Addressing the Effects of Acid Rain

Introduction

With global warming has come a rise in the level of air pollutants. Certain chemicals, such as sulfur dioxide and nitrogen oxides, are produced by industrial processes and fossil fuel consumption; these compounds react with atmospheric water and oxygen to form sulfuric and nitric acids, which mix with rainwater, lowering its pH from around 5.6 to around 4. This acid-water mixture falls to the ground as acid rain.[14]

Global warming is also associated with increased evaporation and more severe precipitation patterns due to rising temperatures;[5] Cambridge in particular has seen a 10% annual increase in precipitation over the past decade. This climate change-induced increase in rainfall also contributes to acid rain production.

Although it may not seem to be a serious issue, acid rain in fact has very harmful and far-reaching effects on soil chemistry and water quality, aquatic ecosystems, and buildings and other infrastructure.

When soil is exposed to acid rain, basic compounds in the soil can initially act as buffers that protect against pH change. However, different types of soils contain varying levels of these compounds, and once that buffering ability is exceeded, acid rain can leach aluminum from the soil into streams and lakes.[1] This increased acidity and aluminum concentration makes ecosystems uninhabitable for a wide variety of plant and animal species;[2] in the Cambridge area, this sort of runoff would severely impact the water quality of the Charles River and endanger the many fish, bird, and other wildlife species that inhabit the area. Acid rain also strips nutrients from soil and trees and, since soil is an important filtration system for both terrestrial and aquatic ecosystems, renders it unable to regulate water chemistry properly.[1] This would adversely affect MIT’s existing green spaces, most notably Briggs Field, Kresge Oval, and the picturesque Killian Court, and make it more difficult to create new green spaces, both on campus and throughout the rest of Cambridge.

Additionally, acid rain has a corrosive effect on limestone and marble structures, as well as metals such as steel and copper.[11,13] Perhaps most notably, the limestone facade of the Institute’s iconic Great Dome could be at risk. More broadly, increased acid rain exposure could cause significant damage to MIT’s utilities systems, buildings, and public art installations.

 

Solutions

There are two main approaches to addressing the threat of increased acid rain to MIT’s infrastructure: protection and prevention.

The first method, protection, reduces the risk of corrosion by creating a physical barrier between the acid rain and vulnerable infrastructure; for example, exposed metal pipes could be coated with a nonreactive material such as PTFE, an extremely chemically inert fluoropolymer.[10] While this approach may be a feasible method of protecting at-risk components of certain utilities systems, it is not readily applicable to shielding entire buildings, art installations, green spaces, or water sources from the harmful effects of acid rain.

Acid rain presents some unique challenges with respect to protecting the Great Dome specifically. In 2009, the entire limestone layer of the Dome was replaced and a new waterproofing membrane applied between the limestone and the concrete structure below it to protect Barker Library from rainwater, which had begun to leak through the Dome.[3] Although this membrane provides excellent protection to the library and other areas under the Dome,[3] the limestone itself is still at risk of corrosion. Because the Dome is so iconic and significant to MIT, any protective measures implemented would need to be effective without compromising the aesthetic of the building or disrupting the operations of the library and other academic spaces below. Therefore, although applying a chemically inert coating or spray to the limestone directly might be an effective way to protect buildings in general, it would be unfeasible for the Dome specifically because such an addition would noticeably alter the color and texture of the facade. A better solution is to cover the Dome first with a nonreactive waterproof shell and then with a durable architectural finish designed to replicate the appearance of natural limestone.

One suitable material for the shell is UV-resistant HDPE, a durable, chemically resistant, impermeable thermoplastic that is easy to machine and weld.[4] The HDPE covering can be fabricated in segments and welded together to create a robust barrier between the Dome and acid rain. Although untreated HDPE is a relatively non-stick material, a variety of surface treatments and industrial adhesives are available to ensure a high bond strength between the HDPE and the layer of architectural finish.[6]

There are many suitable limestone finishes for this application. One example is LymestoneTM, a commercially available acrylic-based finish that closely mimics the color and texture of limestone blocks[7] but is more resistant to damage due to acid rain, as well as to repeated freezing and thawing, mildew, and extreme weather conditions.[9] Two significant advantages that LymestoneTM and similar materials have over traditional limestone are cost and availability: LymestoneTM is both much cheaper and much lighter than limestone blocks, and can be transported much more quickly and easily.[8] LymestoneTM can also help lower the heating and cooling costs of the building[8] and therefore increase MIT’s energy efficiency. Lastly, it is logistically far simpler and less disruptive to reapply the finish if it ever corrodes significantly than it is to replace the Dome’s limestone blocks again in the same manner as in the 2009 project. Taken together, the HDPE shell and architectural finish offer effective and feasible long-term protection for the Great Dome without compromising the building’s architectural style or normal daily operations. (Of course, this solution would work equally well for MIT’s Little Dome, a similar structure located above the Institute’s main entrance.)

The second potential response to acid rain, prevention, offers more comprehensive protection for MIT and Cambridge as a whole. Since most of the pollutants that eventually become acid rain are produced by factories, automobiles, or power plants, reducing emissions by encouraging the use of cleaner processes and/or decreasing people’s dependence on said processes themselves is an effective way of minimizing the amount of acid rain that is created in the first place. Feasible harm-reduction options include burning low-sulfur coal, installing smokestack filters, and passing stricter regulations for car emissions, while possible pollution prevention methods include subsidizing hybrid/electric cars and public transportation, investing in renewable energy production, and developing more efficient electronics.[15] A widespread, coordinated effort to limit emissions would reduce or possibly even eliminate the risk of damage due to acid rain.

Additionally, there is a third potential solution to the problem of acid rain as it relates to soil and water chemistry. In general, aquatic and terrestrial ecosystems are naturally able to handle some level of acidification because they possess a source of basic compounds (e.g. a rock formation containing calcium carbonate) capable of neutralizing excess acid. Intentionally adding more of these basic compounds to soil and water systems could increase their buffering capacity and make them able to handle larger amounts of acid. In particular, many researchers have focused on the mineral olivine as a financially viable way to combat ocean acidification, acid rain, and other similar processes: when powdered and dispersed in water or spread onto soil, olivine reacts with a variety of acidic compounds (it is especially effective at sequestering carbon dioxide, which is helpful for addressing ocean acidification but unfortunately not acid rain).[12] However, even if this method successfully neutralizes excess acid, much more research needs to be done to determine whether it can actually be implemented on a large scale and without any significant negative consequences (the addition of basic compounds and the neutralization reaction between acids and bases could both adversely affect the chemistry of the environment by altering the concentrations of key ions or introducing new compounds). The implementation itself should be fairly straightforward for both MIT and Cambridge at large, as the olivine (or other similar compounds) would simply need to be distributed periodically to rivers, green spaces, and other at-risk areas.

 

By Kelly Chen

 

References

  1. Apfel, A. (2008, January 30). Acid Rain Is Not Only Changing Soil Chemistry, It Is Affecting Climate Change, Says Geological Survey Scientist. Retrieved November 1, 2017, from http://news.cornell.edu/stories/2008/01/acid-rain-critical-climate-change-studies-says-scientist
  2. Effects of Acid Rain. (2017, June 01). Retrieved November 1, 2017, from https://www.epa.gov/acidrain/effects-acid-rain
  3. Great Dome Renovation, Building 10. (n.d.). Retrieved November 1, 2017, from http://capitalprojects.mit.edu/projects/great-dome-building-10
  4. HDPE Sheet Pipe Grade. (n.d.). Retrieved November 28, 2017, from https://www.curbellplastics.com/Shop-Materials/All-Materials/HDPE/HDPE-Sheet-Pipe-Grade#?Shape=CRBL.SkuSheet
  5. How Does Climate Change Affect Precipitation? (n.d.). Retrieved November 29, 2017, from https://pmm.nasa.gov/resources/faq/how-does-climate-change-affect-precipitation
  6. How to Prepare and Bond Polyethylene. (2016, June 09). Retrieved November 28, 2017, from http://www.permabond.com/2015/03/31/prepare-bond-polyethylene/
  7. Lymestone™. (n.d.). Retrieved November 19, 2017, from http://www.dryvit.com/products/stone-masonry-and-quartz-finishes/lymestone/
  8. Dryvit Systems, Inc. (2005). Lymestone™ [Brochure]. West Warwick, RI: Author.
  9. Lymestone™ Finish: 100% Acrylic-Based Finish with the Appearance of Limestone Blocks (pp. 1-3, Data Sheet). (2017). West Warwick, RI: Dryvit Systems, Inc.
  10. Polytetrafluoroethylene. (2017, October 27). Retrieved November 1, 2017, from https://en.wikipedia.org/wiki/Polytetrafluoroethylene
  11. Kucera, V. (1988). Acidification in Tropical Countries (1st ed., Vol. 37, SCOPE) (H. Rodhe & R. Herrera, Eds.). John Wiley & Sons, Inc.
    Citing only Chapter 6 (“The Effect of Acidification on Corrosion of Structures and Cultural Property”), pages 167 to 194 of the text.
  12. Schuiling, R. D., & Krijgsman, P. (2006). Enhanced Weathering: An Effective and Cheap Tool to Sequester CO2. Climatic Change, 74(1-3), 349-354. doi:10.1007/s10584-005-3485-y
  13. Viles, H. A. (1999). Acid Corrosion (of Stone and Metal). In D. E. Alexander (Ed.), Environmental Geology (p. 1). Springer Netherlands. Retrieved November 1, 2017, from https://link.springer.com/referenceworkentry/10.1007%2F1-4020-4494-1_1
    Environmental Geology was published in 1999 as a volume in the Encyclopedia of Earth Science series.
  14. What Is Acid Rain? (2017, March 01). Retrieved November 1, 2017, from https://www.epa.gov/acidrain/what-acid-rain
  15. What Is Being Done? (n.d.). Retrieved November 1, 2017, from https://www3.epa.gov/acidrain/education/site_students/beingdone.html

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