Acid rain, or acid deposition, is defined or a term that includes any form of precipitation with acidic components, like sulphuric or nitric acid, that falls from the atmosphere in wet or dry forms, including rain, snow, fog, hail and dust. 

What Causes Acid Rain?

Acid rain is a result of sulphur dioxide (SO2) and nitrogen oxides (NOX) being emitted into the atmosphere and transported by wind and air, making it a problem for all, not just those who live near the sources of the pollution.  Acid rain consists of sulphuric and nitric acids, formed when aforementioned gases mix and react with water, oxygen and other chemicals, which is then mix with water again and other materials before falling to the ground. 

While a small portion of the SO2 and NOX that cause acid rain comes from natural sources such as volcanoes and rotting vegetation, most of it comes from human activities, like the burning of fossil fuels, through electricity generation, oil refineries and vehicles. Over the past few decades, humans have emitted so many different chemicals that the mix of gases in the atmosphere has been changed. 

As stated, acid deposition can occur in wet and dry forms. When wet, it is what we most commonly think of as acid rain. This falls as rain, snow, fog or hail. Dry deposition occurs when acidic particles and gases may deposit on surfaces (water bodies, vegetation, buildings) quickly or may react during atmospheric transport to form larger particles that can pose a risk to human health. When these accumulated acids are washed off the surface by the next rain, the acidic water flows over and through the ground and could be harmful to plants and wildlife. The amount of acidity in the atmosphere that occurs through dry deposition depends on the amount of rainfall an area receives. 

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What are the Effects of Acid Rain?

While sulphur dioxides actually have a cooling effect on the atmosphere, nitrogen oxides contribute to the formation of ozone, which can be harmful to people and plants. Both of these gases are causes for concern because they can spread easily via air pollution and acid rain. 

Acid rain makes waters more acidic, posing a risk to lakes, streams, wetlands and other aquatic environments. Increasing a body of water’s acidity results in more aluminium absorption from soil, which is then transported to lakes and streams. This makes waters toxic to crayfish, clams, fish and other aquatic animals. 

While some species can handle acidic waters better than others, in connected ecosystems, there will be a domino effect throughout the food chain, whereby one species dying off because it cannot tolerate acidic rain will leave another without food, killing that species off as well.

Acid rain and fog also damage forests, especially those at higher elevations. The acid deposits rob the soil of nutrients such as calcium and cause aluminium to be released into the soil, which makes it difficult for trees to suck up water. The results are less healthy trees and planets that are more vulnerable to cold temperatures, insects and disease. Additionally, the trees’ ability to reproduce is compromised. 

Physical structures such as limestone buildings and cars can also be affected by acid rain. When it takes the form of fog, it can be inhaled and cause health problems, like eye irritation and asthma. 

How Do We Measure It?

Acid rain is measured using a pH scale for which 7.0 is neutral. The lower a substance’s pH (less than 7), the more acidic it is; the higher it is (greater than 7), the more alkaline it is. 

Normally, rain has a pH of about 5.6, making it slightly acidic because CO2 dissolves into it, forming a weak carbonic acid. However, normal precipitation reacts with alkaline chemicals that can be found in air, soils, bedrock, lakes and streams, the reactions of which usually neutralise natural acids. Acid rain has a pH of between 4.2 and 4.4. 

When acid deposition is washed into lakes and streams, it can cause some to turn acidic. 

How Do We End It?

Unfortunately, the only way to reverse the occurrence of acid rain is to curb the release of the pollutants that cause it, meaning that we need to burn fewer fossil fuels and set effective air quality standards. 

In the US, the Clean Air Act of 1990 implemented pollution limits that helped cut SO2 emissions by 88% between 1990 and 2017. Additionally, air quality standards have also lowered nitrogen dioxide emissions by 50% in the same time period. 

Landscapes that have been affected by acid rain can recover from its damage, however it takes time.