In one of the most well-known scenes from The Simpsons (specifically, the beginning of episode 276 of season 13 titled “Brawl in the Family”), the inhabitants of Springfield flee in panic trying to avoid a caustic rain that corrodes their clothes and burns their skin. One of the characters who fares the worst is groundskeeper Willie, who ends up half-naked and writhing in pain on the ground because of the terrible rain. At the same time, Homer Simpson is left unable to watch television, as the acid rain literally disintegrates the antenna.
The TV series was reflecting a severe environmental disaster, a phenomenon capable of seriously damaging ecosystems, living beings, and our buildings: acid rain. What is it? How does it form? What are its effects? Do you want to know everything about acid rain? Let’s dive in!
What is acid rain?
This term, which is the most commonly used, was coined by Robert Angus Smith in 1872 to describe the acid precipitation falling near the industrial city of Manchester. This British chemist was one of the first to draw attention to the environmental dangers of acid rain.
A more technical term that can be found in specialized publications is “acid deposition.” This term is more accurate since it encompasses not only acid rain but also acid snow, acid hail, acid fog, and acid dust. In other words, the concept of acid deposition includes any type of atmospheric precipitation that contains high concentrations of acidic components, primarily sulfuric acid (H2SO4) and nitric acid (HNO3).
How is acid rain distinguished from ordinary rain?
The best way to determine if the rain is acidic or normal is by measuring its hydrogen potential or pH, which indicates the concentration of H+ ions. Remember that the pH scale ranges from 0, representing an extremely acidic substance, to 14, representing a very basic or alkaline substance. Conversely, if a substance has a value around 7, like pure water, it is considered neutral.
Normal rain typically has a slightly acidic pH, ranging from 5 to 5.5. This is due, among other things, to atmospheric CO2 reacting with water to form carbonic acid (H2CO3), which lowers the pH of the rain. In contrast, the pH of acid rain is significantly lower, usually between 4.2 and 4.4.
How is acid rain generated?
To produce acid deposition, the presence of certain ingredients in the atmosphere is necessary, specifically those that serve as the base for the formation of H2SO4 and HNO3. These ingredients are sulfur dioxide (SO2) and nitrogen oxides (NOx).
Our planet has been naturally producing these molecules for eons. For example, volcanoes are prolific generators of SO2 and NOx. Oceans and biological decomposition also produce millions of tons of NOx. Thus, acid deposition is a natural phenomenon. However, like many other natural phenomena such as climate change or wildfires, its frequency and intensity have alarmingly increased due to human activities.
Through the burning of fossil fuels in industry and transportation (for instance, coal is especially rich in sulfur), humans emit enormous quantities of SO2 and NOx, sometimes surpassing natural sources. It is estimated that one-quarter of all NOx reaching the atmosphere comes from fossil fuel combustion. On a global scale, human emissions of SO2 are similar in magnitude to natural emissions. However, on a regional scale, anthropogenic emissions can substantially exceed natural ones, as seen in eastern North America and northern Europe, where 90% of emissions are of human origin.
Once in the atmosphere and with the help of sunlight, these gases (SO2 and NOx) react with oxygen and water, forming the aforementioned acids. When the temperature drops sufficiently, the water vapor condenses and mixes with the acids, forming acid rain.
Where does it occur most frequently?
Considering the above, it can be deduced that the places most affected by this phenomenon are those with a higher degree of industrialization. In fact, Europe, China, and eastern North America hold the dubious honor of recording the most acidic rainfall pH levels, as can be seen in the following map.
Unfortunately, this is not a problem restricted to the most industrialized regions. Air currents can easily carry human emissions across country borders and reach areas far from the original emission point. For example, it is known that a significant portion of acid deposition in Scandinavian countries originates from emissions in central Europe and the United Kingdom. Similarly, Canadian emissions contribute considerably to acid deposition in the eastern United States, while large amounts of sulfur emitted by the U.S. end up precipitating over Canada.
What are the consequences for the environment?
Acid deposition can have highly detrimental impacts on aquatic and terrestrial ecosystems and biodiversity. One of the most well-known effects is the loss of soil nutrients. Soil acidification stimulates the exchange between the H+ ion and nutrients bound to soil particles, such as potassium, magnesium, and calcium. Once released, these nutrients are lost as they dissolve in rainwater and are carried away by runoff, reducing soil fertility. This, in turn, can decrease plant growth, alter the plant communities of affected ecosystems, and, consequently, disrupt the functioning of these ecosystems.
In other cases, what is mobilized are not nutrients but heavy metals that can be toxic to life, such as aluminum, iron, zinc, cadmium, and mercury. These metals can contaminate nearby aquatic ecosystems through runoff from rain, or via underground water currents and aquifers that feed these ecosystems. The increased toxicity caused by these elements can severely harm aquatic and terrestrial organisms and alter food chains.
Another ecosystem process that can be affected by acid deposition is the decomposition of plant debris by terrestrial and aquatic microorganisms, which is crucial for maintaining nutrient cycle stability in both types of ecosystems. Acidification of water or soil can inhibit the metabolism and enzymes of microorganisms, reducing or halting their decomposing activity.
Acid deposition can corrode plant organs, weakening them and making them more susceptible to diseases and adverse environmental conditions, such as droughts or extreme temperatures. This damage negatively impacts their growth, development, and fertility. If the damage from acid deposition is widespread across the plant community, forests and crops can become severely degraded.
In aquatic ecosystems, a substantial drop in pH due to acid deposition can deeply degrade aquatic communities, which are highly sensitive to changes in this parameter, and, consequently, affect the functioning of these ecosystems and the ecosystem services they provide. In such cases, various studies have reported concerning increases in mortality, reproductive failures, and heavy metal accumulation in fish, as well as behavioral changes that affect the survival of aquatic animals and drastic reductions or complete disappearances of entire populations of invertebrates, macrophytes, and phytoplankton.
How does acid rain affect us?
All these ecological disturbances associated with acid deposition negatively impact our welfare. For example, heavy metals released from the soil can contaminate our drinking water supplies. Additionally, we can ingest these metals through our diet when aquatic and terrestrial plants and animals that are part of our food chain accumulate them in their bodies.
Acid deposition can also directly harm our health if exposure is prolonged, although not as dramatically as depicted in The Simpsons episode. Precipitated acids can irritate our skin, eyes, and mucous membranes, and breathing in high concentrations of these acids can lead to respiratory issues.
Our buildings and structures also suffer from the effects of acid deposition, especially those constructed from marble, limestone, or sandstone. These materials erode rapidly when exposed to acids, jeopardizing the integrity of infrastructures and cultural heritage.
What can we do?
The increasing frequency and intensity of acid deposition is another facet of atmospheric pollution linked to human activities. Therefore, the best solution to address acid deposition is to reduce the burning of fossil fuels and transition to more sustainable and environmentally friendly energy sources. Evidence shows that in regions where the incidence of this phenomenon has been reduced, both aquatic and terrestrial ecosystems have managed to recover to some extent, with some of the lost biodiversity returning to its niche.
As is often the case, the well-being of our environment —and by extension, our own—depends on our actions. It is crucial to take action before reaching the point of no return.
References
Fischer K (2023). What is acid rain? [online] November 15. Available in: https://www.webmd.com/lung/acid-rain-what-is-it
Richardson GM, Egyed M, Currie DJ (1995). Does acid rain increase human exposure to mercury? A review and analysis of recent literature. Environ. Toxicol. Chem. 14, 809-813. https://doi.org/10.1002/etc.5620140510
Rodhe H, Dentener F, Schulz M (2002). The global distribution of acidifying wet deposition. Environ. Sci. Technol. 36, 4382-4388. https://doi.org/10.1021/es020057g
Rodríguez I, Ortiz A, Caldevilla P, Giganto S, Búrdalo G, Fernández-Raga M (2023). Comparison between the effects of normal rain and acid rain on calcareous stones under laboratory simulation. Hydrology 10, 79. https://doi.org/10.3390/hydrology10040079
Schindler DW (1988). Effects of acid rain on freshwater ecosystems. Science 239, 149-157. https://doi.org/10.1126/science.239.4836.149
Singh A, Agrawal M (2008). Acid rain and its ecological consequences. J. Environ. Biol. 29, 15-24. https://pubmed.ncbi.nlm.nih.gov/18831326/
United States Environmental Protection Agency (2024). What is acid rain? EPA [online] May 7. Available in: https://www.epa.gov/acidrain/what-acid-rain
