Acid rain is often introduced with a simple formula: atmospheric pollutants react with water, forming acids that return to Earth in rain, snow, fog or dry particles. The chemistry is straightforward enough to write on a classroom whiteboard, but its environmental meaning is far more complex.
The commonly cited formula is:
SO₂ and NOₓ + atmospheric oxygen and water → sulfuric acid (H₂SO₄) and nitric acid (HNO₃)
These acids can lower the pH of precipitation and alter the chemistry of soils, rivers, lakes and groundwater. They can also mobilise toxic metals, damage aquatic ecosystems and increase the vulnerability of forests and freshwater species.
For a blog focused on water quality and persistent contaminants such as PFAS, acid rain offers an important lesson: pollution rarely stays where it is released. Air emissions can become water pollution, while changes in water chemistry can influence how other contaminants move and behave.
What is the acid rain formula?
“Acid rain” is not a single chemical reaction. It describes a group of atmospheric processes involving sulfur dioxide, nitrogen oxides, oxygen, water vapour and sunlight.
Sulfur dioxide, or SO₂, has historically come mainly from burning coal and oil, particularly in power stations, industrial facilities and metal smelting. Nitrogen oxides, commonly written as NOₓ, are produced during high-temperature combustion. Road traffic, aviation, shipping, power generation and industrial activity all contribute to NOₓ emissions.
In the atmosphere, these gases undergo oxidation. A simplified pathway for sulfur pollution is:
SO₂ + OH radicals → sulfur trioxide (SO₃)
SO₃ + H₂O → H₂SO₄
In reality, sulfur dioxide can also be oxidised in cloud droplets and atmospheric particles. The resulting sulfuric acid may remain dissolved in rainwater or become part of fine particulate matter.
Nitrogen pollution follows several pathways. A simplified representation is:
NO₂ + OH radicals → nitric acid (HNO₃)
These reactions are influenced by sunlight, temperature, humidity, cloud chemistry and the presence of other atmospheric compounds. The formula is therefore useful as an explanation, but it is not a complete description of what happens in the atmosphere.
Why normal rain is already slightly acidic
Pure water has a pH of 7, but natural rainwater is not pure water. As raindrops fall, they absorb carbon dioxide from the atmosphere. This creates carbonic acid:
CO₂ + H₂O ⇌ H₂CO₃
For this reason, uncontaminated rain typically has a pH of around 5.6. Acid rain is generally described as precipitation with a pH below this natural level, although definitions can vary depending on the scientific and regulatory context.
The pH scale is logarithmic. A drop from pH 5.6 to pH 4 represents a substantial increase in acidity, not a minor change. Each one-unit decrease corresponds to approximately ten times greater hydrogen ion concentration.
That distinction matters. A rain event with a pH of 4 is not simply “a little more acidic” than normal rain. It can place additional chemical stress on soils and surface waters, especially in regions with limited natural buffering capacity.
Where acid rain has the greatest impact
Acid deposition includes both wet deposition, such as rain and snow, and dry deposition, involving acidic gases and particles that settle directly onto land, buildings and vegetation.
The environmental effect depends on more than rainfall acidity alone. Several factors determine how serious the damage becomes:
- Bedrock geology: Limestone and other carbonate-rich rocks can neutralise acids. Granite and many other rocks provide much less buffering.
- Soil composition: Soils with limited alkaline minerals are more vulnerable to acidification.
- Rainfall patterns: Frequent precipitation can increase the total acid load delivered to an ecosystem.
- Pollution levels: Emissions of sulfur dioxide and nitrogen oxides vary by region and season.
- Water-body size and chemistry: Small, poorly buffered lakes may respond more rapidly than larger systems.
This explains why two lakes receiving similar rainfall can experience very different effects. One may remain relatively stable because its surrounding geology neutralises acidity. Another may undergo a dramatic decline in pH because its watershed has little chemical protection.
What acid rain does to freshwater ecosystems
Acidification changes the chemical conditions on which freshwater organisms depend. Some species are highly sensitive to lower pH, while others tolerate a wider range.
Fish eggs and young fish are often particularly vulnerable. Acidic water can interfere with reproduction, development and the availability of essential minerals. In some cases, declining pH allows toxic forms of aluminium to become more soluble. Aluminium can damage fish gill tissues, reducing their ability to absorb oxygen and regulate salts.
Acidification can also reduce biodiversity. Sensitive species may disappear first, followed by changes in the food web. A lake may still appear clear and clean, yet support far fewer insects, amphibians and fish than it did before its chemistry changed.
This is one reason visual inspection is not enough to assess water quality. A clear stream is not automatically a healthy stream. Monitoring pH, alkalinity, conductivity, dissolved metals and biological indicators provides a much more reliable picture.
Acid rain and soil health
Soils can temporarily absorb acidic inputs, but their buffering capacity is not unlimited. As acidity increases, essential nutrients such as calcium, magnesium and potassium may be displaced and leached away.
At the same time, aluminium and other metals can become more mobile. This creates a double problem: plants lose access to nutrients while their roots may be exposed to higher concentrations of potentially toxic metals.
Forests affected by acid deposition may become more vulnerable to drought, cold weather, pests and disease. Acid rain is rarely the only cause of forest decline, but it can weaken the systems that help trees withstand other pressures.
There is also a connection with soil and groundwater transport. When acidity changes mineral surfaces and organic matter, it can influence how chemicals attach to or detach from soil particles. This matters for a wide range of pollutants, including metals, pesticides and industrial chemicals.
How acid rain connects to PFAS and other water contaminants
PFAS are not created by acid rain, and the acid rain formula should not be confused with PFAS chemistry. PFAS are a large family of manufactured chemicals containing highly stable carbon-fluorine bonds. Their use has included firefighting foams, non-stick products, stain-resistant materials, textiles and industrial applications.
However, acid deposition and PFAS contamination share an important environmental principle: contaminants move through connected systems.
Air emissions can reach water through rainfall. Contaminated soils can release pollutants into rivers and aquifers. Wastewater discharges can transport persistent chemicals downstream. A substance released in one location may therefore affect communities and ecosystems far from the original source.
Changes in pH may also influence the behaviour of co-contaminants. The exact effect depends on the chemical, soil type, organic matter, mineral surfaces and surrounding water chemistry. PFAS transport is especially complex because different compounds behave differently. Short-chain PFAS are generally more mobile in water, while longer-chain compounds may interact more strongly with sediments and organic matter.
This does not mean that lowering pH automatically makes PFAS more dangerous or more mobile in every situation. It does mean that water-quality assessments should avoid looking at contaminants in isolation. Measuring only one chemical or one parameter can miss important interactions.
Does acid rain still matter today?
In many parts of Europe and North America, sulfur dioxide emissions have fallen substantially since the introduction of cleaner fuels, emissions controls and national air-quality regulations. The United Kingdom’s Clean Air Act and later European measures helped reduce the severe acid deposition associated with coal combustion in the twentieth century.
That progress is significant. Some lakes and soils have begun to recover, although recovery can take decades. In certain ecosystems, stored pollutants and depleted nutrients continue to affect water chemistry long after atmospheric emissions decline.
Acid deposition also remains a concern in regions experiencing rapid industrialisation, coal combustion, shipping activity or weak emissions controls. Nitrogen pollution remains particularly important because NOₓ contributes not only to acid deposition but also to ground-level ozone, particulate matter and nutrient enrichment.
Modern environmental monitoring must therefore track both historical recovery and emerging pollution sources. Cleaner air does not instantly produce fully recovered ecosystems.
How scientists measure acid deposition
Researchers do not rely on a single pH reading. They collect precipitation and analyse its chemical composition, often measuring:
- pH and electrical conductivity;
- sulfate, nitrate and ammonium;
- chloride and other major ions;
- alkalinity and buffering capacity;
- trace metals such as aluminium and lead;
- rainfall volume and the total deposition load.
The total amount of acidic material matters as much as its concentration. A small amount of highly acidic rain may have a different effect from repeated, moderately acidic rainfall over an entire season.
Scientists also examine long-term trends in lakes, streams and soils. Biological surveys can reveal ecological damage that a short-term chemical sample might miss. Changes in fish populations, aquatic insects, amphibians and plant communities often provide early evidence of environmental stress.
What can be done to reduce the problem?
The most effective response is to prevent sulfur dioxide and nitrogen oxide emissions at their sources. Key measures include:
- Replacing high-sulfur fuels with lower-emission energy sources;
- Improving energy efficiency in buildings and industry;
- Using flue-gas desulfurisation and other industrial emission controls;
- Reducing vehicle NOₓ emissions through cleaner engines and public transport;
- Expanding renewable electricity and reducing reliance on fossil fuels;
- Strengthening monitoring and enforcement across national borders.
Liming can sometimes help restore acidified lakes and soils by adding alkaline material. However, this is a treatment rather than a permanent solution. It must be carefully managed, repeated when necessary and combined with emissions reductions.
The same prevention-first principle applies to PFAS. Advanced water treatment, including activated carbon, ion exchange and high-pressure membrane technologies, can reduce PFAS concentrations in contaminated water. Yet treatment is only one part of the response. Identifying sources, restricting unnecessary uses, managing contaminated waste and monitoring vulnerable communities are equally important.
What the formula really teaches us
The acid rain formula is valuable because it makes an invisible process visible. Sulfur dioxide and nitrogen oxides may begin as emissions from a chimney or exhaust pipe, but atmospheric chemistry transforms them into acids that can enter water, soil and living organisms.
It also demonstrates why environmental health cannot be divided into neat categories. Air pollution becomes water pollution. Soil chemistry affects groundwater. Industrial policy influences aquatic biodiversity. Public health depends on all of these connections.
For readers concerned about PFAS and drinking-water safety, the message is practical: pollution prevention, comprehensive monitoring and transparent regulation matter more than any single filtration device or laboratory result. Understanding the chemistry is the first step. Protecting the systems connected by that chemistry is the work that follows.
Reliable information is available through the UK Environment Agency, the European Environment Agency, the US Environmental Protection Agency and the World Health Organization. Their data and technical guidance provide essential context for interpreting pH measurements, acid deposition trends and the wider consequences of chemical pollution.