Background Information
-What is Acid Rain?
Is rain that has been made acidic by certain pollutants in the air. Acid rain is a type of
acid deposition, which can appear in many forms. Wet deposition is rain, sleet, snow, or fog that
has become more acidic than normal. Dry deposition is another form of acid deposition, and this
is when gases and dust particles become acidic. Both wet and dry deposition can be carried by
the wind, sometimes for very long distances. Acid deposition in wet and dry forms falls on
buildings, cars, and trees and can make lakes acidic.
What is acidity?
Acidic and basic are two ways that we describe chemical compounds. Acidity is measured using
a pH scale. A pH scale runs from zero (the most acidic) to 14 (the most basic or alkaline). A
substance that is neither basic nor acidic is called "neutral", and this has a pH of 7.
- The Average pH of the Surface Water in Hawaii, Bermuda, and the Canary Islands
The average pH of the surface water in Hawaii is 8.1. The average pH of the surface
water in Bermuda is 8.09. The average pH of surface water in the Canary Islands is 8.06.
-How and Why Acid Rain Forms
Sources of Acid Rain
Acid rain is caused by a chemical reaction that begins when compounds like sulfur
dioxide and nitrogen oxides are released into the air. These substances can rise very high into the
atmosphere, where they mix and react with water, oxygen, and other chemicals to form more
acidic pollutants, known as acid rain. Sulfur dioxide and nitrogen oxides dissolve very easily in
water and can be carried very far by the wind. As a result, the two compounds can travel long
distances where they become part of the rain, sleet, snow, and fog that we experience on certain
days.
Human activities are the main cause of acid rain. Over the past few decades, humans
have released so many different chemicals into the air that they have changed the mix of gases in
the atmosphere. Power plants release the majority of sulfur dioxide and much of the nitrogen
oxides when they burn fossil fuels, such as coal, to produce electricity. In addition, the exhaust
from cars, trucks, and buses releases nitrogen oxides and sulfur dioxide into the air. These
pollutants cause acid rain.
Acid Rain is caused by Reactions in the Environment
Nature depends on balance, and although some rain is naturally acidic, with a pH level of
around 5.0, human activities have made it worse. Normal precipitation—such as rain, sleet, or
snow—reacts with alkaline chemicals, or non-acidic materials, that can be found in air, soils,
bedrock, lakes, and streams. These reactions usually neutralize natural acids. However, if
precipitation becomes too acidic, these materials may not be able to neutralize all of the acids.
Over time, these neutralizing materials can be washed away by acid rain. Damage to crops, trees,
lakes, rivers, and animals can result.
- How People Monitor Environmental pH
Scientists use instruments like Secchi disks, probes, nets, gauges, and meters to determine
how fit the water is. They take measurements of the physical and chemical condition of the water
and the fitness of the animals that live in it.
Scientists collect water in a lot of different ways. They use boats to go out in the middle
of lakes, they wade into streams wearing rubber boots that go up to their chests, they drop
buckets over the sides of bridges—they’ll do almost anything to get a sample.
Water samples aren’t the only things scientists collect. They take photographs from
airplanes and even satellites. They observe what’s happening along streams, lakes, and bays to
get an overall sense of the health of the water. They also collect fish, plants, dirt, and aquatic
bugs, and study what’s happening on the land that’s next to the water.
Scientists measure pH to determine the concentration of hydrogen in the water (The p
stands for “potential of” and the H is hydrogen.) pH ranges from 0 (very acidic) to 14 (very
basic), with 7 being neutral. Most waters range from 6.5 to 8.5. Changes in pH can affect how
chemicals dissolve in the water and whether organisms are affected by them. High acidity can be
deadly to fish and other aquatic organisms.
-What is being done to Reduce Acid Rain?
Regulations and new technologies are helping reduce acid rain.
EPA’s Acid Rain Program
Power plants generate the electricity we use every day. Unfortunately, power plants also produce
large amounts of nitrogen oxides and sulfur dioxide—the pollutants that cause acid rain—when
they burn fossil fuels, especially coal, to produce energy. Congress passed a law called the Clean
Air Act Amendments of 1990, and this law said that EPA should start the Acid Rain Program.
The program limits the amount of sulfur dioxide that power plants can release into the air and
issues allowances to the power plants to cover their sulfur dioxide emissions. It also reduces the
amount of nitrogen oxides that power plants can release.
Reducing Pollution
Scientists have found different ways to reduce the amount of sulfur dioxide released from coalburning power plants. One option is to use coal that contains less sulfur. Another option is to
“wash” the coal to remove some of the sulfur. The power plant can also install equipment called
scrubbers, which remove the sulfur dioxide from gases leaving the smokestack. Because nitrogen
oxides are created in the process of burning coal and other fossil fuels, some power plants are
changing the way they burn coal.
Other Sources of Energy
A great way to reduce acid rain is to produce energy without using fossil fuels. Instead, people
can use renewable energy sources, such as solar and wind power. Renewable energy sources help
reduce acid rain because they produce much less pollution. These energy sources can be used to
power machinery and produce electricity.
Cleaner Cars
Cars and trucks are major sources of the pollutants that cause acid rain. While one car alone does
not produce much pollution, all the cars on the road added together create a lot of pollution.
Therefore, car manufacturers are required to reduce the amount of nitrogen oxides and other
pollutants released by new cars. One type of technology used in cars is called a catalytic
converter. This piece of equipment has been used for over 20 years to reduce the amount of
nitrogen oxides released by cars. Some new cars can also use cleaner fuels, such as natural gas.
Cars that produce less pollution and are better for the environment are often labeled as low
emissions vehicles.
- The Consequences of Failing to Control the Problem
Acid Rain Damages Lakes and Streams
Without pollution or acid rain, most lakes and streams would have a pH level near 6.5.
Acid rain, however, has caused many lakes and streams in the northeast United States and certain
other places to have much lower pH levels. In addition, aluminum that is released into the soil
eventually ends up in lakes and streams. Unfortunately, this increase in acidity and aluminum
levels can be deadly to aquatic wildlife, including phytoplankton, mayflies, rainbow trout, small
mouth bass, frogs, spotted salamanders, crayfish, and other creatures that are part of the food
web. This problem can become much worse during heavy downpours of rain or when the snow
begins to melt in the spring. These types of events are known as episodic acidification.
Although the ocean's ability to take up carbon dioxide prevents atmospheric levels from
climbing even higher, rising levels of carbon dioxide dissolved in the ocean can have a negative
effect on marine life. Carbon dioxide reacts with sea water to produce carbonic acid. The
resulting increase in acidity (measured by lower pH values) reduces the availability of minerals
such as aragonite, which is a form of calcium carbonate. Declining pH and reduced availability
of minerals can make it more difficult for animals to thrive. This can lead to broader changes in
the overall structure of ocean and coastal ecosystems, and can ultimately affect fish populations
and the people who depend on them.
-What Is Happening Now
The ocean plays an important role in regulating the amount of carbon dioxide in
the atmosphere. As atmospheric concentrations of carbon dioxide rise, the ocean absorbs more
carbon dioxide; thus causing an increase in acidity levels. Because of the slow mixing time
between surface waters and deeper waters, it can take hundreds to thousands of years to establish
this balance. Over the past 250 years, oceans have absorbed approximately 40 percent of the
carbon dioxide produced by human activities.
Measurements made over the last few decades have demonstrated that ocean carbon dioxide
levels have risen in response to increased carbon dioxide in the atmosphere, leading to an
increase in acidity levels. Historical modeling suggests that since the 1880s, increased carbon
dioxide has led to lower aragonite saturation levels (less availability of minerals) in the oceans
around the world. The largest decreases in aragonite saturation have occurred in tropical waters.
However, decreases in cold areas may be of greater concern because colder waters typically have
lower aragonite levels to begin with.
This experiment focuses on surface waters, which absorb carbon dioxide from the
atmosphere within a few months. It can take much longer for changes in pH and mineral
saturation to spread to deeper waters, so the full effect of increased atmospheric carbon dioxide
concentrations on ocean acidity may not be seen for many decades, if not centuries. Studies
suggest that the impacts of ocean acidification may be greater at depth, because the availability
of minerals like aragonite is naturally lower in deeper waters.