The acid rain is wet or dry precipitation of substances which generate a pH below 5.6. This precipitation can be wet (diluted in rainwater) or dry (particulate or aerosol depositions).
The term “acid rain” was first proposed by the English researcher Robert Angus Smith in 1850, in the middle of the Industrial Revolution . The most abundant acids that are formed in the atmosphere are nitric and sulfuric by oxidation of natural or artificial pollutants.
The most relevant pollutants are oxides: NO2, NO3, SO2, whose natural sources are volcanic eruptions, forest fires and bacterial degradation. Artificial sources are gas emissions from the burning of fossil fuels (industrial activity and automotive traffic).
Acid rain causes negative impacts on the environment such as acidification of soils and waters, affecting living beings, including humans. Also, soils and water are contaminated with heavy metals, and eutrophication occurs in water bodies.
At the vegetation level, direct damage to the leaves occurs and plant growth is affected. In addition, acidification of the soil immobilizes nutrients and affects mycorrhizae (soil fungi). Similarly, buildings, machinery, monuments and works of art exposed to the elements are severely oxidized or eroded by the effect of precipitated acids.
To remedy the effect of acid rain, some essential measures can be taken such as protecting monuments and correcting acidification of soils and waters. However, the basic solution for acid rain is to reduce the emission into the atmosphere of chemical compounds that are the precursors of acid formation.
How acid rain is formed?
The phenomenon of acid rain begins with the emission into the atmosphere of chemical compounds that are precursors to the formation of acids. These compounds can be emitted by natural or artificial sources.
Natural sources include volcanic eruptions, vegetation fires, and ocean emissions. As artificial sources act industrial emissions, emissions from combustion motor vehicles or the burning of waste.
These sources emit various compounds that can generate acids in the atmosphere. However, the most important are nitrogen oxides and sulfur oxides.
Nitrogen oxides are known as NOx, and include nitrogen dioxide (NO2) and nitrous oxide (NO). For its part, sulfur oxide is SO2 or sulfur dioxide.
Tropospheric process and acids produced
The phenomenon of acid rain occurs in the troposphere (atmospheric zone that goes from the earth’s surface to a height of 16 km).
In the troposphere, air currents can carry these compounds over any part of the planet, making it a global problem. In this process, nitrogen and sulfur oxides interact with other compounds to form nitric acid and sulfuric acid respectively.
Chemical reactions can be carried out either on solid particles in suspension or in drops of water in suspension.
Nitric acid is formed mainly in the gas phase, due to its low solubility in water. For its part, sulfuric acid is more soluble in water, being the main constituent of acid rain.
For the formation of nitric acid (HNO3), nitrogen oxides react with water, with radicals such as OH (to a lesser extent with HO2 and CH3O2), or with tropospheric ozone (O3).
In the case of the production of sulfuric acid (H2SO4), the radicals OH, HO2, CH3O2, water and ozone also participate. Additionally, it can be formed by reacting with hydrogen peroxide (H2O2) and various metal oxides.
H2CO3 is formed by the photochemical reaction of carbon dioxide with atmospheric water.
HCl represents only 2% of acid rain, and its precursor is methyl chloride (ClCH3). This compound comes from the oceans and is oxidized by OH radicals to form hydrochloric acid.
Once the acidic compounds (nitric acid or sulfuric acid, and to a lesser extent hydrochloric acid) have been formed, they will precipitate.
Precipitation can be by deposition of the suspended particles in which the acidification reaction has taken place in the gas phase. Another way is that in the rain the condensed water where the acids were formed precipitates.
The natural acidity of rain is close to a pH of 5.6, although in some uncontaminated areas values of 5. These low pH values have been associated with the presence of acids of natural origin.
It is considered that depending on the pH level, the rain can be classified into:
a) Slightly acidic (pH between 4.7 and 5.6)
b) Medium acid (pH between 4.3 and 4.7)
c) Strongly acidic (pH less than or equal to 4.3).
If the rain has a concentration> 1.3 mg / L for nitrates and> 3 mg / L for sulphates, the contamination is considered to be high.
Acid rain is composed in more than two thirds of cases by the presence of sulfuric acid, followed in abundance by nitric acid. Other components that can contribute to the acidity of the rain are hydrochloric acid and carbonic acid.
Chemical reactions of acid rain
Formation of sulfuric acid (H2SO4)
The production of sulfuric acid can occur in the gas phase or in the liquid phase.
Only 3 to 4% of SO2 is oxidized in the gas phase to produce sulfuric acid. There are many routes for the formation of sulfuric acid from gaseous precursors, here the reaction of SO2 with tropospheric ozone is shown.
The reaction occurs in two stages:
1.- Sulfur dioxide reacts with tropospheric ozone, generating sulfur trioxide and releasing oxygen.
SO2 + O3 = SO3 + O2
2.- Then the sulfur trioxide oxidizes with water vapor and produces sulfuric acid.
SO3 + H2O = H2SO4
In the drops of water that will form the rain, sulfuric acid can be produced in several ways:
1.- SO2 dissolves in water generating sulfurous acid, and this is oxidized by hydrogen peroxide:
SO2 + H2O = H2SO2
H2SO2 + H2O2 = H2SO4 + H2O
2.- Photocatalytic mechanism: In this case metal oxide particles (iron, zinc, titanium) are activated thanks to the action of sunlight (photochemical activation) and oxidize SO2 generating sulfuric acid.
Nitric acid (HNO3) formation
Tropospheric ozone O3 produces the transformation of NO2 to HNO3 in a three-stage process:
1.- NO2 + O3 = NO3 + O2
2.- NO3 + NO2 = N2O5
3.- N2O5 + H2O = 2HNO3
Effects on the environment
Soil acidification and its effects on vegetation
The effect of acid rain on the soil varies depending on its composition. For example, soils of calcareous, basaltic and igneous origin have a greater capacity to neutralize acidity.
For their part, soils rich in quartz as an inert material are not capable of regulating the acid content. Thus, in soils where acid rain increases acidity, metal ions that are toxic to plants and animals are released and carried away.
A relevant case is the dissolution of aluminosilicates, which release aluminum ions that are very harmful to vegetation.
In general, the acidity of the soil reduces the availability of nutrients for plants. In addition, it promotes the release and washing of calcium, which causes deficiencies in plants.
Effect on aquifers and human health
In most cases, acid rain does not look or taste different from normal rain, nor does it create sensations on the skin. Its effects on human health are indirect, and it rarely causes skin damage due to extreme acidity.
One of the problems with acid rain is that by lowering pH values below 5, heavy metals are released and carried away. These pollutants such as aluminum and cadmium can enter underground aquifers.
If the water from these polluted aquifers passes into wells used for human consumption, it can cause serious damage to health.
Deterioration of buildings, monuments and materials
Calcareous type stones
Constructions, monuments and sculptures made with limestone or marble are severely affected by acid rain. This is quite serious, since many historical buildings and works of art are built with these materials.
In the case of limestone, acid rain causes dissolution of limestone and causes recrystallization of calcite. This recrystallization produces whitish tones on the surface.
In the specific case of rain with sulfuric acid, the phenomenon of sulfation occurs. Through this process, the rock surface is transformed into gypsum and CO2 is released.
Marble, although more resistant, is also affected by acid rain. In this case, the exfoliation of the stone occurs, which is why superficial layers of it are detached.
Other non-corrosive materials
In some buildings the structural deterioration is minor, but also with negative effects. For example, dry acid deposits make the walls dirty, thus increasing maintenance costs.
Acid rain causes corrosion of metals due to the phenomenon of oxidation. This causes huge economic losses, since structures, equipment, machinery and vehicles with metal parts are seriously affected.
Flora and fauna
Acid rain modifies the natural balance of aquatic and terrestrial ecosystems.
Plants and animals in lentic bodies of water
Lentic bodies of water are more susceptible to acidification, because they are closed ecosystems. In addition, the accumulation of acids in the water has negative consequences on the life it harbors.
Another consequence of acidification is the precipitation of nitrates through rain, which causes eutrophication in bodies of water. Excess nutrients reduce the available oxygen and adversely affect the survival of aquatic animals .
Another indirect negative effect is the entrainment of heavy metal ions from the terrestrial environment into water bodies. These ions are released into the soil by the action of hydronium ions when acidity increases.
Vegetation and nutrient availability
The most serious problems caused by soil acidification are the immobility of essential nutrients and the increase in toxic metals.
For example, aluminum and magnesium are released from soil particles by being replaced by hydrogen. Aluminum affects the structure and function of the roots and reduces the absorption of calcium essential for plants.
On the other hand, soil acidification causes damage to mycorrhizae (root-associated fungi), which are essential in the dynamics of the forest.
Direct damage to plants and animals
Sulfuric acid causes direct damage to leaves by degrading chlorophyll and producing chlorosis (yellowing of the leaf). In some species growth and production of viable seeds decrease.
Amphibians (frogs and toads) are particularly susceptible to the effects of acidity in water. Some damages are direct injuries and decreased defense against pathogens (especially skin fungi).
The bottom line for acid rain is to reduce emissions of acid precursor chemicals to the environment. The most important of these are sulfur and nitrogen oxides.
However, this has some difficulties, since it implies affecting the economic and development interests of companies and countries. For example, one of the main sources of sulfur dioxide is the burning of coal, which represents more than 70% of the energy in China.
There are some technological alternatives that can help reduce emissions. For example, in the industry the so-called “fluidized beds” incorporate absorbents (limestone or dolomite) that retain SO2. In the case of motor vehicles and combustion engines in general, catalytic converters comply also help to reduce SO2 emissions.
On the other hand, some countries have been implementing specific programs to reduce acid rain. For example, the United States developed the National Acid Precipitation Assessment Program (NAPAP). Among some of the measures contemplated by NAPAP is the implementation of the use of low sulfur fuels.
Another possible measure is the replacement of the fleet with electric cars to reduce both acid rain and global warming. However, although the technology exists to achieve this, pressure from the automotive and oil industries has delayed decisions in this regard. Other factors that influence are cultural elements related to the speed that a vehicle is expected to reach.
Apply acidity correction measures
In some cases, the pH of soils and waters can be increased by adding alkalis, for example by incorporating large amounts of lime. However, this practice is not feasible in very large areas of land.
There are various methods to protect or at least reduce the deterioration of the stone under the effect of acid rain. One of these methods is to wash it with steam or hot water.
Chemical agents such as hydrofluoric acid or ammonium bifluoride can also be used. Once washed, the stone can be sealed by applying special products that clog the pores, such as barium hydroxide.
Metal surfaces liable to corrode can be protected by coating them with a non-corrosive metal such as zinc.
For this, electrodeposition can be applied, or the metal structure to be protected can be immersed in the protective metal in liquid state .
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