Acid Rain and Acid Deposition
Acid rain is a common name for the deposition of acidic material from
the atmosphere either as:
- Wet deposition of acid in precipitation (rain, snow, or fog); or
- Dry deposition of acidic material on dust, smoke, or other aerosols
(small, microscopic particles in the air).
Acid rain was a major problem in Europe and the USA in the last few
deacdes of the 20th century. Strong emission control laws have greatly
reduced the problem in these areas. However, acid rain continues to be
a major problem in some developing countries, especially China.
Here I will use acid rain to cover both types of deposition.
Sources of Acid rain
The acidic materials come from sulfur dioxide (SO2), ammonia
(NH3), nitrogen oxides (NOx) and acidic particles emitted
into the atmosphere by burning of fossil fuels in power plants and cars.
In the United States, roughly 2/3 of all SO2 and
1/4 of all NOx come from burning of fossil fuels, especially coal, in
electric power plants.
power plants emit large quantities of acid pollutants into the atmosphere,
although the volume of pollutants has been decreasing as scrubbers
that remove pollutants from exhaust gases have become more widely used.
The image shows exhaust from the American Electric Power's Gen. James
M. Gavin Plant in Chesire in Gallia County, in the Ohio Valley. It
is one of the largest coal-fired power plants in Ohio. Most of the
visible exhaust is condensed vapor, but the brownish haze includes
From Ohio.Com of the Akron Beacon Journal, article Ohio
EPA cites area for soot problems.
at base of Georgia Power's Bowen Plant removes 95% of the sulfur dioxide
in the plant's exhaust gas. Click on the image to bring up a
diagram of how a scrubber works from Scrubber
freshens smokestack by
Wade Rawlins, Staff Writer for the News Observer.
Acid rain occurs when these gases react in the atmosphere with water,
oxygen, and other chemicals to form various acidic compounds. The result
is a mild solution of sulfuric acid and nitric acid. When sulfur dioxide
and nitrogen oxides are released from power plants and other sources,
prevailing winds blow these compounds across state and national borders,
sometimes over hundreds of miles.
is Acid Rain, EPA.
Most acid rain falls downwind of power plants. In the USA, many are
located in the mid-west, and acid rain is common there and throughout
the east coast. As power plant emissions are increasingly regulated,
the amount of acid rainfall has decreased. Total annual emissions of
SO2 in the USA dropped from 28.8 × 106 metric tons in 1978
to 17.8 × 106 metric tons in 1998.
Acid rain deposition in the USA from 1983 through 1997. From: Driscoll
Acidity of Rain
Acidity of precipitation is measured in pH units, where
where H+ is the dissolved hydrogen ion concentration
in a weak solution in water. The lower the pH the more acidic the precipitation,
the higher the pH the more basic the precipitation. Pure water water
has a pH of 7.0, and pure rain has a ph of 5.6 because carbon dioxide
dissolved in water forms a weak acid, carbonic acid, H2CO3.
H2O + CO2 –> H2CO3
pH scale from Environmental Protection Agency, pH
The pH of precipitation from very polluted air can be less
than 2 in extreme cases. Mostly, the pH of precipitation ranges from
4.4 to 5.8.
Acidity of precipitation measured by the National Atmospheric Deposition
Program in 2006. Notice that precipitation is most acidic downwind of the
large concentration of power plants in the Ohio Valley.
Effect on Soils
The effect of acid rain on the environment
depends greatly on the ability of soils to neutralize the acid. Limestone
and other rocks and soils containing calcium carbonate are most effective.
Acids react with calcium carbonate to produce neutral compounds and carbon
dioxide. For example, sulfuric acid and hydrochloric acid react with
CaCO3 + H2SO4 –> CaSO4 +
H2CO3 –> CO2 gas
2HCl + CaCO3 -> CO2 +
CaCl2 + H2O
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.
In the slightly acidic soils in typical evergreen forests in the Northeastern
U.S., Canada, and Europe, two other effects can neutralize the acid
rain. The acid can be immobilized as the soil or vegetation retains
the sulfate and nitrate ions (from sulfuric and nitric acids). Very
deep soils have a large capacity to retain sulfate and nitrate ions.
From Virtual ChemBook, Elmhurst College: Acid
Rain – Soil Interactions.
Mature forest soils are also able to neutralize the acids
in rain. Such soils are acidic and rich in humic acids. Krug (1983)
reports that mature soils
in New England or Norwegian forests have a pH of 3.8, and they may contain
as much acid as would fall in 1000 years of rain at 1m/year with a pH
of 4.3. These soils are highly resistant to acid
In contrast, thin alpine soils
lacking carbonates and humic acid and overlying granite rocks are
not able to neutralize acid rain. The same is true for disturbed forests
where forest fires and logging have reduced the organic material in the
soil, which also make the soil more susceptible to damage by acid
Leaching of Nutrients
Acids leach nutrients from the soil. They are carried deeper into the ground
or into streams, depriving plants of essential elements calcium, potassium,
magnesium and trace metals. Normally, metals are attached to clay particles
produced by weathering of rocks. The H+ anion of the acid
replaces the metal ions in the clay in a process called ion exchange.
The metals occur in groundwater as sulphates and nitrates. Two H+ anions
displace one Ca2+ or
or two K+ anions.
Release of Aluminum
Aluminum in soils and rock is very insoulable if the pH is greater than
5. More acidic solutiona dissolve aluminum from the soil, and the aluminum
is carried into streams and lakes by runoff and groundwater.
Effect on Vegetation
Bavaria, Germany, high-altitude forest damaged by acid rain. Photo by
Spitzbergler. From AccuWeather.
More photos of Bavarian forest by Spitzbergler.
In some regions, especially regions where
granite is close to the surface and where soils have been degraded by
logging and forest fires, the soil has little ability to neutralize
the acid. In these regions, acid deposition depletes the available
plant-nutrient cations Ca2+, Mg2+, and K+,
it increases the leaching of aluminum, and it increases the amount of
sulfur and nitrogen in the soil. All lead to weakening of trees, leading
to their death by bark beetle infestations and disease.
Some of the most dramatic effects on forests
have been observed in Europe. In 1983, a survey in West Germany showed
that 34% of the country's total forest is damaged by air pollution.
This included about one half of the famous Black Forest. Switzerland
has recorded damage to 14 % of her forest trees...
Vogelmann, measured the reproductive capacity of the forest by counting
the total trees in an area. Red spruce dropped from 6,000 trees to 1,000
trees, a decline of 80 % from 1965 to 1983. Very few pine cones and young
trees were found. Sugar maple tree counts dropped 84 % and beech tree
counts dropped 63% over the same time period...
Acid rain or acid cloud droplets that fall on the leaves and needles
of trees leaches the nutrients from them. Calcium, magnesium, and potassium
ions may be removed from the leaves faster than the roots can resupply
them. Acid rain in combination with ozone may damage the waxy coating
on leaves and needles. This may weaken, damage them, and provide opportunities
for diseases to enter the tree.
From Virtual ChemBook Elmhurst College Charles E. Ophardt Acid
Rain Effects on Forests.
Effect on Aquatic Animals
When nutrients and metals, including heavy metals and aluminum,
are leached from the soil, they are carried by runoff and groundwater into
streams and lakes where they kill aquatic life. Aluminum dissolved by
acid rain is highly toxic to many aquatic animals, especially young animals
including eggs and larvae.
Not all species of animals are equally sensitive to acids
and aluminum. Some fish species (such as (brown bullhead, yellow perch,
golden shiner, brook trout, and white sucker) are tolerant of water with
pH < 6, while others (such as Atlantic salmon, tiger trout,
redbreast sunfish, bluegill, tiger
musky, walleye, and alewife) cannot tolerate such waters. Most fish are
killed if pH falls below 5.2 (Driskoll, 2001).
In areas where soils have little capacity
to buffer acids in water, acidic precipitation can be a problem because
the infiltrating acidic water can increase the solubility of metals,
which results in the flushing of high concentrations of dissolved
metals into surface water. Increased concentrations of naturally
occurring metals such as aluminum may be toxic to aquatic organisms.
Studies of watersheds have indicated that the length of subsurface
flow paths has an effect on the degree to which acidic water is buffered
by flow through the subsurface. For example, studies of watersheds
in England have indicated that acidity was higher in streams during
storms when more of the sub-surface flow moved through the soil rather
than through the deeper flow paths. Moreover, in a study
of the effects of acid precipitation on lakes in the Adirondack Mountains
of New York, the length of time that water was in contact with deep
subsurface materials was the most important factor affecting acidity
because contact time determined the amount of buffering that could
– US Geological Survey. Circular 1139, Ground Water and Surface Water
A Single Resource. Effects
of Atmospheric Deposition on the Quality of Ground Water and Surface
- A good review
article article on acid rain in North America by Driscoll (2001)
This is a 700kByte PDF file.
- For information on acid rain deposition check the acid
rain web pages at the EPA. More information is at the National
Atmospheric Deposition Program web pages.
- To understand how emissions trading works and how it
has led to a reduction of acid rain in the US, read Clearing
The Air: The Truth About Capping and Trading Emissions (a 0.6
MByte PDF file).
- The USGS has a site on acid
rain with examples from Washington DC, especially the effects
of acid precipitation on buildings. There is an active
monitoring program in the US called the National Acid Precipitation
Assessment Program (NAPAP) and they have a brief summary on their
web site. Environment Canada has also a very informative web page
on acid rain (as
well as other environmental issues including ozone hole over the
- The EPA publishes Emission
Reports listing the amounts of pollutants emitted
by individual power plants.
- A European perspective with
many good links can be found at the Swedish NGO Secretariat on Acid
DRISCOLL, C. T., G. B. LAWRENCE, et al. (2001). Acidic Deposition
in the Northeastern United States: Sources and Inputs, Ecosystem Effects,
and Management Strategies. BioScience 51(3): 180--198.
Krug, E. C. and C. R. Frink (1983). Acid Rain on Acid Soil: A
New Perspective. Science 221 (4610):
Acid rain is widely believed to be responsible for acidifying soil and water
in areas of North America and northern Europe. However, factors commonly considered
to make landscapes susceptible to acidification by acid rain are the same factors
long known to strongly acidify soils through the natural processes of soil formation.
Recovery from extreme and widespread careless land use has also occurred in regions
undergoing acidification. There is evidence that acidification by acid rain is
superimposed on long-term acidification induced by changes in land use and consequent
vegetative succession. Thus, the interactions of acid rain, acid soil, and vegetation
need to be carefully examined on a watershed basis in assessing benefits expected
from proposed reductions in emissions of oxides of sulfur and nitrogen.
5 January, 2009