogen emissions are from human activities.
The use of coal for electricity, fuel for vehicles, and many other factors contribute to the pollution.  Once these toxins are in the air, they are converted to nitric and sulfuric acid by reacting with water, oxygen, and oxidants.

The basic reactions for each follow:

“SO2 + 2OH — H2SO4 (sulfuric acid) <NO2 + OH — HNO3 (nitric acid).

Both of these acids are easily dissolved in water.  Consequently, acidic water returns to Earth in the form of rain snow or fog.  The sulfur dioxide and nitrogen oxide emissions can also chemically transform into sulfur and nitrogen salts.
” In this state, they are deposited as “dry” acid rain in the form of gases or particles.  This “dry” rain causes the same type of damage as regular acid rain, but it can be transferred hundreds of miles by prevailing winds.

History


The corrosive effect of polluted, acidic city air on limestone and marble was noted in the 17th century by John Evelyn, who remarked upon the Arundel marbles “miserably neglected, & scattredup & downe about the Gardens & other places of Arundell-house, & how exceedingly the corrosive aire of London impaired them”. Since the Industrial Revolution, emissions of sulfur dioxide and nitrogen oxides to the atmosphere have increased.In 1852, Robert Angus Smith was the first to show the relationship between acid rain and atmospheric pollution in Manchester, England. Though acidic rain was discovered in 1852, it was not until the late 1960s that scientists began widely observing and studying the phenomenon. The term “acid rain” was generated in 1972.Canadian Harold Harvey was among the first to research a “dead” lake. Public awareness of acid rain in the U.S increased in the 1970s after the New York Times promulgated reports from the Hubbard Brook Experimental Forest in New Hampshire of the myriad deleterious environmental effects demonstrated to result from it.


Occasional pH readings in rain and fog water of well below 2.4 have been reported in industrialized areas. Industrial acid rain is a substantial problem in Europe, China, Russia and areas down-wind from them. These areas all burn sulfur-containing coal to generate heat and electricity.The problem of acid rain not only has increased with population and industrial growth, but has become more widespread. The use of tall smokestacks to reduce local pollution has contributed to the spread of acid rain by releasing gases into regional atmospheric circulation.Often deposition occurs a considerable distance downwind of the emissions, with mountainous regions tending to receive the greatest deposition (simply because of their higher rainfall). An example of this effect is the low pH of rain (compared to the local emissions) which falls in Scandinavia.

how acid rain is formed