Acid rain is a type of precipitation, such as rain or snow, that is unusually acidic due to the presence of pollutants like sulfur dioxide and nitrogen oxides in the atmosphere. These pollutants react with water vapor to form acidic compounds, posing environmental risks to ecosystems, soil, and water bodies.
Simply put, acid rain means rain that is acidic in nature due to the presence of certain pollutants in the air due to cars and industrial processes.
Acidity is determined based on the pH level of the water droplets by assigning it a number between 0 and 14, where 0 represents extreme acidity, and 14 represents superlative basicity (the opposite of acidity).
According to the EPA,
“Acid rain, or acid deposition, is a broad term that includes any form of precipitation with acidic components, such as sulfuric or nitric acid that fall to the ground from the atmosphere in wet or dry forms. This can include rain, snow, fog, hail or even dust that is acidic.“
Saying sulfates do not cause acid rain is the same as saying that smoking does not cause lung cancer.
~ Andrew L. Lewis, Jr.
Normal rainwater is slightly acidic with a pH range of 5.3-6.0 because carbon dioxide and water in the air react together to form carbonic acid, a weak acid. When the pH level of rainwater falls below this range, it becomes acid rain.
When these gases react with water molecules and oxygen, among other chemicals found in the atmosphere, mild acidic chemical compounds such as sulfuric and nitric acid are formed, resulting in acid rain.
Now, the effects of acid rains are very detrimental, ranging from the weathering of buildings and corrosion of metals to the peeling of paints on surfaces.
But then, what causes this type of rain?
Well, there are several reasons that can be attributed to it. One is erupting volcanoes. These volcanoes contain some chemicals that can cause acid rain.
The burning of fossil fuels is another reason and perhaps one of the major causes. The running of factories and automobiles due to human activities are a few other reasons behind the acidity we witness in downpours.
Presently, large amounts of acid deposition are witnessed in Southeastern Canada, Northeastern United States, and most of Europe, including portions of Sweden, Norway, and Germany. In addition, some amount of acid deposition is found in parts of South Asia, South Africa, Sri Lanka, and Southern India.
Forms of Acid Rain
There are two forms in which acid deposition occurs – wet and dry. Both are discussed below:
1. Wet Deposition
When the wind blows the acidic chemicals in the air to the areas where the weather is wet, the acids fall to the ground in the form of rain, sleet, fog, snow, or mist. It removes acid from the atmosphere and deposits them on the earth’s surface.
When this acid flows through the ground, it affects a large population of plants, animals, and aquatic life. The water from the drain flows into rivers and canals mixed up with seawater, affecting marine habitats.
2. Dry Deposition
If the wind blows the acidic chemicals in the air to the dry weather areas, the acidic pollutants slip into dust or smoke and fall to the ground as dry particles.
These stick to the ground and other surfaces, such as cars, houses, trees, and buildings. Dry deposition is just as prevalent as the wet type. In fact, almost 50% of the acidic pollutants in the atmosphere fall back through dry deposition. These acidic pollutants can be washed away from the earth’s surface by rainstorms.
Dry deposition was discovered way back in the 1800s during the Industrial Revolution. A Scottish chemist, Robert Angus Smith, was the first to discover this phenomenon in 1852 as a relationship between acid rain and atmospheric pollution in Manchester, England.
However, it gained public attention mainly in the 1960s. The term was coined in 1972 when the NY Times published reports about the climate change effects that started arising due to the occurrence of acid rain in the Hubbard Brook Experimental Forest in New Hampshire.
Causes of Acid Rain
Both natural and man-made sources are known to play a role in the formation of acid rain. But, it is mainly caused by the combustion of fossil fuels, which results in emissions of sulfur dioxide (SO2) and nitrogen oxides (NOx).
1. Natural Sources
The major natural causal agent for acid rain is volcanic emissions. Volcanoes emit acid-producing gases to create higher than normal amounts of acid rain or any other form of precipitation, such as fog and snow, to the extent of affecting vegetation cover and the health of residents within the surrounding.
Decaying vegetation, wildfires, and biological processes within the environment also generate acid-rain-forming gases. Dimethyl sulfide is a typical example of a major biological contributor to sulfur-containing elements in the atmosphere.
Lighting strikes also naturally produce nitric oxides that react with water molecules via electrical activity to produce nitric acid, forming acid rain.
2. Man-made Sources
Human activities leading to chemical gas emissions such as sulfur and nitrogen are the primary contributors to acid rain. The activities include air pollution sources emitting sulfur and nitrogen gases like factories, power generation facilities, and automobiles.
In particular, the use of coal for electrical power generation is the biggest contributor to gaseous emissions, leading to acid rain.
Automobiles and factories also release high scores of gaseous emissions daily into the air, especially in highly industrialized areas and urban regions with large numbers of car traffic.
These gases react in the atmosphere with water, oxygen, and other chemicals to form various acidic compounds such as sulfuric acid, ammonium nitrate, and nitric acid. As a result, these areas experience exceedingly high amounts of acid rain.
The existing winds blow these acidic compounds over large areas across borders, and they fall back to the ground as acid rain or other forms of precipitation. Upon reaching the earth, the acidic deposition flows across the surface, absorbs into the soil, enters lakes and rivers, and finally gets mixed up with seawater.
Effects of Acid Rain
Acid rain has significant effects on the world environment and public health.
1. Effect on The Aquatic Environment
Acid rain either falls directly on aquatic bodies or runs off the forests, roads, and fields to flow into streams, rivers, and lakes. Over a period of time, acids accumulate in the water and lower the overall pH of the water body.
The aquatic plants and animals need a pH of about 4.8 to survive. If the pH level falls below that, the conditions become hostile for the survival of aquatic life.
Acid rain’s tendency to alter pH and aluminum concentrations greatly affects pH concentration levels in surface water, thereby affecting fish and other aquatic life forms. At pH levels below 5, most fish eggs cannot hatch. Lower pHs can also kill adult fish.
Acid rain runoff from catchment areas into rivers and lakes has also reduced biodiversity as rivers and lakes become more acidic.
Species, including fish, plant, and insect types in some lakes, rivers, and brooks, have been reduced, and some even eliminated owing to excess acid rain flowing into the waters.
2. Effect on Forests
Acid rain makes trees vulnerable to disease, extreme weather, and insects by destroying their leaves, damaging the bark, and arresting their growth. Forest damage from acid rain is most evident in Eastern Europe — especially Germany, Poland and Switzerland.
3. Effect on Soil
Acid rain highly impacts soil chemistry and biology. It means soil microbes, biological activity, and soil chemical compositions, such as the pH, are damaged or reversed due to the effects of acid rain.
The soil needs to maintain an optimum pH level for the continuity of biological activity. When acid rains seep into the soil, it means lower soil pH, which damages or reverses soil biological and chemical activities. Hence, sensitive soil microorganisms that cannot adapt to changes in pH are killed.
High soil acidity also denatures enzymes for the soil microbes. On the same breadth, hydrogen ions of acid rain leach away vital minerals and nutrients such as calcium and magnesium.
4. Vegetation Cover and Plantations
The damaging effects of acid rain on soil and high levels of dry depositions have endlessly damaged high-altitude forests and vegetation cover since acidic fogs and clouds mostly encircle them.
Besides, the widespread effects of acid rain on ecological harmony have led to stunted growth and even death of some forests and vegetation cover.
5. Effect on Architecture and Buildings
Acid rain on buildings, especially those constructed with limestone, reacts with the minerals and corrode them away. This leaves the building weak and susceptible to decay. Modern buildings, cars, airplanes, steel bridges and pipes are all affected by acid rain. Irreplaceable damage can be caused to the old heritage buildings.
6. Effect on Public Health
When in the atmosphere, sulfur dioxide and nitrogen oxide gases and their particulate matter derivatives, like sulfates and nitrates, degrade visibility and can cause accidents, leading to injuries and deaths.
Acid rain does not directly affect human health because acid rainwater is too dilute to cause serious health problems.
However, the dry depositions, also known as gaseous particulates in the air, which in this case are nitrogen oxides and sulfur dioxide, can cause serious health problems when inhaled. Intensified levels of acid depositions in dry form in the air can cause lung and heart problems such as bronchitis and asthma.
7. Other Effects
Acid rain leads to weathering of buildings, corrosion of metals, and peeling of paints on surfaces. Buildings and structures made of marble and limestone are especially damaged by acid rain due to the reactivity of the acids in the rain and the calcium compounds in the structures.
The effects are commonly seen on statues, old gravestones, historic monuments, and damaged buildings. Acid rain also corrodes metals like steel, bronze, copper, and iron.
Solutions to Acid Rain
Some of these solutions include:
1. Cleaning up Exhaust Pipes and Smokestacks
Most of the electric power supporting the modern-day energy requirements comes from combusting fossil fuels such as oil, natural gas, and coal that generate nitrogen oxides (NOx) and sulfur dioxide (SO2) as the chief contributors to acid rain. Burning coal largely accounts for SO2 emissions, while NOx emissions are mostly from fossil fuel combustion.
Washing coal, use of coal comprised of low sulfur, and use of devices known as “scrubbers” can provide technical solutions to SO2 emissions. “Scrubbing”, also called flue-gas desulfurization (FGD) typically works to chemically eliminate SO2 from the gases leaving smokestacks.
It can eliminate up to 95% of SO2 gases. Power generation facilities can also shift to using fuels that emit much less SO2, such as natural gas, instead of burning coal. These methods are called emission reduction strategies.
Similarly, NOx emissions from automobile fossil fuel combustions are mitigated by using catalytic converters. Catalytic converters are fixed on the exhaust pipe system to reduce NOx emission. Improvement of gasoline that combusts cleaner is also a strategy for reducing the emission of NOx gases.
2. Restoring Damaged Environments
The use of limestone or lime in a process called liming is a practice that people can do to repair the damage caused by acid rain to lakes, rivers and brooks. Adding lime into acidic surface waters balances the acidity. It’s a process that has extensively been used, for instance, in Sweden, to keep the water pH at optimum.
Even though liming is an expensive method and has to be done repeatedly, it only offers a short-term solution at the expense of solving the broader challenges of SO2 and NOx emissions and risks to human health. Nevertheless, it helps to restore and allow the survival of aquatic life forms by improving chronically acidified surface waters.
3. Alternative Energy Sources
Besides fossil fuels, there is a wide range of alternative energy sources that can generate electrical power. These include wind energy, geothermal energy, solar energy, hydropower, and nuclear power.
Harnessing these energy sources can offer effective electrical power alternatives to fossil fuels. Fuel cells, natural gas, and batteries can also substitute fossil fuels as cleaner energy sources. As of today, all energy sources have environmental and economic costs and benefits. The only solution is using sustainable energy that can protect the future.
4. Individual, National/State, and International Actions
Millions of people, directly and indirectly, contribute to SO2 and NOx emissions. Mitigation of this challenge requires individuals to be more informed about energy conservation and ways of reducing emissions, such as turning off lights or electrical appliances when not using them, using public transport, using energy-efficient electrical appliances, and using hybrid vehicles or those with low NOx emissions.