What is Acid Rain? How Does it Impact Global Warming?

One of the most apparent effects of air pollution is acid rain that occurs when rain, snow, mists, or dust become abnormally acidic as a result of substances emitted into the air. The origin of it is also closely associated with the burning of fossil fuels, such as coal, oil, and gas, which produce sulfur dioxide (SO2) and nitrogen oxides (NOₓ). 

The same energy systems are also emitting massively huge quantities of carbon dioxide (CO2) and other greenhouse gases that contribute to global warming. 

In this way, acid rain and global warming are similar processes, but they have the same industrial roots and tend to aggravate or otherwise obscure each other in both complex and subtle interactions.

What Is Acid Rain?

Any precipitation or deposition of the air that contains a considerably lower pH than the normal rain, typically pH 5.2 or lower, is considered to be acid rain. Natural air exposure of pure water makes it slightly acidic (pH ≈ 5.6) due to the dissolution of carbon dioxide (CO2) in rainwater into weak carbonic acid. 

Nevertheless, as the human emission of sulfuric and nitric acid is dissolved in cloud droplets, the pH decreases drastically, turning the rain toxic to the ecosystem from an ecological and geochemical point of view.

In the past, it has been observed in such studies as the U.S. National Acid Precipitation Assessment Program (NAPAP) that anthropogenic emissions have the potential to reduce the pH of precipitation in highly contaminated areas to 4.2-4.4. 

This amount of acidity is sufficient to damage the ecosystems, corrode infrastructure, and alter the soil chemistry in the long-term.

How Is Acid Rain Formed?

Acid rain starts when SO2 and (NOₓ) are released into the atmosphere through power plant combustion of fossil fuels, refineries, smelters and road transport. These gases react in a sequence of oxidation and hydration processes in the atmosphere. 

Sulfur dioxide, e.g., is reduced to sulfuric acid (H2SO4), and nitrogen oxides are reduced to nitric acid (HNO3) in the presence of water and oxidants, e.g. ozone and hydroxyl radicals.

These acids are then washed away into cloud droplets or deposited onto dust particles as wet or dry deposition (rain, snow) and aerosols and particles of an acidic nature. 

The resulting joint effect is commonly referred to as acid deposition which may cover hundreds or even thousands of kilometers of distance from the original source of emission.

Primary Sources and Areas of Impact

In the present day, coal-fired power plants, industrial combustion processes, and road transport are the major sources of SO2 and (NOₓ) emissions especially in the rapidly industrializing regions. 

The Northeastern United States and Eastern Canada became some of the first regions to experience acid rain in the 20th century as the area winds would blow polluted coal-burning belts in the Midwest and Ohio Valley into sensitive forested and lake rich regions. 

Other cross-border movements in Europe led to the so-called acid rain crisis in Scandinavia and the Alps, whereby distant lakes and woods experienced pronounced reductions in fish stocks and tree well-being.

Countries in Asia like China and India have been under increased acid-deposition stresses as the growth of coal-based power generation and industrial activity has increased but the process has begun to moderate in certain areas owing to national emission-control courses. 

The North American and European experience of regulations has become a world template of how to deal with the acid-rain-precursor emissions and at the same time control climate change.

What are Environmental and Ecological Effects of Acid Rain?

Acid rain causes the lakes, streams and wetlands to become acidic and in the process releases the toxic aluminum (Al) in the soils and rocks around the wetlands. 

This aluminium can harm the gills of fish, cripple reproduction, and lower the numbers of vulnerable species, including brook trout, frogs, and salamanders. In waters that have become quite acidic, complete fish populations can vanish, with more low-PH-tolerant algae and invertebrates replacing them.

Effects on Forests and Soils

Acid deposition absorbs the needed nutrients such as calcium and magnesium in forests which are vital in plant growth and resistance to stress. 

Meanwhile, the dumping of aluminum into soil may poison tree roots, weaken root systems, and retard growth. The stressed trees will be susceptible to drought, pests and diseases, which would lead to a long-term decline of forests in seriously affected areas.

Building and Infrastructure Effects

Acid rain increases the rate of chemical weathering and corrosion of other materials like limestone, marble and steel. The erosion, pitting and discoloration on historic monuments, statues and buildings which have undergone continual exposure are observable in the course of time. 

The contemporary bridges, pipelines and industrial buildings also experience increased cost in maintenance because of the increased corrosion by the processes of acid-deposition.

Acid Rain and Global Warming: What is the Relationship?

The important correlation between acid rain and global warming is the fact that both of them are as a result of the burning of fossil fuels. 

The power plants, refineries and heavy industries that use coal do not only release SO2 and (NOₓ), but also produce vast amounts of CO2 which is the major greenhouse gas that contributes to global warming. 

In such a way, numerous areas that had always had an issue with acid rain were also a large source of climate-change drivers.

Is Acid Rain a Direct Cause of Global Warming?

The precipitated acidic rain, i.e. sulfuric and nitric acid is not a greenhouse gas itself, and it does not trap the heat in the atmosphere as effectively as CO2, methane (CH4), or nitrous oxide (N2O). 

Nonetheless, the industrial processes that produce the precursors of acid-rains nearly always emit greenhouse gases, and, therefore, limiting acid-rain emissions is often associated with the reduction of the climate-related effects.

READ| General Knowledge Quiz Question and Answers on Matter in Our Surroundings