About Acid Rain
The decrease in pH of rains is caused by the reaction between SOx/NOx and water that forms sulfur oxoacids or nitrogen oxoacids. The oxides of sulfur and nitrogen are typically released by the burning of fossil fuels. For example, using coals that are not desulfurized can form SO2 when it’s burning, as illustrated by the following equation:
Another example of the formation of NOx is the combustion of fuels of cars at very high temperatures. The following equation shows some of the reactions between oxygen and nitrogen that can take place in such conditions:
Those oxides can all acts as Lewis acids since they are typically attacked by the lone pair electron in H2O when forming oxoacids. The following equations show some of the common reactions that happen.
Those oxides are acidic because they can release H+ ions. When those oxides react with water, the lone pair electron of H2O can attack the central atom (sulfur or nitrogen) and form S-O-H or N-O-H bonds. Because sulfur and nitrogen are very electronegative, they can pull electrons towards them from the H atom. Therefore, [H+] ions are released when those oxoacids are dissolved in water and gives an acidic property to the solution.
S and N both have very high electronegativity comparing to other elements.
Examples and Effects
Beijing and the Nature
Beijing is surrounded by factories and power plants that burn coal during its production. While the four main power plants that are inside the urban area has already been closed down, others in the suburban areas are still operating currently. They may be a source of SOx and NOx in the air. Nevertheless, the government has already imposed a certain value of pollution tax on them to reduce their incentive of overproducing.
Car emissions is another major source of SOx and NOx in Beijing. There are more than 5,510,000 cars currently owned in Beijing, more than any city in China. As explained before, NOx can be found in the car emission. Regulations in Beijing includes subsidizing hybrid vehicles and limiting the citizen’s usage of their car.
Besides the major factories around Beijing, small factories spread in Beijing that provide heat to houses in Beijing still produces a large proportion of sulfur and nitrogen oxides. Most of them are built near to residents and operate on the combustion of coals. There is actually one in BNDS, and the smoke comes out of the chimney every winter.
Generally, acid rain will make the environment less habitable to animals and plants. On the one hand, acid rains cause the necessary metal ions for plants to become soluble in water and washes them away. Without those minerals, plants may not be able to grow sufficiently. On the other hand, the increased amount of minerals in water bodies can also cause the problem of over-fertilization. Acid rains are corrosive so that they can be dangerous for buildings and sculptures as well (Brown&Ford 397).
Effects on Soil and Vegetation
The impact to the water bodies
Acid rain can decrease the pH of the water body. Furthermore, aluminum from rocks can be released while contacting the acidic water body. The combination of acidic environment and the relatively high concentration of aluminum is toxic to aquatic species such as fishes, crayfishes, and clams (National Geographic). It is also important to notice that the decrease of population in one species can affect other species living in the same ecosystem, so even though some species have a higher toleration to the acidic environment, the impact of acid rain on the whole aquatic ecosystem can still be catastrophic.
The impact to the soil
The effect of acid rain can vary based on the buffering ability of the soil. For example, soil containing more limestone or marble is more likely to neutralize the acid rain by their high pH.
If the soil is rich in neither limestone nor marble, then the acid rain dropped into the soil won’t be neutralized and will accumulate. The microbial equilibrium exists within the ecosystem in the soil may also be broken by the decrease in pH. Furthermore, like for the water bodies, aluminum can be released to the acidic soil as well, making the soil less habitable to plants.
Nevertheless, there are controversies regarding the significance of the effect of acid rain to the soil. Tabatabai and Olson argued that the “Additions of acidity in precipitation to soils are insignificant in comparison with the acidity produced in natural processes and by agricultural soil amendments” (1).
The Impact to the plants
There are two ways that acid rain can impact plants: the direct way and the indirect way. The direct effect refers to any damage caused by the direct contact between the acid rain and the plant while the indirect effects are those with mediums such as soil and water.
Like introduced above, aluminum is dissolved into the water or the soil with acid rains. Aluminum ions are usually toxic to the plants, so the aggregation of free aluminum ions in the soil or the water body can cause the plants in that region to grow more slowly or even die. Other similar ions that are released into soil or water bodies with the help of acid rain include cadmium and mercury. This type of indirect effect has raised problems in the agriculture sector since it not only decreases the productivity but also hinders sustainable development as those free ions can aggregate in the environment.
Because acid itself might be corrosive, direct contact between vegetation and acid precipitation can also be harmful. For example, the plants living on high altitude bathes in acid fog very often. The acid fog causes the loss of the wax protection layer of the leaves, leaving leaves unprotected (“Air Pollution”). Another example is illustrated by Du et al., showing that the amount of chlorophyll (which is essential for the photosynthesis) in leaves will decrease with more direct contact with acid precipitation (768).
Status, Problems, and Concerns
Based on the data from both Seip et al. and Solberg et al., the most affected place in China is Chongqing because it has both a very low pH of the precipitation and a low buffering capacity of its soil. Therefore, the soil at eastern Sichuan and Chongqing city would be very sensitive to acid rains. Moreover, because Sichuan Province has a very large forest coverage rate and great biodiversity, solving the problem of acid rain is even more urgent.
Similarly, Yunnan province has a very low average pH of the water precipitation. Because the majority of the areas in the Yunnan province are used for agricultural purposes, the problem of acid rain can impact the local economy significantly.
Development and the protection of the environment are sometimes considered to be contradictory to each other, especially when an area’s economy starts to boost for the first time. This is the case for Yunnan Province, as its economy has just started to grow but is already facing the problem of acid rain. Though limiting the burning of fossil fuels and promoting clean energy is a solution to the problem of acid rain, the direct cost of doing that is much higher than using fossil fuels. As we can see from the previous acid rain distribution diagram, this problem exists in almost all of the southern sun-rising cities in China, and will continue to be a problem in the future.
Protection of large area of forest from acid rain is also a very hard challenge for the future as people cannot simply cover all of the trees. There’s no doubt that the acid rain problem in those areas has to take a long time to solve, but the situation of the animals living in those areas should also be monitored because they are usually impacted as well. Sometimes, protecting the balance between animals is even harder than solving the problem of acid rain.
“Acid Rain.” Acid Rain Facts, Information, Pictures and Effects. National Geographic, 09 Oct. 2009, www.nationalgeographic.com/environment/global-warming/acid-rain/. Accessed 13 Sept. 2017.
Air Pollution. www.air-quality.org.uk/. Accessed 13 Sept. 2017.
Brown, C. & Ford, Mike, (2014). Pearson Baccalaureate Chemistry Higher Level 2nd Edition. Pearson Education.
Cox, Lyndon, et al. Nitrogen oxides (NOx) why and how they are controlled. DIANE Publishing, 1999.
Du, Enzai, et al. “Direct effect of acid rain on leaf chlorophyll content of terrestrial plants in China.” Science of The Total Environment, 605, 2017, pp. 764-769.
Foust, Richard. Atmospheric Reactions of Sulfur and Nitrogent. N.p., n.d, mtweb.mtsu.edu/nchong/NS-reactions-atm.htm. Accessed 13 Sept. 2017.
“Nitrogen Oxide (NOx) Pollution.” Icopal Noxite. Icopal, n.d., www.icopal-noxite.co.uk/nox-problem/nox-pollution.aspx. Accessed 13 Sept. 2017.
Seip, H.M., et al. (1999). “Acid deposition and its effects in China: An overview.” Environmental Science & Policy, 2.1, 1999, pp. 9-24.
Solberg, Svein, et al. “Acid rain in China.” Environmental science & technology, 40.2, 2006, pp. 418-425.
Tabatabai, M. Ali, and Olson, R. A. “Effect of acid rain on soils.” Critical Reviews in Environmental Science and Technology 15.1, 1985, 65-110.