Also called acid deposition, acid rain is an umbrella term referring to any kind of precipitation involving acidic constituents, such as nitric acid or sulfuric acid, which descend upon earth in different forms – including rain, fog, snow, hail and also dust. Contrary to common perception, acid rain not just affects areas notorious for polluting the air with harmful gases.

Since winds can blow nitrogen oxides (NOX) and sulfur dioxides (SO2) across regions, acid rain can be an issue for everyone, including regions that aren’t guilty of polluting the air with nitrogen oxide and sulfur dioxide. This is why acid rain cannot be ignored as a region-specific issue. By the way, acid rain doesn’t mean pure acids falling from the sky onto the ground.

The term “acid rain” first came into reckoning in the 1800s when the industrial revolution was in its beginning stages. In 1872, the term was coined by Robert Angus Smith, a Scottish chemist, who was referring to acidic precipitation happening in Manchester, England. However, acid rain grabbed public attention much later in the 1960s. In America, acidic precipitation was first detected officially only in 1972. In fact, during the 1980s, there was tension between the American and Canadian governments over sulfur dioxide and nitrogen dioxide emission controls.

Formation

The base for acid rain is laid when chemicals such as NOX and SO2 enter the atmosphere. When these chemicals elevate and come in contact with the cloud’s oxygen, oxidants, and water, nitric and sulfuric acid are formed. And these pour down as acid rain. When fossil fuels are burnt, sulfur dioxide is emitted. It then reaches the atmosphere and oxidizes to turn into sulfate ion. This sulfate ion merges with hydrogen atoms and transforms as sulfuric acid before descending upon earth as acid rain. This oxidation is usually more when the air is heavily polluted. The polluted air comprises other compounds such as ozone and ammonia that catalyze or accelerate the conversion of sulfur dioxide to sulfuric acid.  

Causes

There are different sources of NOand SO2 – fossil fuels (oil, coal, and natural gas) is just one source. Natural causes of acid rain is a volcano and also rotting vegetables that release gases into the air. However, the majority of NOand SO2 emission is courtesy man-made activities that entail vehicles, electric generators, oil refineries, manufacturing industries, etc. The majority of sulfur emanates from electric power stations, which are into making electricity, and also volcanoes and decaying vegetation. Vehicle exhausts are primary responsible for generating nitrogen oxides. Nitrogen oxides are also generated when a thunderstorm causes lightning, which results in the air getting extremely hot.

Other sources or causes of acid rain could be volatile organic compounds or VOCs. These chemicals comprise carbon that easily turn into gases or vapors. Solvents such as paint thinners and degreasers, and coal, gasoline, wood, and natural gas also give out VOCs.

Types

Acid rain can be wet or dry. The wet form is usually rain, fog, hail, or snow. The moist variant of acid rain happens when the acid traps itself in fog or cloud droplets. This is usually the case in coastal regions and at high altitudes.

Dry depositions are basically dust particles and gases turning acidic. Just like wet depositions, the dry variant can also be carried across distances by the wind. Dry depositions happen when the acidic chemicals are blown away to dry weather areas by air. As a result, the dry pollutants blend into the smoke or dust and head to the ground in the form of dry particles.

Both dry and wet depositions have the same impact on the objects they fall. However, humans are more vulnerable to dry depositions, since they are not visible to the naked human eye and can be inhaled unknowingly.

Measuring Acidity

The acidity of the rain can be measured with pH scale. On a pH scale, anything below 7.0 is acidic. The smaller the number, the more acidic the substance. Regular rain usually has a pH value of 5.6. It’s marginally acidic because of the carbon dioxide presence in the atmosphere that reacts with water to create carbonic acid. Acid rain, on the other hand, has a pH lower than 5, usually falling in the 4.2-4.4 range. The actual number could vary with the region.

Measuring dry depositions is difficult and costlier than measuring wet depositions, since there are different techniques employed for measurement. Also dry deposition is based on multiple factors, such as deposited or atmospheric pollutants’ characteristics, meteorological conditions (wind speed, solar radiation, surface wetness) relative humidity, air temperature, etc., and the traits of the surface where the deposition happens.

Impact

Acid rain is corrosive in nature and can erode stone buildings, statues, disturb aquatic life, kill trees, and turn fertile lands into barren areas. However, the impact is not immediate or the active corrosion process is not visible to the human eye. Strong acids would usually burn the skin when in contact and can also destroy metals. Acid rain isn’t so strong, which means it cannot burn skin. Pure lemon juice has a pH value of 2.3 but it doesn’t burn the skin upon contact. And acid rain has never been more acidic than lemon juice. However, acid rain does pose issues elsewhere.

Forest and Soil

As aforementioned, acid rain can seriously impact trees, soil, water and buildings. It’s believed acid rain impacts the speed with which trees grow. However, trees that grow slower than usual could also have other things to blame for their slow growth such as air pollutants, diseases, insects and drought. Acid rain could affect trees in different ways:

  • Robs the soil’s minerals and nutrients (such as magnesium and calcium) and also disturbs its natural pH balance, which are essential for the growth of trees. With the introduction of acid rain, the soil pH goes up, which means soil microorganisms that aren’t able to adapt to the pH changes get killed. Also, acid rain releases aluminum and other harmful substances into the soil, which harm plants.
  • Damages leaves by wearing away their waxy protective coating, thereby creating bottlenecks for proper photosynthesis. In other words, it becomes difficult for the trees to absorb minerals. Also, acid rain could cause leafy plants to hold higher quantities of potentially toxic materials such as cadmium. Tree barks are also hurt by acid rain.

All of these things work in combination to weaken the trees, increasing their susceptibility to insects, diseases, bad weather, etc. Not to mention, plants are not safe from the effects of acid rain too.

Aquatic Life and Acid Rain

Acid rain has a negative impact on aquatic life as well. In fact, lakes tend to be more susceptible, thanks to the lake beds’ low alkaline content. By the way, alkaline tends to neutralize the acid effect. Though acid rain cannot be completely blamed for disturbing or eradicating life under water since some fishes and water plants can survive acidic water fairly well, the impact is quite substantial nonetheless.

Water acidity could cause deformities in young fishes and also prevent proper hatching of eggs since pH levels lower than 5 disable most fish eggs from hatching. Even fishes that are not directly hurt by acid rain could feel the brunt, if the fishes they used to feed on got affected and extinct by the acidic water. Similarly, if the fishes that served as feed to other water animals happen to survive the acid water onslaught, it can increase in population and disturb the entire ecosystem, as a result.

Human-Made Structures

Every material on earth is naturally designed to erode or lose its shine with age if it’s at the mercy of nature constantly. In other words, wind, water, snow, and ice contribute to erosion mildly. But acid rain tends to increase the pace of erosion considerably. As a result, buildings, statues, cables, pipes, etc. wear off much quickly than they normally would upon exposure to acid rain. The building materials decay rapidly and the paint work becomes more prone to peeling.

Acid rain also wears down statues made from stone. Usually, acid rain tends to hurt structures made from sandstone or limestone the most, making them look much older than they actually are. These materials are equally susceptible to both wet acid rain and acidic gases. This is why heritage buildings and historic structures are put through restoration work time and again.

Human Life

As aforementioned, acid rain doesn’t have much impact on human upon skin-contact. However, it’s a different ball game when the particulates relating to acid rain are inhaled. These constituents could be fine sulfur oxide (SOx) and nitrates. They are capable of traveling far and coming in contact with the lungs when inhaled. Inhaling acid rain pollutants could cause asthma and bronchitis in kids. Similar health issues are also likely to affect individuals in their sixties, pregnant women, and people with a history of heart disease.

Solutions

Acid rain is a grave environmental issue. Thankfully, governments across the world have started to realize this and are finding ways to reduce their country’s dangerous emission levels. The emissions can be reduced by:

  • Finding a way to burn fossil fuels that doesn’t lead to much air pollution;
  • Fitting vehicles with catalytic converters that sip out hazardous chemical substances from exhaust gases;
  • Finding new, environment-friendly sources of energy to generate electrical power – such as wind energy, solar energy, geothermal energy, etc.
  • Encouraging people to use public transport by subsidizing fares and modernizing vehicles so that even the elite class don’t mind commuting by buses; and
  • Saving electricity and walking whenever possible.