It's Raining Acid: Acid Rain in Malta and Beyond

Acknowledgements

President: Louisa Degabriele

Social Policy Officer and Policy Paper Leader: Juergen Valletta

S-Cubed Administrative Board: Cristina Stafrace, Julian Formosa, Julian Bondin, Dindora Mercieca

S-Cubed Executive Board: Martina Debono, Gabriel Grima, Valerie Esposito, Lara Bugeja, Luke Said, Leanne Axisa, Aiden Grima

Social Policy Subcommittee: Amy Buttigieg, Federica Grech, Megan Abdilla, Phillip Johannesen, Karl Caruana, Matthew Clark, Ranwa Almusrati, Sabrina Pace

Special Mentions: Amy Borg on behalf of DESA (Department of English Students Association)

Rainwater is an essential resource for life on Earth. It provides us with water for drinking, cooking, and cleaning, and it is fundamental for the growth of trees, plants, and crops in agriculture. Since rainwater is such a vital resource, we are instinctively always on the lookout for anything that may threaten it. One such threat is acid rain, and research on the source of this problem and on its adverse effects will help us come up with solutions to minimise its impact.

To understand acid rain, we must first understand the concept of acidity. Acidity has been defined over and over again by different chemists, and different theoretical approaches have been suggested along the years. The most common approach taken for common aqueous (taking water as a solvent) situations is the Brønsted-Lowry Acid-Base Theory, which defines acidity as the presence of hydrogen ions or protons, H+, in a solution. These ions are generated by the dissociation of the acid in a solution, as shown by the dissociation of hydrochloric acid in Equation1. 1

HCl → H+ + Cl-

Equation1

The most common method of quantifying acidity is the logarithmic function of the concentration of protons, also known as the pH scale, which most commonly runs from a value of 0, indicating the highest acidic conditions, up to a value of 14, indicating the least acidic or most basic conditions. pH levels lower than 0 or higher than 14 are not that common since they also protonate or deprotonate water, the solvent in aqueous solutions, respectively Water tends to be neither basic, nor acidic, having a pH of 7 at the centre of the pH scale. This is because, as shown by Equation 2, minimal amounts of water molecules dissociate to form equal amounts of hydrogen ions and hydroxide ions, which neutralise each other 2

H2O ⇌ H+ + OH-

Equation2

Nevertheless, over the years, scientists have observed that the pH of rainwater is very unlikely to have a value of 7. Many studies show that the pH tends to be below 7, indicating acidity and the presence of acids in rainwater 2

Research suggests that the major cause of these acids in our atmosphere and in rainwater is the burning of fossil fuels, in industry, automobiles, and in other combustion processes. During such activities, several gaseous emissions are released into the atmosphere, which generally include sulfur dioxide (SO2), hydrogen chloride (HCl), and even nitrous oxides (NOx).

These chemical substances may also be found in soot, ash, and other particulate matter which transports the acid precursors up into the atmosphere 3–5

The gases would then mix and react with atmospheric water via different mechanisms of wet deposition. One such process is rain-out, whereby the substances mix with water and water vapour in clouds. Another process is wash-out, which happens when substances are caught up in downpouring precipitation, this being rain, hail, or snow.

Once the emitted chemical substances are interacting with the water, a variety of reactions which yield the acidic gases that generate the acidity in rainwater take place. One pathway is that followed by sulfur dioxide which is generated during the burning of fuels containing sulfur, as illustrated by Equation 3. However, sulfur-containing precursors and SO2 are also generated naturally in our atmosphere, such as from volcanic eruptions and from sulfates present in our oceans.SO2 reacts with water to form the weakly acidic sulfurous acid, H2SO3, as indicated by Equation4. Furthermore, sulfur dioxide may also be slowly oxidised by oxygen in the air to form sulfite ions, which then react with the water they are dissolved in, forming sulfuric acid, which is a stronger acid. This is shown by Equation5 and Equation6 3–6

Equation3

Equation4

Equation5

Equation6

A second type of acid rain precursor is nitrous oxides. High temperatures such as those generated during combustion processes in automotive engines enable the reaction between nitrogen and oxygen gas in the air. This produces nitrogen monoxide as shown in Equation 7, which can be further oxidised to nitrogen dioxide as illustrated by Equation 8. The nitrogen dioxide is oxidised even more and also hydrolysed to form nitric acid, as shown in Equation9 3,5

Equation7

Equation8

Equation9

Furthermore, nitrogen dioxide may alternatively be oxidised by ozone to produce nitrogen trioxide as seen in Equation 10. Nitrogen dioxide and nitrogen trioxide react with each other to form dinitrogen pentoxide as illustrated by Equation11, which itself serves as an additional source of nitric acid when hydrolysed via Equation12 3

Hydrogen chloride gas is another acidic pollutant that contributes to a lower pH in atmospheric water and in rainwater. This is usually emitted during the burning of polyvinylchloride (PVC) or any fuels or substances containing chloride compounds.

When all these pollutants are mixed with rainwater, they dissolve and dissociate into the constituent ions, as described previously in Equation 1. One of these ions would be the hydrogen cation which lowers the pH value of the water and leads to acidity.

Nevertheless, unpolluted rainwater is not exactly neutral. It generally has a pH value which is lower than 7. This is caused by the presence of carbonic acid in rainwater, which is formed by the reaction between carbon dioxide gas in our atmosphere and water, as illustrated by Equation13. 4,7

As a result, when considering natural levels of carbon dioxide in air, the pH value of rainwater is typically reported as being approximately 5.6 due to the natural carbonic acid present. Hence, precipitation is only considered to be acidic if the recorded pH value is below 5.6, and not below 7.4,7

Acid rain usually precipitates far from the site of its production, due to winds carrying the vapour and other gases produced with them. Therefore, the stronger the winds, the further the acid rain travels This essentially makes the acid rain problem an international one 8

Acid Rain in Malta

The frequently strong winds in the Mediterranean area continuously transport substantial amounts of natural contaminants across the region, such as desert sand, ash, and gases from active volcanic cones, and marine aerosols from the sea.9

Over the years, through the increase in industrial processes and demand for materials, man-made pollution has also had a significant increase, especially from heavy industries and from pollutant transportation methods. As a result, the strong prevailing winds now also distribute significant quantities of such man-made pollutants, including the ones which were previously mentioned, across the globe.9,10

Situated between Africa and Europe, Malta faces significant pollution from these continents. A large part of said pollution originates from the oil industries in Sicily, Libya, Algeria, and Egypt, and from numerous other heavy industries situated in these and other Mediterranean-bordering nations.9

Apart from the industries in neighbouring countries, Malta has experienced a growth in its own industrial sector over the years, contributing to its own pollution levels. The primary contributors to pollution are the power plants essential for electricity production, given that currently the generated electricity stems mainly from these facilities. As the demand for electricity has surged due to an uncontrolled growth in the population and heightened water consumption, stemming from both population growth and limited freshwater availability, the reliance on electricity-powered distillation plants has intensified.

The rise of sulfur dioxide in precipitation is evident through the growing presence of sulfate on limestone surfaces found on the exteriors of buildings, particularly those situated near power stations. A 1996 study which analysed limestone powder collected from the surfaces of Maltese churches revealed the presence of shiny black particles identified as soot. The research also established a direct correlation between the level of sulfation and the presence of these particles, both significantly influenced by the prevailing wind patterns.11

The population upsurge has also fuelled the increase in transportation activity across the islands, exacerbated by inefficient public transport. Therefore, the production of nitrogen oxides generated by the internal combustion engines and carbon dioxide emissions have seen a substantial increase.9,12 Multiple scientific studies consistently link the rise in road-traffic pollution to higher rates of asthma, also showing the seriousness of the adverse effects on human health caused by such pollution.

However, land traffic is not the sole source of pollution. As an island nation without direct physical links to mainland Europe, Malta heavily relies on air services and infrastructure for global connectivity. Additionally, having two natural ports greatly facilitates the movement of sizable ships, further boosting the growth of sea transport industries. The Valletta and Marsaxlokk ports play a pivotal role not only in Malta’s transport of passengers and goods but also within the bordering Mediterranean and global logistics network. Furthermore, the coastline features numerous small harbours and landing points, catering to the ferry service between Malta and Gozo, fishing vessels, pleasure crafts, and large yachts.12

Changes in the global environment have also played a role in the rise of pollution. Malta has experienced soaring temperatures during the summer

due to the overall increase in global temperatures. Consequently, there has been a heightened reliance on electricity-powered air conditioning systems. Several studies show that Malta’s emissions of sulfur dioxide reach their peak levels in the summer months.

Hence, it can be inferred that there has been a notable rise in atmospheric pollutants in recent years. Sulfur oxides and nitrogen oxides, when dissolved with rainwater, lead to acidic precipitation, distinct from other aerosolbased air pollution. The increase in acid rain levels also lead to an increase in the problems it causes, which can be summarised as compromised health, and adverse effects on agriculture and architecture, among others.13

Effects of Acid Rain on Human Health

Acid rain is a form of pollution that affects people’s health in many ways, from direct exposure to its complex impacts on food and water sources.3 Sulfur dioxide is the most significant gaseous pollutant when it comes to human health, having different effects depending on how it interacts with the human body.14 Nitrous oxides, on the other hand, are found to have little to no effect on human health at small doses. However, they sometimes tend to react with organic compounds and free radicals to form ozone, which has irritating effects to the nose, eye and throat at levels as low as 0.1 ppm.15 Symptoms of cardiovascular and respiratory disease may also develop, with severity increasing with the concentration of ozone 16

When sulfur dioxide is ingested in small quantities, up to 1.0 g, it is mostly harmless, leading to its use as a food stabiliser. The use of sulfur dioxide and sulfites for this purpose suggests their safety when incorporated into food processing. However, inhaling sulfur dioxide is extremely dangerous as it shows cumulative effects with chronic exposure, implying that prolonged exposure to sulfur dioxide through breathing can lead to an increasing impact on health over time.14 Apart from that, inhaling it would produce sulfurous acid, which is toxic, in the mucus along the respiratory tract. This is why the effects caused by sulfur dioxide are more noticeable when it is in gas or aerosol form.3

Sulfur dioxide’s presence can only be detected at concentrations above 0.2 ppm. The threshold for nose and throat irritation begins above 6-12 ppm. However, the threshold for eye irritation is higher, at 20 ppm, and skin irritation starts at 10, 000 ppm (1 %) concentration. Due to the irritating nature of sulfur dioxide, acute poisoning is extremely rare. This shows how sulfur dioxide acts as a protective mechanism, alerting individuals of its

presence before serious injury occurs. Therefore, the industrial threshold limit value (TLV) is safely set at 5 ppm.14

The mechanism of acute poisoning by sulfur dioxide is still controversial as there are many conflicting opinions about it within the scientific community In fact, a workshop was done in 1985 by the National Institute of Environmental Health Sciences (NIEHS) in the United States of America regarding this topic, and other human health effects of acid rain were discussed too. The report from the workshop also emphasises the indirect phase of acid rain, where acidic substances deposited on dry surfaces or in water interact with materials, potentially mobilising toxic elements and heavy metals enhancing human exposure.17

From the workshop, it was clear that the effects of acid rain go beyond simple physical contact. In fact, the impact of acid rain on human health has also been divided into two components by Goyer et al. in 1985, which are the pre-depositional phase which involves the air pollutants responsible for forming acid rain (NOx, SO2), as discussed earlier in this section, and the postdepositional phase which involves the mobilising of toxic elements, mostly metals, from their ores or from insoluble deposits.17

During the post-depositional phase, acidification plays a critical role in the movement of metals from fixed locations like ores and insoluble deposits to substances humans are exposed to, such as water and food. This process also involves the conversion of metals into more harmful forms through metal-ion interactions and is facilitated by organic ligands. Metal-ion interactions may be classified into three categories: fast exchange, intermediary exchange, and slow exchange. Metal-ions in fast exchange, such as alkali and alkali earth metals and the hydrogen ion, are especially significant. Ions of heavy metals, such as lead, cadmium and aluminium can be substituted by the fast exchange ions. Acidification also supports the bioconversion of mercury to methylmercury.17 Thus, upon acidification the heavy metals are displaced from their harmless state in the soil, and dissolution in water becomes a possible risk.3 This results in the contamination of water sources and bioconcentration in food chains, which can lead to accumulation of heavy metals in the human body.17 The aforementioned metals (mercury, lead, cadmium and aluminium) are the most significant metals which are liberated by the post-depositional processes.

As has been previously stated, mercury forms an organic compound called methylmercury, which is especially damaging to the nervous system.18 A change in pH, brought about by acidification, is one of the known factors which influence methylmercury levels. The compound accumulates in fish, with its highest concentration in predatory fish, until it is ingested by humans.17

Initial effects of the compound include non-specific symptoms such as paraesthesia, malaise, and blurred vision. Symptoms may escalate into deafness, visual field constriction, and muscle-coordination conditions such as dysarthria and ataxia. Whilst milder symptoms can go away over time, methylmercury toxicity may even induce a coma or lead to death.18

Multiple studies carried out by Nordberg and Strangert concluded that consumption of 50 μg of methylmercury a day in adults leads to a 0.3 % chance of developing the initial symptom of paraesthesia. However, quadrupling the intake level to 200 μg increases the risk of paraesthesia to 8 %; hence the initial risk (0.3 %) is magnified by a factor greater than 26.17

It is worth noting that developing central nervous systems are more susceptible to harm.17 Foetuses with slightly poisoned mothers had greater rates of being born with severe cerebral palsy.19 Further foetal risks associated with maternal blood poisoning include microcephaly, hyperreflexia, deafness, blindness, and gross motor and mental impairment.20

One cannot quantify the amount of lead exposed to humans through longterm acid precipitation as it has many other sources. However, lead’s toxicity is well documented. Effects of lead include heme biosynthesis and erythropoiesis, kidney dysfunction, central and peripheral nervous system toxicity, derangement of vitamin D, and essential trace metal metabolism. Lead may also lead to a reduced speed of nerve conductions, a hinderance to intelligence, and a diminished learning capacity. Children are also more susceptible to damage than adults, as they intake and retain a higher percentage of lead per unit weight 17

The body has no mechanism to remove cadmium as it does for other metals, thus all of the ingested cadmium is retained. Long-term ingestion of low levels of cadmium, as is facilitated through acid rain, harms the kidneys, and causes renal disease. People with the highest risk of cadmium toxicity include the elderly, cigarette smokers, and vegetarians. The leading explanation for the risk of smokers and vegetarians lies within the fact that tobacco and leafy vegetables have a high tendency to absorb cadmium from acidified soil.21

Whilst aluminium is generally regarded as being non-toxic, it has been associated with development of diseasein patients with chronic renal failure undergoing dialysis. Accumulation of aluminium in the patient’s brain, muscle, and bone is concordant with present of dialysis encephalopathy, which is usually fatal within 6 months.22 Patients with high aluminium levels in their bones are also subject to developing osteodystrophy. Dialysis encephalopathy is also found in children with renal failure, not undergoing dialysis, but instead ingesting aluminium hydroxide 17

Effects of Acid Rain on Agriculture

Agriculture is also negatively impacted by acid rain. There is evidence that soil and groundwater compositions have changed with increase in acid rain throughout the years, shown as the lowering of the pH of the soil and an increase in microbial biomass that is present in the soil.23 Apart from that, soil samples found with higher levels of sulfur and phosphorus demonstrated increased dead crop biomass as a result due to decreased nutrient intake. Apart from that, the decreased pH in the soil could also lead to increased solubility, and thus an increased uptake, of elements such as manganese and iron which are harmful to plants when they are in higher concentrations than is needed. Decreased vital mineral and element uptake may impact crop growth and health, however more research is required in the field in order to demonstrate a clear correlation.24

A decrease in soil pH can lead to hindered plant growth, which has many implications. The decrease in plant growth can lead to disruption in many ecosystems and food chains, as the consumers which are placed above the producer plants in the chain would also decrease in number. This could then lead to less meat production for human consumption, and less animals to produce vital resources. Apart from that, plant-based foods and resources could also dwindle due to a lack of growth of the respective plants.25

The nitrogen cycle is also heavily affected by soil pH, since it is mainly driven by nitrifying, denitrifying, and nitrogen fixing bacteria, although the latter is not only dependant on bacteria in the soil, but also on other natural phenomena such as lightning. It was found that the nitrification process in nitrifying bacteria such as Nitrosomonas and Nitrobacter decreases with around 13 % for every pH unit the soil pH decreases between 5.0 and 9.0. Below 5.0, the decrease is significantly larger, which is most often the case when considering acid rain, since this is considered to be precipitation with pH under 5.6 as mentioned previously 26 Nitrates are vital to plant functioning, since they are the main source of nitrogen for the plant, which is important for metabolism, signal transferring, growth, development, and adaptation to various environments.27

Effects of Acid Rain on Architecture

The impact of acid rain on architectural structures, particularly on those which are built with limestone, as are the majority of the older buildings in Malta and the megalithic temples, and those which are built with particularly vulnerable materials, is a matter of considerable concern This is worrying both from the perspective of cultural preservation and that structure strength. The acid rain developed from air pollution leads to the acceleration of the weathering process of vulnerable materials due to the corrosive nature of acid on these materials. As will be noted, this precipitation has particular effects on architectural works, both within the Maltese context and beyond.

Effectively all materials used for construction, which thus serve as the backbone and foundation of all architectural structures, undergo some form of notable deterioration due to acid rain specifically. Concrete substrates undergo corrosion and thus crack, metals exhibit accelerated rusting, and even materials which are traditionally considered more weather-resilient deteriorate relatively quicker due to acid rain. Before proceeding with analysis, it is worth contrasting the effects of normal rainfall to that of acid rain when it comes to their respective effects on exposed surfaces as to enable a reference point for said effects. While conventional precipitation incrementally manifests in the typical weathering of exposed objects over a large period of time, acid rain operates as a much more rapid and corrosive agent, not only facilitating and catalysing the erosion process but also altering and intensifying its effects.

Considering the Maltese context, where most architectural heritage is built using Globigerina Limestone, it is worth considering particular cases of the effects of acid rain on the material. The corrosive effects on sedimentary rock and marble, a metamorphic rock formed by limestone alterations, are also extremely relevant to the case in Malta since they too are common materials in Maltese structures.28

Limestone is mainly made up of calcite and dolomite,29 and marble of calcite crystals and mica grains. When calcite in particular interacts with sulfuric and nitric acid, often found in acid rain, it dissolves. Architecturally, this results in roughened surfaces, loss of detail, and general removal of a given building’s exposed material. This is particularly evident when looking at older buildings and statues made mainly out of these materials. This loss of material and detail will usually occur in certain spots of the buildings that are more susceptible and reactive but may also occur on the entire structure given over time, indicating structure vulnerabilities integrity-wise.30

Interestingly, this reaction does not only occur in sites that are explicitly exposed to the elements but also to those in sheltered areas. This can be seen in the form of blackened crusts that form in certain areas which peel off to reveal crumbling deteriorating stone. This blackened crust forms during a reaction between the calcite, water, and sulfuric acid and is mainly made of gypsum. Gypsum is water soluble and is usually washed away, however it is easily formed when carbonate stone surfaces are exposed to sulfur dioxide gas.30

Furthermore, the weathering of limestone doesn’t stop at the surface, for it is also present beneath it. Processes such as salt crystallisation, dissolution, and leaching can all affect the internal structure of the limestone due to the porosity and capillary uptake of the stone which weakens this. The porosity of Lower Globigerina Limestone in Malta was found to vary between 23.9 % and 40 % which implies that it is very susceptible to being weakened from within.31

Conclusion

It is no secret that acid rain is one of the most dangerous and widespread forms of pollution, due to the multitude of harmful effects it has on many sectors which have been discussed, and since it is transported through precipitation. These effects end up causing large monetary losses too: money is needed for more pH buffers for soils as they get more acidic, restoration of weathered surfaces and buildings, and treatment for disease caused by acid rain among others. Apart from monetary losses, many other losses are caused due to the aforementioned effects, such as loss in cultural heritage and structural integrity in used buildings, which poses a serious hazard to the people present inside or close to it, a decrease in plant-based products considering acid rain’s effect on plant growth, an increase in several diseases, and a negative impact on human health overall, to name a few

Several studies on acid rain and its effects have discussed potential solutions to the problem. These include using fuels which burn more cleanly, and if it is necessary to use coal, ensuring that its combustion is moreefficient Using smokestack scrubbers in chimneys of industrial plants reduces about 90 –95 % of the sulfur released through smoke, as does regular maintenance. Using public transport is recommended too, so as to reduce the number of vehicles on the road 32 The public should be educated more thoroughly on the negative effects of acid rain,32,33 and stricter penalties should be put into place for non-compliance with sulfur dioxide and nitrous oxide emission regulations, since some of the fines are currently actually cheaper than

treating pollutants.33 Another paper disagreed with these suggestions, claiming instead that there is not enough information on acid rain to determine and implement suggestions which are certain to minimise it successfully It thus calls for more research on the topic.34 Even though some of the suggestions put up in the past to help with this problem, such as the use of scrubbers which is now even enforced by law in most countries worldwide, have been accepted, much more effort can be done. However, this would only be possible through cooperation between countries and the people themselves.32

Bibliography

(1) Ruthenberg, K.; Chang, H. Acidity: Modes of Characterization and Quantification. Studies in History and Philosophy of Science Part A 2017, 65–66, 121–131. https://doi.org/10.1016/j.shpsa.2017.04.003.

(2) Hammett, L. P. The Theory of Acidity. J. Am. Chem. Soc. 1928, 50 (10), 2666–2673. https://doi.org/10.1021/ja01397a011.

(3) Singh, A.; Agrawal, M. Acid Rain and Its Ecological Consequences. J EnvironBiol 2008, 29 (1), 15–24.

(4) Charlson, R. J.; Rodhe, H. Factors Controlling the Acidity of Natural Rainwater. Nature 1982, 295, 683–685. https://doi.org/10.1038/295683a0.

(5) Newbery, D. M. Acid Rain. Economic Policy 1990, 5 (11), 297–346. https://doi.org/10.2307/1344480.

(6) Zhang, L.; Wang, J.; Wang, S.; Wang, C.; Yang, F.; Li, T. Chemical Characteristics of Long-Term Acid Rain and Its Impact on Lake Water Chemistry: A Case Study in Southwest China. JEnvironSci(China) 2024, 138, 121–131. https://doi.org/10.1016/j.jes.2023.03.028.

(7) Bogan, R.; Ohde, S.; Arakaki, T.; Mori, I.; Mcleod, C. Changes in Rainwater PH Associated with Increasing Atmospheric Carbon Dioxide after the Industrial Revolution. Water Air and Soil Pollution 2009, 196, 263–271. https://doi.org/10.1007/s11270-008-9774-0.

(8) Back, L.; Bretherton, C. The Relationship between Wind Speed and Precipitation in the Pacific ITCZ. JournalofClimate-JCLIMATE 2005, 18. https://doi.org/10.1175/JCLI3519.1.

(9) Bonanno, A.; Cassar, J. The Deterioration of Cultural Property by Airborne Pollutants : A Case Study of a Mediterranean Island. 1991

(10) Sammut, G.; Sinagra, E.; Helmus, R.; de Voogt, P. Perfluoroalkyl Substances in the Maltese Environment - (I) Surface Water and Rain Water. Sci Total Environ 2017, 589, 182–190. https://doi.org/10.1016/j.scitotenv.2017.02.128.

(11) Vella, A. J.; Camilleri, A.; Tabone Adami, J. P. Limestone Surfaces in Builtup Environment as Indicators of Atmospheric Pollution. Environ GeochemHealth 1996, 18 (4), 165–170. https://doi.org/10.1007/BF01771239.

(12) Attard, M. The Impact of Global Environmental Change on Transport in Malta. 2015

(13) Desira, N. (2012). Sulfur Dioxide Trends in Malta : A Statistical Computing Approach. masterThesis, University of Malta, 2012. https://www.um.edu.mt/library/oar/handle/123456789/103364 (accessed 2024-02-10).

(14) Demanuele, J.; Caruana, S. Sulphur Dioxide in Air, 1991, pp. 29-35.

(15) Ferris Jr., B. G. Health Effects of Exposure to Low Levels of Regulated Air Pollutants. JournaloftheAirPollutionControlAssociation 1978, 28 (5), 482–497. https://doi.org/10.1080/00022470.1978.10470621.

(16) Nuvolone, D.; Petri, D.; Voller, F. The Effects of Ozone on Human Health. Environ Sci Pollut Res Int 2018, 25 (9), 8074–8088. https://doi.org/10.1007/s11356-017-9239-3.

(17) Goyer, R. A.; Bachmann, J.; Clarkson, T. W.; Ferris, B. G.; Graham, J.; Mushak, P.; Perl, D. P.; Rall, D. P.; Schlesinger, R.; Sharpe, W. Potential Human Health Effects of Acid Rain: Report of a Workshop. Environ HealthPerspect 1985, 60, 355–368. https://doi.org/10.1289/ehp.8560355.

(18) Mercury; United Nations Environment Programme, World Health Organization, Eds.; Environmental health criteria; World Health Organization ; obtainable from Q Corp.]: Geneva : [Albany, N.Y, 1976, pp. 1-131.

(19) Harada, M. Congenital Minamata Disease: Intrauterine Methylmercury Poisoning. Teratology 1978, 18 (2), 285–288. https://doi.org/10.1002/tera.1420180216.

(20) Amin-Zaki, L.; Elhassani, S.; Majeed, M. A.; Clarkson, T. W.; Doherty, R. A.; Greenwood, M. Intra-Uterine Methylmercury Poisoning in Iraq. Pediatrics 1974, 54 (5), 587–595.

(21) EPA. Health Assessment Document for Cadmium; 600/8-79–003; Environmental Protection Agency: Environmental Criteria and Assessment Office, 1979.

(22) Alfrey, A. C.; LeGendre, G. R.; Kaehny, W. D. The Dialysis Encephalopathy Syndrome. Possible Aluminum Intoxication. NEnglJMed 1976, 294 (4), 184–188. https://doi.org/10.1056/NEJM197601222940402.

(23) Liu, Z.; Li, D.; Zhang, J.; Saleem, M.; Zhang, Y.; Ma, R.; He, Y.; Yang, J.; Xiang, H.; Wei, H. Effect of Simulated Acid Rain on Soil CO2, CH4 and N2O Emissions and Microbial Communities in an Agricultural Soil. Geoderma 2020, 366

https://doi.org/10.1016/j.geoderma.2020.114222.

(24) Wang, C.; Fang, Y.; An, W.; Zeng, C.; Wang, W.; Sardans, J.; FernándezMartínez, M.; Peñuelas, J. Acid Rain Mediated Nitrogen and Sulfur Deposition Alters Soil Nitrogen, Phosphorus and Carbon Fractions in a Subtropical Paddy. CATENA 2020, 195, 104876. https://doi.org/10.1016/j.catena.2020.104876.

(25) Barrow, N. J.; Hartemink, A. E. The Effects of PH on Nutrient Availability Depend on Both Soils and Plants. Plant Soil 2023, 487 (1), 21–37. https://doi.org/10.1007/s11104-023-05960-5.

(26) Villaverde, S.; García-Encina, P. A.; Fdz-Polanco, F. Influence of PH over Nitrifying Biofilm Activity in Submerged Biofilters. Water Research 1997, 31 (5), 1180–1186. https://doi.org/10.1016/S0043-1354(96)00376-4.

(27) Zhang, G.-B.; Meng, S.; Gong, J.-M. The Expected and Unexpected Roles of Nitrate Transporters in Plant Abiotic Stress Resistance and Their Regulation. Int J Mol Sci 2018, 19 (11), 3535. https://doi.org/10.3390/ijms19113535.

(28) Cassar, J.; Vella, A. Methodology to Identify Badly Weathering Limestone Using Geochemistry: Case Study on the Lower Globigerina Limestone of the Maltese Islands. Quarterly Journal of Engineering Geologyand Hydrogeology - Q J ENG GEOL HYDROGEOL 2003, 36, 85–96. https://doi.org/10.1144/1470-923602-007.

(29) Limestone: Characteristics, Uses And Problem https://www.gsa.gov/real-estate/historic-preservation/historicpreservation-policy-tools/preservation-tools-resources/technicalprocedures/limestone-characteristics-uses-and-problem (accessed 2024-02-12).

(30) Howdoesacidprecipitationaffectmarbleandlimestonebuildings?| U.S. Geological Survey. https://www.usgs.gov/faqs/how-does-acidprecipitation-affect-marble-and-limestone-buildings (accessed 202402-12).

(31) Grøntoft, T.; Cassar, J. An Assessment of the Contribution of Air Pollution to the Weathering of Limestone Heritage in Malta. Environ Earth Sci 2020, 79 (12), 288. https://doi.org/10.1007/s12665-020-09027-x.

(32) Bhardwaj, M. Acid Rain-A Specific Type of Pollution: Its Concept, Causes, Effects and Tentative Solutions. 2016

(33) Bao, Y.; Fang, H.; Yang, X.; Zhang, Z. The Impact of Acid Rain in Central China and Possible Solution. MATEC Web of Conferences 2023, 386. https://doi.org/10.1051/matecconf/202338603017.

(34) Eney, A. B.; Petzold, D. E. The Problem of Acid Rain: An Overview. Environmentalist 1987, 7 (2), 95–103. https://doi.org/10.1007/BF02240291.