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Acid Rain

What Is Acid Rain?

Acid deposition, commonly known as acid rain, is a broad term that includes both wet deposition and dry deposition of acidic components. Wet deposition, in this case, represents any form of precipitation (e.g. rain, snow, fog or hail) mixed with acidic components, such as sulfuric or nitric acid, that falls to the ground from the atmosphere. Dry deposition represents the settling of these acidic components out of the atmosphere in the absence of precipitation. Rain has a natural pH of about 5.6; it is slightly acidic because carbon dioxide (CO2) dissolves into it forming carbonic acid, a weaker acid than the components driving acid rain. Nevertheless, there are strong acidic components in the natural environment, such as hydrogen sulfide and sulfur dioxide from volcanic eruptions, dimethyl sulfide from the ocean and nitrogen oxides from lightning. These chemical compounds can naturally lower the pH of rainwater to 5.0. Therefore, "acid rain" is often defined as rainwater with a pH less than 5.0, implying some impact from human activities.

SO2 and NOX are the main precursors of acid rain. SO2 mainly originates from burning of fossil fuels to generate electricity. NOx pollution is emitted by vehicles as well as industrial sources such as power plants, industrial boilers, cement kilns, and turbines. SO2 and NOx react with water, oxygen and other chemicals to form sulfuric and nitric acid, and lower the pH to form acid rain.


Schematic diagram of the article

Acid rain impacts the environment both directly and indirectly. For instance, as sulfate deposits to the ground via precipitation, the calcium and magnesium ions in the soil are leached out. This large amount of mineral loss in soil also affects tree growth, where in the worst cases, the plants wither and can even die due to lack nutrients.

Wildlife are also impacted by acid rain as it deposits on lakes and rivers, where sulfuric acid prevents fish from absorbing essential oxygen, salt and nutrients in the water. In addition, acid rain ionizes elemental aluminum, which is then mobilized and flows into the water, posing a lethal threat to the fish.

Acid rain also directly impacts our manmade environment by eroding buildings and statues, while suspended acidic particles can reach even less exposed, but particularly vulnerable items inside of buildings such as centuries-old literature and ancient artworks.

Air Pollution Control Policy and Acid Rain

In the past 30 years, Taiwan Ministry of Environment (MOENV) has adopted various air pollution control strategies, and the acid rain situation has progressively improved. In 1992, MOENV established the air pollutants emission standards for stationary pollution sources and began levying air pollution fees in 1995. MOENV then implemented a permit management system, a continuous emission monitoring system, and announced more rigorous emission standards. As a result, large pollution sources gradually installed or improved pollution control equipment to reduce emissions. In terms of mobile pollution sources, MOENV also reduces the air pollution emitted from vehicles through measures such as oil composition control, more rigorous emission standards, regular inspections, subsidies for the replacement of old vehicles, etc. Thus far, MOENV has established 14 acid rain monitoring sites around Taiwan (see Figure 2), which observed a decrease in the average concentration of non-sea-salt sulfate ions (nss-SO42-) by 61% in the past 33 years. This in turn has been driving the rainwater pH upward, where in 2021-2023, the annual average pH value of all stations was greater than 5.0, proving the effectiveness of the adopted pollution control strategies.


Schematic diagram of the article
Schematic diagram of the article
Schematic diagram of the article

The improvement of rainwater pH required (and still requires) the participation of all people, such as through regular inspections of automobiles and motorcycles, taking more public transportation, and developing energy-saving habits, which all reduce air pollution emissions and decrease the likelihood of acid rain.

Acid Rain in various countries

According to the monitoring results from the Acid Deposition Monitoring Network in East Asia (EANET) and the European Monitoring and Evaluation Programme (EMEP), rainwater pH has increased to over 5.0 in most of countries. The average pH value of rainwater in Taiwan was 5.77 in 2021.

Table 1. The 2019 worldwide rain chemistry monitoring result

Country Period pH Concentration
µeq L-1
Wet deposition
kg ha-1yr-1
Precipitation
mm
Investigator
(Data released year)
SO42- NO3- SO42 NO3-
China, Chongqing 2019 5.10 92.8 48.6 37.2 34.2 1131 EANET(2021)
China, Xiamen 2019 5.40 57.9 35.3 28.5 22.4 2040 EANET(2021)
Finland 2019 4.88 10.5 12.3 3.5 5.2 686.2 EMEP(2021)
France 2019 5.48 12.4 12.7 6.6 9.0 1143.4 EMEP(2021)
Germany 2019 5.36 16.7 20.7 6.7 10.7 910.9 EMEP(2021)
Indonesia 2019 5.36 16.7 20.7 6.7 10.7 910.9 EANET(2021)
Japan 2019 4.97 27.4 11.2 20.9 11.2 1834.8 EANET(2021)
Mongolia 2019 5.49 69.2 56.8 11.2 1.2 33.7 EANET(2021)
Philippines 2019 5.40 20.4 13.1 23.1 19.2 2367.0 EANET(2021)
Republic of Korea 2019 5.21 41.7 31.4 14.9 14.9 796.3 EANET(2021)
Taiwan 2019 5.73 40 31 26 23 2309 MOENV(2020)
Thailand 2019 5.52 16.7 18.0 6.5 9.0 813 EANET(2021)
United Kingdom 2019 5.60 15.4 11.5 8.8 7.5 1139.6 EMEP(2021)
USA 2019 5.0-6.4 2-27 1-19 0.4-13 0.7-16 100-2749 NADP(2020)
Vietnam 2019 5.51 60.5 28.5 50.8 28.2 1822 EANET(2021)

Table 2. The 2020 worldwide rain chemistry monitoring result

Country Period pH Concentration
µeq L-1
Wet deposition
kg ha-1yr-1
Precipitation
mm
Investigator
(Data released year)
SO42- NO3- SO42 NO3-
China, Chongqing 2020 5.11 56.7 42.9 34.1 33.4 1254.9 EANET(2022)
China, Xiamen 2020 5.59 42.3 39.6 9.1 12.1 556.4 EANET(2022)
Indonesia 2020 5.37 29.2 16.2 31.2 23.3 2256.0 EANET(2022)
Japan 2020 5.00 25.3 10.0 21.1 11.6 1931.4 EANET(2022)
Mongolia 2020 6.10 86.8 41.8 5.7 3.6 139.2 EANET(2022)
Philippines 2020 5.60 22.9 11.3 27.7 17.7 2519.3 EANET(2022)
Taiwan 2020 5.60 33 27 17 17 1768 MOENV(2021)
Thailand 2020 5.85 15.0 20.1 9.5 16.8 1311.9 EANET(2022)
USA 2020 5.1-6.4 1-35 1-19 0.2-9 0.4-10 83.2447 NADP(2021)
Vietnam 2020 5.84 72.0 32.4 65.2 36.3 2090.3 EANET(2022)

Table 3. The 2021 worldwide rain chemistry monitoring result

Country Period pH Concentration
µeq L-1
Wet deposition
kg ha-1yr-1
Precipitation
mm
Investigator
(Data released year)
SO42- NO3- SO42 NO3-
China, Chongqing 2021 5.29 50.8 38.4 34.6 33.7 1424.0 EANET(2023)
China, Xiamen 2021 5.10 34.7 34.0 9.2 12.1 591.0 EANET(2023)
Finland 2021 5.02 8.9 10.5 2.7 4.1 637.9 EMEP(2023)
France 2021 5.53 9.8 9.8 5.0 6.9 1130.5 EMEP(2023)
Germany 2021 5.39 16.1 18.7 6.6 9.9 943.8 EMEP(2023)
Indonesia 2021 5.41 33.5 16.8 33.6 22.8 2136.0 EANET(2023)
Japan 2021 5.03 58.2 14.0 52.0 15.9 2107.0 EANET(2023)
Mongolia 2021 6.1 86.8 41.8 5.7 3.6 139.2 EANET(2022)
Philippines 2021 5.00 35.9 13.2 44.1 21.0 2556.0 EANET(2023)
Taiwan 2021 5.77 34 33 20 22 2112 MOENV(2022)
Thailand 2021 5.49 17.9 25.1 7.6 13.8 883.0 EANET(2023)
United Kingdom 2021 5.64 13.9 9.0 7.2 5.6 1062.5 EMEP(2023)
USA 2021 5.1-6.5 1-25 1-22 0.2-13 0.2-11 65-3785 NADP(2022)
Vietnam 2021 5.81 53.4 23.8 52.1 28.7 2131.0 EANET(2023)

*Data source:The Acid Deposition Monitoring Network in East Asia (EANET)、European Monitoring and Evaluation Programme (EMEP)、 National Atmospheric Deposition Program (NADP) and Ministry of Environment, Executive Yuan, R.O.C(Taiwan).


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