The
most important gas which leads to acidification is sulfur dioxide.
Emissions of nitrogen oxides which are oxidised to form Nitric acid are
of increasing importance due to stricter controls on emissions of sulfur
containing compounds. 70 Tg(S) per year in the form of SO2 comes from
fossil fuel combustion and industry, 2.8 Tg(S) from wildfires, and 7-8
Tg(S) per year from volcanoes.
Natural emissions
The
principal natural phenomena that contribute acid-producing gases to the
atmosphere are emissions from volcanoes and those from biological
processes that occur on the land, in wetlands, and in the oceans. The
major biological source of sulfur containing compounds is Dimethyl
sulfide. The effects of acidic deposits have been detected in glacial
ice thousands of years old in remote parts of the globe.
Human emissions
The
principal cause of acid rain is sulfur and nitrogen compounds from
human sources, such as electricity generation and motor vehicles. The
gases can be carried hundreds of miles in the atmosphere before they are
converted to acids and deposited.
Gas phase chemistryIn
the gas phase sulfur dioxide is oxidised by reaction with the hydroxyl
radical via a termolecular reaction:SO2 + OH· + M→ HOSO2· + M
which is followed by:HOSO2· + O2 → HO2· + SO3
In the presence of water sulfur trioxide is converted rapidly to sulfuric acid:SO3 + H2O + M → H2SO4 + M
Nitric acid is formed by the reaction of OH with Nitrogen dioxide:NO2 + OH· + M → HNO3 + M
Chemistry in cloud droplets
When
clouds are present the loss rate of SO2 is faster than can be explained
by gas phase chemistry alone. This is due to reactions in the liquid
water droplets
Hydrolysis
Sulfur dioxide dissolves in water and then, like carbon dioxide, hydrolyses in a series of equilibrium reactions:
SO2 (g)+ H2O ? SO2·H2O SO2·H2O ? H++HSO3- HSO3- ? H++SO32- Oxidation
There
are a large number of aqueous reactions of sulfur which oxidise it from
S(IV) to S(VI) leading to the formation of sulfuric acid. The most
important oxidation reactions are with ozone, hydrogen peroxide and
oxygen (reactions with oxygen are catalysed by Iron and Manganese in the
cloud droplets).
For more information see Seinfeld and Pandis (1998).
Aerosol formation
In
the gas phase sulfuric and nitric can condense on existing aerosols or
nucleate to form new aerosols. The nucleation process is an important
source of new particles in the atmosphere and so emissions of sulfur
containing compounds, as well as causing acidification also have a
climate effect.
Acid deposition
Wet deposition
Wet
deposition of acids occurs when any form of precipitation (rain, snow,
etc) removes acids from the atmosphere and delivers it to the Earth's
surface. This can result from the deposition of acids produced in the
raindrops (see aqueous phase chemistry above) or by the precipitation
removing the acids either in clouds or below clouds. Wet removal of both
gases and aerosol are both of importance for wet deposition.
Dry deposition
Acid
deposition also occurs via dry deposition in the absence of
precipitation. This can be responsible for as much as 20 to 60% of total
acid deposition. This occurs when particles and gases stick to the
ground, plants or other surfaces.
Adverse effects
Decades
of enhanced acid input has increased the environmental stress on high
elevation forests and aquatic organisms in sensitive ecosystems. In
extreme cases, it has altered entire biological communities and
eliminated some fish species from certain lakes and streams. In many
other cases, the changes have been more subtle, leading to a reduction
in the diversity of organisms in an ecosystem. This is particularly true
in the northeastern United States, where the rain tends to be most
acidic, and often the soil has less capacity to neutralize the acidity.
Acid rain also can damage certain building materials and historical monuments.
Some scientists have suggested links to human health, but none have been proven.
Effects on lake ecology
There
is a strong relationship between lower pH values and the loss of
populations of fish in lakes. Below 4.5 virtually no fish survive,
whereas levels of 6 or higher promote healthy populations. Acid in water
inhibits the production of enzymes which enable fish's larvae to escape
their eggs. It also mobilizes toxic metals such as aluminium in lakes.
Aluminium causes some fish to produce an excess of mucus around their
gills, preventing proper ventilation. Phytoplankton growth is inhibited
by high acid levels, and animals which feed on it suffer.
Many lakes
are subject to natural acid runoff from acid soils, and this can be
triggered by particular rainfall patterns that concentrate the acid. An
acid lake with newly-dead fish is not necessarily evidence of severe
air-pollution.
Effects of acid rain on soil biologySoil
biology can be seriously damaged by acid rain. Some tropical microbes
can quickly consume acids (Rodhe, 2005) but other types of microbe are
unable to tolerate low pHs and are killed. The enzymes of these microbes
are denatured (changed in shape so they no longer function) by the
acid.
The hydronium ions of acid rain also mobilize toxins and leache away essential nutrients.
Forest
soils tend to be inhabited by fungi, but acid rain shifts forest soils
to be more bacterially dominated. In order to fix nitrogen many trees
rely on fungi in a symbiotic relationship with their roots. If acidity
inhibits the growth of these mycorrhizae associations this could lead to
trees struggling to fix nitrogen without their symbiotic partners.
Other adverse effects
Trees
are harmed by acid rain in a variety of ways. The waxy surface of
leaves is broken down and nutrients are lost, making trees more
susceptible to frost, fungi, and insects. Root growth slows and as a
result fewer nutrients are taken up. Toxic ions are mobilized in the
soil, and valuable minerals are leached away or (as in the case of
phosphate) become bound to aluminium or iron compounds, or to clay.
The
toxic ions released due to acid rain form the greatest threat to
humans. Mobilized copper has been implicated in outbreaks of
diarrhea/diarrhoea in young children and it is thought that water
supplies contaminated with aluminium cause Alzheimer's disease.
Acid
rain can cause corrosion of ancient and valuable statues and has caused
considerable damage. This is because the sulfuric acid in the rain
chemically reacts with the calcium in the stones (lime stone, sandstone,
marble and granite) to create gypsum, which then flakes off. This is
also commonly seen on old gravestones where the acid rain can cause the
inscription to become completely illegible.
Acid rain also causes an increased rate of oxidation for iron.
Prevention methods
Technical solutions
In
the United States, many coal-burning power plants use Flue gas
desulfurization (FGD) to remove sulfur-containing gases from their stack
gases. An example of FGD is the wet scrubber which is commonly used in
the U.S. and many other countries. A wet scrubber is basically a
reaction tower equipped with a fan that extracts hot smoky stack gases
from a power plant into the tower. Lime or limestone in slurry form is
also injected into the tower to mix with the stack gases and combine
with the sulfur dioxide present. The calcium carbonate of the limestone
produces pH-neutral calcium sulfate that is physically removed from the
scrubber. That is, the scrubber turns sulfur pollution into industrial
sulfates.
In some areas the sulfates are sold to chemical companies
as gypsum when the purity of calcium sulfate is high. In others, they
are placed in a land-fill.
International treaties
A
number of international treaties on the long range transport of
atmospheric pollutants have been agreed e.g. Sulphur Emissions Reduction
Protocol and Convention on Long-Range Transboundary Air Pollution.
Emissions trading
An
even more benign regulatory scheme involves emission trading. In this
scheme, every current polluting facility is given an emissions license
that becomes part of capital equipment. Operators can then install
pollution control equipment, and sell parts of their emissions licenses.
The main effect of this is to give operators real economic incentives
to install pollution controls. Since public interest groups can retire
the licenses by purchasing them, the net result is a continuously
decreasing and more diffused set of pollution sources. At the same time,
no particular operator is ever forced to spend money without a return
of value from commercial sale of assets.
