A Critical Review of Acid Rain Causes, Effects, and Mitigation Measures.pdf

________________________________________________________________________
a
State Key Laboratory of Environmental Geochemistry, Institute of Geochemistry, Chinese Academy of
Sciences, Guiyang, Guizhou, 550002, China.
b
University of Chinese Academy of Sciences, Beijing 100049, China.
c
CAS Key Laboratory of Urban Pollutant Conversion, Institute of Urban Environment, Chinese Academy
of Sciences, Xiamen 361021, China.
d
School of Science and Engineering, Tongji University, Shanghai 200092, P. R. China.
e
School of Traffic and Transportation, Lanzhou Jiaotong University, Lanzhou, 730070, China.
f
Institute of Environmental Engineering and Building installations, Faculty of Environmental Engineering
and Energy, Poznan University of Technology, Berdychowo 4, 60-965 Poznan, Poland.
g
School of Civil Engineering, Fujian University of Technology, Fujian, 350108, P.R. China.
h
State Key Laboratory of Geomechanics and Geotechnical Engineering, Institute of Rock and Soil
Mechanics, Chinese Academy of Sciences, Wuhan 430071, Hubei Province, China.
*Corresponding author: E-mail: tgratien0@gmail.com;
Chapter 2
Print ISBN: 978-81-19102-88-4, eBook ISBN: 978-81-19102-85-3
A Critical Review of Acid Rain: Causes,
Effects, and Mitigation Measures
Gratien Twagirayezu a,b*
, Jean Claude Nizeyimana b,c
,
Olivier Irumva d
, Charles Ntakiyimana e
, Fasilate Uwimpaye f
,
Ritha Nyirandayisabye g
, Habasi Patrick Manzi b,c
and Theogene Hakuzweyezu b,h
DOI: 10.9734/bpi/npgees/v6/5127A
ABSTRACT
The acidity of rainwater has recently been revealed as a major environmental
danger to ecosystems and human health around the world. Therefore, it is very
important to know what causes acid rain, its effects, and what might be done
about it. This review is based on open-access papers with "acid rain" as a
keyword and the present situation of acid rain around the world. Typically, acid
rain appears in the form of tiny bits of dry material, snow, and fog that land on
Earth. Acid rain commonly has a pH between 4.2 and 4.4, whereas normal rain
has a pH of 5.6. The production of electricity, the usage of vehicles, agricultural
and industrial activities, etc., all contribute to wet and dry deposition. Acid rain
has effects on aquatic environments, animals, plants, soil, global warming,
vegetation cover, monuments, buildings, and health. The findings of this research
showed that optimizing the use of fossil fuels, transitioning to renewable energy
sources, limiting the use of fertilizers and pesticides, conserving energy, restoring
the environment, lowering consumption levels, convincing others, educating
people, and following regulations are sustainable ways of controlling acid rain. In
addition, collaborations between the scientific and policy communities can be
considered as a useful instrument. Finally, the issues of acid rain and air pollution
should be among the worldwide big projects that require everyone to reach the
Sustainable Development Goals.

Novel Perspectives of Geography, Environment and Earth Sciences Vol. 6
A Critical Review of Acid Rain: Causes, Effects, and Mitigation Measures
24
Keywords: Acid rain; sulphuric; air pollution; ecosystems; nitrogen; monitoring.
1. INTRODUCTION
Since the beginning of civilization, humans have used many of the earth's
resources to improve their standard of living. However, what makes it easier is
damaging the environment by releasing contaminants. Once rain reacts with
some air contaminants, including sulfur or nitrogen oxides, the water vapor
transforms into very diluted versions of sulfuric or nitric acids, which is renamed
"acidic rain" [1]. “Rain-containing acid has been falling for at least the last twenty
to thirty years. This happened more than a hundred years ago. Since the majority
of the globe commenced its process of industrialization many years ago, the
impacts of pollution have been a problem for all countries. Acid rain causes
extensive damage to aquatic habitats, including lakes, streams, and forests, as
well as the plants and animals that call these places home. Automobiles,
factories, and other sources of industrial and electrical waste are the primary
contributors to acid rain. The northeastern region of the United States is home to
some of the most acidic environments on the continent [2]. The huge number of
cities, the high population density, and the concentration of power and industrial
units in the Northeast are the primary contributors to this pattern of high acidity.
Moreover, every day, New Delhi adds 1,500 new, poorly controlled automobiles
to its roadways, so it should come as no surprise that the city is suffocating under
the weight of auto emissions. Acids that are carried in the air are hazardous to
one's health and may corrode limestone, marble, and metal [3]. Every year, air
pollution in Asia is responsible for the deaths of 2 million people. Acid rain can
still cause damage to the soil, especially in forests and lakes, even if they are far
away from any kind of industrial activity [4]. Acid rain occurs when pollutants are
blown into the troposphere, where they condense into an acidic state and
eventually rain back down to Earth (typically as rain or snow) [5]. Acid
precipitation causes the leaves of plants to burn, and it makes lakes so acidic
that fish and other forms of life cannot survive in them. Due to this, further
information regarding acid rain should be made publicly available.
Recently, acid rain emerged as a major issue that requires scientific and policy-
wise attention. Many people, particularly in the 1980s, believed that acid rain was
one of the most serious environmental threats at the time [6]. This perception
persisted for a period of time. Media attention was focused mostly on reports of
fish disappearing from Scandinavian surface waters and the loss of forest cover
across Europe. Indeed, even throughout the North American continent, there was
a significant amount of public and governmental attention focused on acid rain.
During the cold war, communication between East and West was minimal. The
Convention on Long-range Transboundary Air Pollution (often referred to as
CLRTAP; in this article, we call it the Air Convention) was endorsed in 1979 as a
treaty under the United Nations Economic Commission for Europe (UNECE).
Information on the specific regions and resources affected by acidic deposition,
the locations and mechanisms by which acid-derived emissions are converted
and transmitted, the consequences of acid deposition, and the urgency of taking
action to mitigate it have all been uncovered as part of this study's mission to

25
inform public policy [2]. Most of the time, we learn from our own experiences and
from the stories of people who were involved in science and politics during the
early decades of the event's history. Therefore, this review paper describes acid
rain in details, the factors that contribute to acid rain, and evinces how the issue
of acid rain has evolved into a primary concern for the environment in
industrialized nations. It then enumerates mitigation measures for acid rain.
2. OVERVIEW OF ACID RAIN HISTORY
Swedish scientist Svante Odén highlighted a new and dangerous environmental
hazard called acid rain in a deliberately provocative piece titled "An Insidious
Chemical Warfare Among the Nations of Europe" that appeared in the Swedish
newspaper Dagens Nyheter in October 1967. He attributed the considerable
reduction in the pH of precipitation and surface waters during the preceding
decade to Europe's rising sulfur dioxide emissions.
The discovery was brought to the attention of the Swedish government right
away, and a few weeks following Odén's paper, the minister of industry reported
the problem to the Organization for Economic Cooperation and Development
(OECD), although it did not gain any political attention at that point. Go ran
Persson, Sweden's representative raised the issue in the OECD's Air Pollution
Management Committee as well. Again, the message was met with suspicion,
and the committee agreed that sulfur dioxide was a local problem that could be
remedied by erecting towering stacks. It wasn't until Persson thought he was
about to "lose the case" that he "played his final card" and cited the Chinese
nuclear bomb trials' findings of transcontinental radiation transfer. After that,
there was a shift in thinking, and everyone at the conference concluded that acid
rain should be something that needed more investigation. The Organization for
Economic Co-operation and Development (OECD) and the western world
concluded that air pollution may be an issue with international political aspects at
this point.
The findings made by Odén were in considerable part based on the regional
precipitation networks that were operational throughout Sweden and Europe at
the time. Hans Egner, a Swedish scientist, established a network in 1947 to
explore the significance of atmospheric deposition concerning the fertilization of
crops. Egner, Carl Gustav Rossby, and Erik Eriksson had all contributed to the
expansion of the network into what is now known as the European Air Chemistry
Network (EACN) By 1954 [7,8]. Odén used the information gathered from these
networks and his own Scandinavian surface water network set up in 1961 to
conclude the rate at which acidification is progressing [9].
However, evidence of acid rain and many of its environmental consequences had
been gathered long before 1967–1968. An English scientist named Robert Angus
Smith first noticed many of the hallmarks of acid rain in the middle of the
nineteenth century! In the year 1852, Smith wrote and published a
comprehensive study on the chemical composition of the rain that fell in and
around the city of Manchester in England. Smith introduced the phrase "acid

26
rain" for the first time twenty years later, in a thorough book titled "Air and Rain:
The Beginnings of a Chemical Climatology," and enunciated many of the main
principles that are part of our current knowledge of this phenomenon [10].
Regrettably, on the other hand, the pioneering work of Smith was largely
disregarded by practically all of the investigators who came after him.
In the early 1900s, the number of salmon caught in Norway dropped by a lot. In
1927, Professor Knut Dahl suggested that the acidity of surface waters was a
major reason why fish were going extinct. Later, Alf Dannevig assumed that "the
acidity of a lake is dependent on the acidity of precipitation and the contribution
from the soil" [11]. From 1955 to 1963, Eville Gorham and his team of
researchers in England and Canada studied acid rain's origins and impacts on
agriculture, aquatic ecosystems, human health, and soils [12]. With the help of
Dahl and Dannevig, Gorham and his team have uncovered key components of
the reasons for modern alterations in the chemistry of atmospheric emissions
and deposition and their effects on aquatic ecosystems.
However, neither scientists nor the general public have generally acknowledged
these ground-breaking contributions, such as those Smith made a century ago.
This was the case with Smith's work as well. Gorham's studies, much like Smith's
studies from a century earlier, were received with what Gorham himself admitted
was a "thundering silence" not just from the scientific community but also from
the general population at large. In 1967 and 1968, Svante Oden wrote a
purposely inflammatory article in Dagens Nyheter and a well-documented
Ecological Committee Report, respectively, which brought the topic of acid rain to
the attention of the public and academics [9]. There was a lot of scientific and
policy-relevant information in the study that showed that people who were
moving away from the pollutant-emitting source regions were still having negative
ecological and environmental effects because of long-distance transportation and
deposits of acidifying contaminants. All of the major air pollutants have had their
emissions cut by a lot, and the most essential acidifying compound, sulfur
dioxide, has seen its emissions in Europe drop by at least 80% since around
1980 -1990 (Fig. 1).
3. MEASURING ACID RAIN
pH is a scale used to quantify acidity and alkalinity, with 7.0 being a neutral pH
value. In general, substances with a pH value less than 7 are considered acidic,
whereas those with a pH value over 7 are considered alkaline, as shown in Fig.
2. In most cases, carbon dioxide (CO2) dissolves in rainwater to form weak
carbonic acid, lowering the pH to 5.6. The typical pH range of acid rain is 4.2 to
4.4. Researchers, ecologists, politicians, and computer models all rely on the
data collected by the NADP's National Trends Network (NTN) to better
understand precipitation patterns in the atmosphere. The National Acid
Deposition Program/National Terrestrial Network monitors acid rain in over 250
locations in the United States, Canada, Alaska, Hawaii, and the United States
Virgin Islands. Dry deposition, in contrast to wet deposition, is hard to quantify
and may be costly. The Clean Air Status and Trends Network (CASTNET) offers

27
estimates of the dry deposition of nitrogen and sulfur pollutants. Some lakes and
streams may become acidic if the acid deposition is allowed to wash into them.
The Long-Term Monitoring (LTM) Network collects data on the chemistry of
surface water at more than 280 sites across the world. This research helps
researchers gain important insights into the state of aquatic ecosystems and how
different bodies of water react to shifting levels of acid-causing emissions and
acid deposition.
Fig. 1. European emissions during 1880–2020 for SO2 ( ), NOx taken as
NO2 ( ), and NH3 ( ) (taken from the study of Schöpp et al.[13]).
Fig. 2. Representation of different acid-base levels of different species on
the pH scale
4. CLASSIFICATION OF ACID RAIN
Acid rain is sometimes known as "acid deposition" since it encompasses other
kinds of acidic precipitation, like snow. There are two forms of deposition, which
are explained below.
4.1 Wet Deposition
Wet deposition encompasses acidic precipitation, fog, and snow. Acids may
descend to the ground in the form of rain, snow, fog, or mist if airborne acid

28
compounds are pushed into regions with wet weather. Many species of flora and
fauna are threatened by the corrosive water that flows over and through the soil.
4.2 Dry Deposition
When moisture is not present, a process called dry deposition can occur, which
can lead to the formation of acidic particles and gases. Rapid deposition of acidic
particles and gases onto surfaces (water bodies, plants, or structures) is
possible, or they may react with one another in the atmosphere en route to form
larger particles that could be harmful to humans. There is concern that this acidic
water, as it moves over and through the earth, could be hazardous to aquatic
organisms and other life forms. The acidity of rainwater is caused by the
neutralization of the acids that have accumulated on a surface and are then
rinsed away by the rain. Rainfall in a region is directly correlated with the amount
of acidity in the air that falls to the ground as dry deposition. For instance, the
proportion of dry-to-wet deposition that occurs in desert regions is greater than
that that occurs in regions that get several inches of precipitation on an annual
basis.
5. CAUSES OF ACID RAIN
Acid rain has several potential origins, but the most common are natural, power
production, transportation, agriculture, industry, and high levels of consumption.
Fig. 3 shows how the release of sulfur dioxide (SO2) and nitrogen oxides (NOX)
into the atmosphere and their subsequent transport by wind and air currents
result in acid rain. When combined with water and oxygen, SO2 and NOX form
sulfuric and nitric acids, respectively. Then, they combine with water and other
substances before dropping to the earth. Only a small fraction of the sulfur
dioxide (SO2) and nitrogen oxides (NOX) that contribute to acid rain come from
naturally occurring sources like volcanoes; the great majority comes from the
combustion of fossil fuels. The combustion of fossil fuels for energy generation is
the primary contributor to atmospheric SO2 and NOX concentrations. Electric
power plants, transportation (cars, trucks, and buses), industry (factories, oil
refineries, etc.), and agriculture (livestock) are the primary sources of SO2 and
NOX emissions, respectively. The combustion of fossil fuels like coal and
petroleum, as well as many other types of industrial activities, are man-made
sources of SO2 emissions [14]. Smelting metal (zinc and copper) ores, creating
sulfuric acid, and running acid concentrators in the petroleum industry are other
significant contributors. Winds can transport SO2 and NOX over great distances
and international boundaries, making acid rain a danger for everyone, not just
those who live near the origins.

29
Fig. 3. Demonstrates the process by which acid rain occurs in our
environment: (1) SO2 and NOx emissions are discharged into the
atmosphere, (2) where the pollutants are turned into acidic particles that
may be carried over great distances. (3) These acidic particles then fall to
the ground as wet and dry deposition (dust, rain, snow, etc.), and (4) may
have negative impacts on soil, forests, streams, and lakes
(Source: https://www.epa.gov/acidrain/what-acid-rain)
Although NOx emissions are relatively modest compared to those of SO2, their
share in the production of acid rain is rising. pH, an abbreviation for "hydrogen
potential," is a measure of how acidic something is. The pH of regular rainfall is
also on the acidic side. This is because, as shown in Eq. 1, carbonic acid is
produced when water combines slightly with CO2 in the air. The acidity of typical
precipitation is affected by a trace amount of nitric acid, which is formed by the
oxidation of nitrogen in the water present during lightning storms (Eq. 2). When
the concentration of H
+
ions in the atmosphere rises over 2.5 Eq-1 and then
drops below 5.6 Eq-1, acid rain occurs [15]. Galloway et al. [16] suggested a pH
of 5.0 as the maximum value for natural contribution.
CO2 + H2O H2CO3 (1)

30
2N2 + 5O2 + 2H2O 4HNO3 (2)
5.1 Chemical Reactions during Acid Rain Formation
Acid rain is the product of a chemical reaction between sulfur dioxide (SO2),
nitrogen oxides (NOx), and oxygen (O3). Sulfuric acid and nitric acid mists are
created when molecules of SO2 and NOx in the air are carried by the wind and
react with vapors in the presence of sunshine. At the prevailing high
temperatures, these acids are still in their vaporous forms. When temperatures
drop, condensation forms aerosol droplets, which are black, acidic, and
carbonaceous because of the presence of unburned carbon particles. Reactions
involving O3 in acidic environments are shown in Eqs. (3) - (8).
O3
-
O2 + O (3)
O+H2O OH
•
(4)
OH
•
+ SO2 HSO3 (5)
HSO3
-
+ OH H2SO4 (6)
OH + NO2
-
HNO3 (7)
HSO3 + O2
-
SO3
2-
+ HO
•
2 (8)
Formic and acetic acids, as well as certain other organic acids, are produced
when peroxy radicals react with formaldehyde, acetaldehyde, and other organic
acids, increasing the acidity of rain by 5% to 20%. Eqs. (9) – (14) show acid
reactions involving sulfur. Sulfur is often found in rather high concentrations in
coal. When coal is burned, the components of the coal itself get oxidized. Sulfur
undergoes direct oxidation in the flame, resulting in the production of SO2, which
is then released into the atmosphere through the smoke stacks. At low enough
temperatures, SO2 slowly changes into SO3
2-
when it is transported through the
air by the dominating wind. The rate at which SO3
2-
is reduced to SO4 depends
on the availability of oxidants in the surrounding atmosphere. Sulfur dioxide is
oxidized in clouds and much-polluted air where ammonia and O3 levels are
excessively high. To produce more sulfuric acid, these catalysts are necessary.
S + O2 SO2 (9)
2SO2 + O2 2 SO3
2-
(10)
SO3
2-
+ H2O H2SO4 (11)
SO2 + H H2SO3 (H+ HSO3) (12)
HSO3 + O3 SO4
2+
+ H
+
+ O2 (13)

31
H2O2 + HSO3 -HSO4
-
+ H2O (14)
Acid reactions involving nitrogen are shown in Eq. (15)-(18).
NO2 + OH HNO3 (15)
NO2 + O NO3 (16)
NO2+ NO3 N2O5 (17)
N2O5+ H2O 2HNO3 (18)
6. EFFECTS OF ACID RAIN
Acid deposition has a wide range of serious consequences in natural and man-
made environments. Acid rain has effects on aquatic environments, animals,
plants, soil, global warming, vegetation cover, monuments, buildings, and health.
Dry and wet deposition wash into waterways from forests, farms, and roadways.
In reality, sulfur dioxide has a cooling impact on the atmosphere, so it is not one
of the principal greenhouse gases contributing to global warming, which is one of
the main causes of climate change. Yet nitrogen oxides are a key factor in the
production of ground-level ozone, a significant pollutant that may have negative
health effects on humans. Both gases are harmful to the environment and human
health due to their propensity to pollute the air and precipitate acid rain. Lakes,
streams, wetlands, and other aquatic ecosystems are particularly vulnerable to
the biological consequences of acid rain. Rain with an acidic pH dissolves more
metal in the soil, which eventually washes into waterways. The combination is
toxic to aquatic organisms such as crawfish, clams, fish, and others. Some
aquatic organisms do better under more alkaline conditions than others. But in a
linked ecosystem, the effects of one species trickle down the food chain to
others, including non-aquatic species like birds.
6.1 Effects of Acid Rain on Health
The look, consistency, and taste of acid rain are identical to those of clean rain.
Acid rain has no direct impacts on people but has indirect ones. Acid rain or
swimming in an acid lake pose no greater danger than doing the same things in
clean water. On the other hand, the pollutants that contribute to acid rain, such
as sulfur dioxide (SO2) and nitrogen oxides (NOx), are harmful to human health.
When these gases combine in the atmosphere, they produce minute particles of
sulfate and nitrate that may be carried over great distances by the wind and
breathed deeply into the lungs of individuals. Indoor air may also be
contaminated by fine particles. Numerous scientific investigations have shown a
correlation between rising levels of fine particles and a rise in the number of
cases of disease and premature deaths caused by heart and lung conditions,
including asthma and bronchitis.

32
6.2 Acid Rain Harms Other Plants
Scientists, foresters, and other experts have noticed that the development of
certain forests has slowed down over the years. When they should be green and
healthy, the needles and leaves on the pine tree turn brown and fall off. In the
most severe circumstances, individual trees or even large sections of the forest
might suddenly perish for no apparent cause. The damage caused by acid rain
on trees may also be passed on to other types of plants. Rain and fog that
contain acid can cause harm, particularly to woods located at higher altitudes.
For instance, in 1980, the German scientist Bernhard Ulrich issued a warning
that the deposition of sulfur into the atmosphere posed a significant risk to the
forests of Europe. It should come as no surprise that the acid deposits deplete
the soil of necessary elements like calcium. They also increase aluminum
production in the soil, which limits the ability of trees to take up water. An
extensive testing program in the Solling region led to the conclusion that heavy
pollution from the air had dramatically altered the soil's chemical composition
[17].
A variety of environmental challenges, like acid rain (Fig. 4), weaken trees and
plants, leaving them more vulnerable to things like frost, insects, and disease.
The trees' fertility may be hampered by pollution. Acid rain may also leach
aluminum and other soluble elements from the ground. As the dissolved
aluminum reaches nearby water sources like streams and marshes, it
accumulates and may eventually reach dangerous levels. Acid rain washes away
all of the minerals that plants need to thrive, including magnesium, calcium, and
potassium. Plant and animal life may be harmed by a shortage of nutrients.
Finally, a forest that lacks calcium and has too much aluminum may be more
vulnerable to pests, diseases, and damage from cold temperatures and drought.
Fig. 4. Damaged forests in Poland's Norway spruce are due to acid rain. By
dissolving soil nutrients before plants can utilize them, acid rain weakens
trees. Acid rain is created when raindrops absorb air contaminants like
sulfur and nitrogen oxides
(Source: https://www.nationalgeographic.com/environment/article/acid-rain)

33
6.3 Effects on Stone Buildings and Monuments
Marble and limestone have traditionally been considered the superior
construction materials to use for creating long-lasting structures and monuments.
Both marble and limestone are composed of calcium carbonate (CaCO3); the
main difference between the two is in the way their crystals are organized. In
comparison to marble, the crystals that make up limestone are smaller, and the
stone's porosity makes it more suitable for architectural applications. Because it
can achieve a high polish while having bigger crystals and fewer pores, marble is
the material of choice for use in the construction of monuments and sculptures.
Even though marble and limestone are known to be very long-lasting
construction materials, outdoor monuments and structures built of these
materials are now suffering from progressive erosion due to acid rain. The
reaction between calcium carbonate and sulfuric acid, the principal acid
component of acid rain, results in the dissolution of CaCO3 to provide aqueous
ions, which are then swept away in the water flow. Calcium carbonate
precipitation is a major cause of acid rain (Eq. 19). Generally, the structural
integrity of a building will not be compromised by acid rain, but the surface
characteristics of any relief work (such as the faces on a statue) may be
irreparably damaged.
CaCO3 +H2SO4 Ca2
+
(aq) + SO4
2-
+H2O+CO2 (19)
7. CRITICAL LOADS AND ADVANCED POLICIES
Critical loads are among the best-known features for controlling the acid rain
issue [18,19]. In 1988, the Air Convention's Executive Body (the convention's
highest decision-making body) decided that new discussions on the regulation of
sulfur and nitrogen emissions should be focused on critical loads, and so it
requested that all parties to the Air Convention develop their critical load maps.
The Netherlands took the initiative to take the lead, preparing mapping guides
and launching an international network, both of which proved to be essential for
the concept's acceptability in both the scientific and policy sectors [20,21] (Fig.
5). Integrated Assessment Models (IAMs) supplied a mechanism to assess how
to accomplish a mandated ecological impact to decrease most cost-effectively
when critical loads are now a foundation for future procedures. Although many
competing models were developed, the model developed at the International
Institute for Applied Systems Analysis (IIASA) was adopted as the basis for the
1994 Second Sulphur Protocol [22].
However, the idea could not be used in the same way as it had been for sulfur
and acid deposition in the process of amending or creating a new protocol for
nitrogen oxides. This is because a strategy would need to consider more
components. After all, nitrogen oxide emissions contributed to various effects.
Instead, a more nuanced method was presented that takes many effects and
substances into account at once [23]. The International Institute for Applied
Systems Analysis (IIASA) and other bodies under the Air Convention were
tasked with creating a more comprehensive integrated assessment model that

34
would allow them to consider the consequences of acidic deposition, nitrogen
deposition, and ozone all at once-the so-called multi-pollutant, multi-effect
approach. Measures to be taken to reduce emissions after 2010 were laid out in
both the GP and a similar European Union (EU) directive from 2001, known as
the National Emissions Ceilings (NEC) Directive.
Fig. 5. The result of emission control efforts between 1990 and 2010 for
SO2, NOx, and NH3 is provided as maps of acidity levels that surpass the
critical load threshold. These maps have been quite useful for illustrative
purposes, both in terms of the implications of potential future policies and
of past acts (taken from Maas et al. [24] and Grennfei et al. [25]
8. FUTURE CHALLENGES
Many people throughout the world applaud the efforts made to reduce air
pollution. Sulfur dioxide, nitrogen oxides, volatile organic compounds, and a few
other components have greatly reduced emissions, although this
accomplishment is mostly confined to Europe and North America and a few other
industrialized nations (including Japan and Australia) [24]. However, air pollution
is still a concern, even in places where it has been addressed as a high priority
for decades. Some of the negative effects on ecosystems that prompted the
convention to be established have been mitigated, but the acidification effects of
past emissions will continue to be felt for decades to come [26,27], and ammonia
emissions were only cut by 20-30% in Europe, and much less in North America.
When it comes to health implications, it is tough to speak about success when
hundreds of thousands of people on both continents are expected to die sooner
as a result of air pollution.
However, when one looks outside the typical industrialized world, the issue
becomes much broader and more pressing. Today's emphasis is on big
metropolitan areas in nations experiencing tremendous population expansion
and industrialization. Despite significant attempts to reduce sulfur emissions in
China, the world's biggest emitter, serious hurdles remain. Sulfur emissions

35
continue to rise in India and numerous other nations. According to the World
Health Organization (WHO), more than four million people worldwide die
prematurely as a result of outdoor air pollution
(https://www.who.int/airpollution/ambient/health-impacts/en/). It is generally
believed that fine particles (PM2.5) are the primary factor causing adverse health
impacts. Therefore, limiting exposure to air pollution and the risks it poses to
human health, especially from the tiniest particles, is a pressing and novel
concern.
However, there is a possibility that control measures will only concentrate, to a
limited degree, on the appropriate sources and procedures for dealing with the
problem at hand. In recent years, there have been many instances of severe air
pollution in Paris, characterized by high levels of particulate matter concentration.
These occurrences were at first blamed on local pollution. A deeper analysis has
revealed that particulate nitrate is largely caused by regional emissions and the
buildup of high concentrations over several days, as it is formed when urban
emissions of nitrogen oxides from traffic combine with ammonium emissions from
agricultural areas in the surrounding region. It is necessary to take into
consideration the fact that circumstances quite similar to these often arise in
metropolitan areas of developing nations. One example of this is the burning of
agricultural trash. As was previously mentioned, the intercontinental and
hemispheric distances are linked to the air pollution problem.
It should therefore come as no surprise that the scientific groups studying climate
change and air pollution need to collaborate more closely with one another. Heat
waves are associated with poor air quality because winds are often extremely
low and the atmospheric boundary layer is stationary, which are both factors that
contribute to the heat. Health considerations are important from the viewpoints of
both air pollution and climate change. Heat waves cause soil and vegetation to
dry up, which increases the likelihood of fires and, in the case of wildfires around
the globe, may also produce major air pollution. Heat waves can also cause a
rise in the temperature of the ground (e.g., in California in 2018).
There has been significant progress in the study of atmospheric and air pollution,
but more research is needed to answer fundamental problems and inform the
most effective approaches. The impacts of nitrogen on ecosystems, the
connections between air pollution and climate (through carbon storage in
ecosystems and effects on radiation balances, for example), and the health
implications of air pollution are only a few examples of study priorities in this
area. Due to advances in computing power and the ability to model increasingly
complex systems, such as those involved in climate change research, modern
climate models are formulated as Earth system models, coupling the
atmosphere, ocean, land surface, cryosphere, biogeochemical cycles, and
human activities. Because of this, scientists can now examine both climate
change and air pollution at the same time. The modeling approach can be fine-
tuned by collecting data to determine where the boundaries of Earth System
components lie, which could improve our ability to understand and quantify the
fluxes and interactions between different compartments like terrestrial and

36
aquatic ecosystems. Such models should also account for air pollution. Within
this framework, notions on a global scale such as "planetary boundaries" and
"trajectories of the Earth system vs. planetary thresholds" have been created
[28].
9. MITIGATION MEASURES
Acid rain will continue to be an issue for as long as people continue to burn fossil
fuels, and nations that continue to rely largely on coal for power generation and
steel manufacturing will continue to struggle with its impacts. As a result of the
challenges caused by air pollution and the detrimental impact it has on both the
health of humans and the environment, a variety of measures are now being
implemented to lower sulfur and nitrogen emissions. The only effective strategy
for combating acid rain is to reduce emissions of the chemicals that lead to its
formation. This entails the use of fewer fossil fuels and the establishment of
criteria for air quality. Scrubbers, which absorb pollutants before they are
released into the atmosphere, and catalytic converters, which cut emissions from
vehicles, are two technologies that many countries are currently requiring energy
providers to use to clear smokestacks. The most important processes for
controlling acid rain are optimizing processes that use fossil fuels, transitioning to
energies that use renewable resources instead, limiting the use of fertilizers and
pesticides, restoring environments, conserving energy, lowering consumption
levels, convincing others, educating people, and passing regulations.
In addition, there should be some consideration given to policy intervention on a
global scale. For instance, the effects of acid rain on ecosystems and natural
resources gained widespread attention in the 1970s and 1980s, initially in the
northeastern United States and then in the northwest of Europe. In 1980, several
northeastern states and the Canadian province of Ontario sued the United States
Environmental Protection Agency (EPA), demanding that the EPA do something
to curb emissions of acid precursors produced by the federal government. The
National Acid Precipitation Assessment Programme (NAPAP) was established by
the United States Congress by the Acid Precipitation Act of 1980 and charged
with performing a ten-year scientific, technological, and economic study of the
acid rain problem. In addition, the Clean Air Act of 1990 was passed in the United
States to combat acid rain. By instituting pollution controls, it helped cut down on
sulfur dioxide emissions by 88% from 1990 to 2017. Nitrogen dioxide emissions
in the United States have decreased by half in the same period as a result of air
quality regulations. Because of these tendencies, certain fish populations and the
red spruce forests in New England, for example, have been able to rebound from
the harm caused by acid rain. However, restoration takes time, and the eastern
Canadian and northeastern United States soils have only lately shown
indications of returning to a more stable nutrient balance. The implementation of
regulations for sulfur dioxide emissions has resulted in a decrease of 75% in
China since 2007, while an increase of 50% may be seen in India. By the Air
Convention, eight protocols have been signed, each of which mandates that
parties take significant measures to combat acid rain and a variety of other forms
of air pollution (Table 1) [25].

37
Table 1. The convention on long-range transboundary air pollution and its
protocols
Agreements Content Comment
The 1979 convention on
transboundary long-range
air pollution
Frame convention 51 parties to the
convention
The Gothenburg Protocol
of 1999 to combat
acidification,
eutrophication, and
ground-level ozone.
Emissions of SO2, NOx,
NH3, and VOC,
Amendment also fine
particulates
Amended in 2012
The 1998 Aarhus protocol
concerning persistent
organic pollutants (POPs).
A selected number of
persistent organic
compounds
Amended in 2009
The Aarhus protocol on
heavy metals, which was
established in 1998
Control of a selected
number of heavy metals
Amended in 2012
The treaty was signed in
Oslo in 1994 to further
drastically reduce sulfur
emissions.
The protocol sets ceilings
for SO2 emissions based
on CL and IAM
The Geneva protocol of
1991, was concerned with
the management of
emissions of volatile
organic compounds or
their transboundary fluxes.
30% reduction in VOC
emissions
The Sofia Protocol of
1988, was concerned with
the management of
emissions of nitrogen
oxides or their
transboundary fluxes.
Stipulates no further
increase in NOx emissions
The treaty was signed in
Helsinki in 1985 to reduce
sulfur emissions or their
transboundary fluxes by at
least 30% worldwide.
SO2 control by 30%
between 1980 and 1993
The Geneva Protocol of
1984, provided for the
long-term funding of a joint
initiative to monitor and
evaluate the long-range
transmission of air
contaminants throughout
Europe (EMEP)
Stipulates financial support
to the EMEP centers

38
Some affected sites should be standardized using the "Liming" process, which
eliminates the damage to lakes and other water bodies by adding lime. Popular
substances for increasing the pH of acidified water include caustic soda, sodium
carbonate, slaked lime, and limestone [29]. Liming may reduce some of the
symptoms of acidification; nevertheless, it is both costly and ineffective as a
treatment.
10. CONCLUSION
Acid rain is emerging as a significant environmental threat to ecosystems and
human health. Therefore, science, politics, industry, and the general public
should continue working together to establish the effective management of what
was thought to be one of the most significant environmental crises. The most
successful application of science-based policy advice occurred when the most
up-to-date and accurate information was collected, analyzed, and utilized to
comprehend the particular issues, produce and assess potential solutions, and
keep track of the results of policy implementation. However, the world as we
know it now does not seem to be the same, and we cannot simply apply how the
worldwide scientific community collaborated back then to the issues that we face
today. However, there are things to gain from this experience. The establishment
of reciprocal trust between scientific advisors and politicians is of the utmost
significance, as is the fact that both groups should be forthright about their
respective ideals and objectives. It is possible to have a meaningful conversation
about important issues in society by doing things in this manner. When the
guidance is based on the principle that scientists and policymakers should work
together to jointly produce new information and policy alternatives, it is most
effective. Scrubbers, which absorb pollutants before they are released into the
atmosphere, and catalytic converters, which cut emissions from vehicles, are two
technologies that many countries are currently requiring energy providers to use
to clear smokestacks. We have to adopt using fossil fuels, limiting the use of
fertilizers and pesticides, restoring environments, conserving energy, lowering
consumption levels, educating people, and releasing regulations as pillar
solutions.
COMPETING INTERESTS
Authors have declared that no competing interests exist.
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___________________________________________________________________________________
© Copyright (2023): Author(s). The licensee is the publisher (B P International).
Peer-Review History: During review of this manuscript, double blind peer-review policy has been followed. Author(s) of
this manuscript received review comments from a minimum of two peer-reviewers. Author(s) submitted revised
manuscript as per the comments of the peer-reviewers. As per the comments of the peer-reviewers and depending on the
quality of the revised manuscript, the Book editor approved the revised manuscript for final publication.

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