What is acid rain and why is it dangerous

Acid rain is rain and other precipitation (snow, hail, and even fog) with a high acidic oxide content caused by air pollution from anthropogenic and natural sources. When we were young, many feared acid rain would eventually become part of everyday life. In this lesson of the Windy.app Meteorological Textbook (WMO) for better weather forecasting you will learn what acid rain is, about its main dangers, and why it has disappeared entirely in some places, while in others, it will not go any where any time soon.

The power of hydrogen

As you remember, everything we see consists of tiny particles, called molecules. Molecules consist of atoms, and atoms are made of even smaller particles, the "most important" of which is the proton. The proton, in other words, is the nucleus of the simplest atom — hydrogen.

Acids are solutions in which molecules give away their protons (hydrogen) more often than they take them from other molecules. The more actively a substance gets rid of its protons when dissolved in water, the higher the acidity of the resulting solution.

Acidity is measured by pH value, where pH stands for "power of Hydrogen", that is, the concentration of the "lost" hydrogen in the solution.

Acids are substances with a pH below 7. A neutral pH is equal to 7, and is the pH of absolutely clean water that consists exclusively of H2O molecules. The lower the pH, the higher the acidity. The pH of Coca-Cola, for instance, is around 3, and near zero pH levels can be found in nature, probably only in the geothermal waters of active volcanoes.

The acidity (pH) of common drinks. Valarya Milovanova / Windy.app

Water in nature is rarely neutral, as different contaminants shift its pH to one side of the scale or the other. Rain water, as a rule, has a pH of about 5.7. Carbon dioxide, prevalent in the atmosphere, adds to its acidity.

This value was considered the norm until, at the beginning of the 20th century, US scientists, during routine measurements, discovered rain with a pH between 2 and 3, which is close to the pHof stomach acid or lemon juice!

But where does all this acid in the clouds come from, and why was it only discovered now?

The three horsemen of the Acid Apocalypse

Acid rain happens in nature, most commonly after volcanic eruptions and thunderstorms. Still, the pH values observed by the first acid rain researchers were unexpected and seemed catastrophic.

You can only imagine how much they panicked, and how quickly they ran to the government for more research funds. Soon, they discovered that the main contributors to rain acidity were humans, not nature.

When carbon, sulfur, and nitrogen burn in the presence of oxygen, acidic oxides are formed: carbon dioxide, sulfur dioxide, and different nitric oxides. These gases, in turn, gather in the clouds and dissolve in rainwater, which then falls on the ground as acid. And it doesn't necessarily have to be acid rain; there's also acid snow, and even acid mist.

Oxides of sulfur and nitrogen make the most dangerous acid rains. In the middle of the 20th century, the vast majority of these substances were emitted into the atmosphere by heat power plants, petroleum processing plants, and cars. By their exhaust pipes, to be more precise, as exhaust smoke, along with carbon dioxide, also contains plenty of nitric oxides.

Coal power plants are one of the main culprits of acid rains. Chris Leboutillier / Unsplash

Coal power plants were, among other factors, responsible for acid rain in the US Midwest and Northeast. After a long struggle with corporations and their lobbies, by the end of the 20th century, scientists managed to convince the US government to introduce quotas on emissions containing sulfur and nitrogen. Similar measures were taken by other countries.

Overall, these tactics were successful: since the beginning of the 1990s, sulfur oxide emissions have dropped by 40%, and acid rains — by 65%. And the European Union, for instance, managed to reduce sulfur dioxide emissions by 70%.

But why are we still writing about acid rain if the worst is a thing of the past? The problem is that, for some parts of the globe, they are still part of the present and even the future.

Where and what should we be afraid of

Rain is usually considered acid if the pH of rainwater drops below 4.5.

Compared to the normal 5.7, it doesn't sound so terrifying, until we realize that every drop of 1 point in value on the pH scale represents a ten-fold increase in acidity. That means that the difference between a pH of 6 and 3 is 10*10*10, a thousand times increase in acidity!

The worst acid rain was registered before the introduction of quotas in the 1970s and 1980s, when rain-water acidity in North America could go down to 2.1. The current "worst" of acid rain is about 3.7 pH, and values comparable to those of the 20th century can only be found in countries with more relaxed ecological legislation, such as China, India, and Russia.

As a rule, a one-time acid rain doesn't particularly harm the ecosystem. However, suppose rain pH remains below 4.5 for some time. In this case, acid accumulates in rivers and other water bodies, killing fish and replacing nutrients from the soil, which is detrimental to crops and vegetation. And if acid falls from the sky particularly often, it can corrode stone and metal monuments and buildings.

Acid rain often does not fall in the country that caused it. Acid accumulates in the clouds, and they are constantly moving due to the wind. This way, Scandinavian countries get acid rain "made" in Western Europe, and Russia feels the consequences of emissions in Eastern Europe.

Trees die in Norilsk (Russia) due to acid rain and the poor ecological situation. Anqi Lu / Unsplash

Currently, Eastern Europe, the West Coast of the USA, and the Canadian Southeast are most affected by acid rain. Acid rain is also registered in China, Taiwan, and around Norilsk (Northern Siberia, Russia), where copper, nickel, and other metals are mined.

It's almost impossible to distinguish acid rain from rain with a normal pH value. Despite the scary name that fuels pure nightmares, acid rain doesn't melt everyday objects as it falls and doesn't even particularly influence people's health (not from a one-time exposure, anyway).

Increased concentration of sulfur oxide in the air, however, does promote lung disease, such as asthma and bronchitis. The recommendations here are simple and self-explanatory: try not to live next to coal, metal, and oil processing facilities and busy highways for extended periods.

In general, the situation with acid rain looks optimistic. This phenomenon has explicitly promoted the development of citizen science, and humanity has demonstrated that together we can solve even a challenging ecological problem. This means we can also tackle more significant issues, such as climate change.

Text: Jason Bright, a journalist and a traveller

Cover photo: Osman Rana / Unsplash