A review on acid rain: An environmental threat

Acid rain is one of the utmost serious environmental problems that arose due to air pollution. It also contributes to climate warming. It has a pH of less than 5.6. The main purpose of the review is to emphasize the causes of acid rain, its various effects, and different ways to control acid rain. Data for the last few decades on acid rain is summarized in this review. The primary cause of acid rain is the emission of Sulphur dioxide and nitrogen oxide. These environmental pollutants come from various activities of humans such as the combustion of fossil fuels, burnable waste in automobiles, and thermal power plants. These chemicals become acidic after mixing with the rainwater and precipitate in the form of rain, fog, and snow and exert various effects on the ecosystems, forests, plants, soil, buildings, monuments, and water bodies as well as on human health. Its effects can be reduced by decreasing the Sulphur dioxide and nitrogen oxide emission and by adding lime into lakes, streams, and soil which neutralize the acidity. Several legislations should also make to control acid rain. Its control is necessary, to maintain food sustainability, environmental sustainability, to save the biodiversity of earth and human wellbeing.


Introduction
To make life easier, human beings utilized various natural resources. They have made several facilities that use the energy of the earth's resources. Burning fuels like oil, natural gas, and coal generate this energy. On one hand, this development has made life easier but on the other side, it releases various dangerous substances into the environment which leads to pollution [1]. Industrialization, urbanization, and burning of fossil fuelsrease in the number of pollutants that lead to air pollution. Acid rain is one of the utmost serious problems of the environment [2, 3] that arose due to air pollution [4]. Any kind of precipitation which has acidic nature is called acid rain [5]. Acid rain has a pH of less than 5.6 [6-9]. Acid rain occurs when from various sources, emission of sulfur dioxide and nitrogen oxide take place then [10] these pollutants mix with the oxygen, carbon, and other chemicals and form acidic compounds like sulfuric acid and nitric acid in the atmosphere [11] which precipitate in the form of snow, fog or rain Until the 20 th -century, smith work was mostly forgotten when the studies began to interrelate the atmospheric sulfate and other chemicals constituents deposition with the air pollution near the metal smelter in Canada [25]. In the 1960s and early 1970s in Sweden, observations exposed that nitric oxide and sulfur dioxide emission cause acid rain [26, 27] later on it was also observed in North America [28]. Afterward, various environmental effects of acid rain on aquatic animals and plants were also recognized and it was identified that acidification in fishes was due to longrange transportation of pollutants of sulfur [29]. Acid rain was first reported in North America, Europe, and later on in Asia [30, 31]. In 1936, the first step was taken in London to control the acid rain. Though, the harshness of the problem increased after 1970 due to increase concentration of sulfur dioxide in the atmosphere which resulted from increased use of coal fuel. The acid deposition act in 1980 was passed by the US Congress after ten years of continuous National Acidic Precipitation Assessment Program (NAPAP). This increases the observation for dry deposition and acid rain effects on buildings, aquatic ecosystems, and monuments. In the mid-1990s it was obvious that acid rain also interacts with climate warming and ozone layer depletion which are other environmental problems . This deposition of acids may be dry or wet. Snow, rain, and fog are wet deposition. When air takes the acid chemicals into such an area that has wet weather then acid on the ground falls in the form of snow, fog, and rain. This acidic water flows through the ground and affects the diversity of animals and plants [33]. Acidic particles and gases are called dry deposition. Towards cars, buildings, homes, and trees, these acidic particles and gases are blown by wind which results in sticking of these chemicals with the material. Through the rainstorms, from the other surfaces and trees, these dry deposited particles and gases can be washed out. When this occurs, then running water mixes these acids with the acid rain and makes it more acidic [34, 35].

Causes of acid rain
The primary causes of acid rain are nitrogen oxide, Sulphur dioxide, and ozone. Emission of these Sulphur dioxide and nitrogen oxide pollutants causes acid rain [36]. These environmental pollutants come from various activities of humans such as the combustion of fossil fuels, burnable waste in automobiles, and thermal power plants. The pollutants of Sulphur come from various natural sources such as volcanic eruptions and oceans [37]. Human activities that produce Sulphur dioxide include iron and metallic ores smelting, burning of petroleum and coal, and other industrial processes. While the natural sources of nitrogen oxide pollutants include biological processes and lightening. Human activities which produce nitrogen oxide are power station, emission from industries and, vehicle exhausts. Sulphur in the atmosphere reacts with the hydrogen peroxide and form Sulphur dioxide, while oxidation of nitrogen dioxide produces nitric acid. When these chemicals mix with the rainwater in the atmosphere then fall on the ground in the form of acid rain [38, 39]. Effects of acid rain Acid rain occurs due to the emission of Sulphur dioxide and nitrogen oxide and exerts various effects on, and fall off, canopy cover is also decreased, crown dieback occurs and results in the complete death of trees in forests.

Physiological activities of animals
Various metabolic activities of animals are affected by acid rain. For example, brown trout is very sensitive to acid rain, and in females; its sexual behavior is extremely affected by it. The nest digging behavior in them is also inhibited by the acidity of water [57]. Due to acid rain, the normal function of the human body is also getting affected. Some troubles have also been seen in the immune system due to acid rain, after exposure to acidity, the level of antibodies decreases considerably in plasma [58].

Soil
Acid rain damages the chemistry of the soil [59, 60] and changes the soil quality [61]. Some harmless minerals such as aluminum and mercury are present in the soil to which plants cannot absorb when they react with the acids, then they become easily available to plants for absorption and change the soil biology and chemistry as well as cause harmful effects [62, 63]. They also harm the animals which feed on these plants. Acid rain denatures the enzymes of microbes in the soil and killed them due to their intolerance at low pH [60]. Emissions of greenhouse gas from the soil are altered by the acid rain by changing the composition and biomass of microbial community; as a result acid rain makes changes in the availability of nutrients and soil edaphic properties like pH etc. . These acids lower the pH of water bodies, due to which reproduction of plants and animals is also affected. These acids do not allow fishes to breathe due to which accumulation of heavy metals occur in the body and result in the death of fishes [73,74]. When the birds eat these poisoned fishes then chemicals also enter into their system and when other animals eat these birds then these heavy metals introduce into these animals. In the food chain, chemicals are introduced at each trophic level in this way and chemical concentration also increased at each level [

Human health
The appearance and taste of acid rain are just like clean water. It exerts indirect effects on human health. Acid rain causes the leaching of toxins from the soil, these toxins include Al, Mn, Fe, Pb, and Hg [97], which get dissolve in the soil and reach the groundwater, human drink this water [98] due to which various heavy metals accumulated in their bodies and result in headache, cough, throat and nose irritation. These toxins are also absorbed by the animals and plants, when humans ingest these toxins then kidney problems and damage in the brain occur. These toxins also lead to heart diseases as well as lung diseases such as bronchitis and asthma. It is very hazardous to swim in an acid lake or to walk in acid rain. Wind transport nitrate and sulfate particles present in the atmosphere which are inhaled during breathing and leads to cancer [99]. In Tokyo, skin and eye irritation have also been observed due to polluted droplets [100]. Acid rain and climate change Acid rain has an association with the climate. The emission of sulfur dioxide, nitrogen dioxide, and carbon dioxide causes acid rain. Carbon dioxide is a primary gas that also leads to the greenhouse effect. Human activities produce these chemicals of acid rain which reach the atmosphere, when their concentration becomes high then the temperature of the atmosphere increases and results in climate warming. This warming occurs at a global level. In this way, Acid rain gases also responsible for climate change [101].

Control of acid rain
Acid rain can be controlled through the following ways,

Liming
Liming is a process that is used to neutralize the acid by adding limestone into the lakes, water, and soils and reduces the lethal effects of heavy metals. This method can be applied to a specific area. Liming also allows the survival of the native population of fishes in ponds. The addition of lime reduces the damage to the water bodies. The pH of acidic water is raised by the addition of slacked lime, Caustic soda, limestone, and sodium carbonate [102]. In water bodies, liming enhances the water quality and also increases the productivity of plants and animals. It also restores various species. Liming is an exclusive process but is no real remedy. This process should be periodically repeated to keep effectiveness [103].

Reduce the emission of pollutants
Acid rain can be controlled by reducing the emission of pollutants such as Sulphur dioxide and nitrogen oxide which causes acid rain.

Policy intervention
In Europe and the United States, during the 1970s and 1980s, acid rain effects on various natural resources were a serious issue. In 1980, the environment protection agency in the state of Canada took the action to control the emission of chemicals which causes acid rain [104,105]. In 1990, Legislations to control acid rain effects and control program of acid rain was also introduced [106]. The major aim of this program was to reduce the emission of Sulphur dioxide and nitrogen oxide to achieve health benefits for the environment and the public [107].

Recommendations
To reduce Sulphur dioxide emission such kind of coal should be used which have less Sulphur. Coal should be washed before using it because washing will reduce the Sulphur contents. Clean fuels should be used. Natural gas for the burning should be used rather than coal because less Sulphur dioxide produces from the burning of natural gas. Gases that are leaving the smokestacks contain Sulphur dioxide, scrubbers devices should be used to remove Sulphur dioxide chemically from these gases. In industries, a dust collection system should be fixed to reduce the release of chemicals. To reduce emissions, vehicles should be maintained timely. The burning of fossil fuels generates electricity but leads to emissions of harmful substances that result in acid rain. To prevent these emissions, any other energy sources such as nuclear power and hydropower should be used to produce electricity in the replacement of fossil fuels. In this way, emissions can be controlled.

Conclusion
After the study, the literature of review concludes that acid rain is one of the serious environmental issues. Acid rain has a pH of less than 5.6. It occurs due to the emission of Sulphur dioxide and Nitric oxide which reacts with water and produces sulfuric acid and nitric acid which precipitate in the form of rain, fog, and snow. These environmental pollutants come from various activities of humans such as the combustion of fossil fuels, burnable waste in automobiles, and thermal power plants. Acid rain adversely affects forests, plants, buildings, monuments, aquatic systems, soil, and human health. Acid rain effects can be reduced by adding lime into lakes, streams, and soil which neutralizes the acid. Several legislations should also build to control the acid rain. The buffering capacity of acid and the mechanism which is involved in the plant system treated with simulated acid rain is still unclear and needs deeper study.    Inhibition of nitrogen fixation in alfalfa by arsenate, heavy metals, fluoride, and