Atmospheric pollutants impact historic buildings

Acid rain deteriorates historical monuments, especially those built with calcium carbonate in its different forms (limestone, reef stone, or marble, among others) in the Mayan archaeological sites in the Yucatan Peninsula and Chiapas; El Tajin and the fortress of San Juan de Ulua, Veracruz, as well as the Palace of Fine Arts in Mexico City.

This substance (a humid atmospheric deposit with a pH less than 5.6), explained Rodolfo Sosa Echeverría, from the UNAM's Institute of Atmospheric Sciences and Climate Change (ICAyCC), is what falls on the Earth in dry form (gases or particles) and humid form: rain, snow or hail.

When the pH of rain has a value greater than 5.6, it is considered non-acidic; however, knowing that the neutral pH is 7, this slight acidity is due to the presence of a weak one, carbonic acid, which in turn was formed by the reaction of carbon dioxide with water contained in the atmosphere. A pH below 5.6 in the rain indicates the presence of stronger acids.

Atmospheric pollutants can not only affect human health, water bodies, and ecosystems but also deteriorate building materials that represent a commercial or cultural value. Those that are emitted interact with meteorology and atmospheric chemistry, generating reactions that turn the atmosphere into a reactor that produces acid rain with sulfuric and nitric acids, whose precursors are sulfur and nitrogen oxides expelled into the atmosphere by natural and anthropogenic sources.

Deterioration of structures

The archaeological zone of El Tajín, located on the coast of the Gulf of Mexico, was built with limestone, which is made of calcium carbonate, a material susceptible to react with acid rain. Meanwhile, the fortress of San Juan de Ulua was built with a coral reef (múcara stone), which is also calcium carbonate. One of the parameters to be determined is the loss of material in terms of length units in a given time. They are measured in micrometers (a micrometer is one-millionth of a meter) per year.

Based on research initiated by Humberto Bravo Álvarez, founding researcher of the then Center for Atmospheric Sciences, and from the doctoral research of Rogelio Soto Ayala, from the School of Engineering, it was found that in Tulum, Quintana Roo, there are reductions of four micrometers per year, and in El Tajín, 4.15.

However, micrometer reduction is even more critical in structures with hieroglyphs, such as lintels and stelae, as well as in the ball game at El Tajin. As they are made of stone and designed to provide information over time, data is lost and it is more difficult to decipher these symbols. It also happens because of vandalism.

Sometimes, "people arrive practically with slicers" and take away the hieroglyphs written on the stelae, as has happened in the archaeological zone of Calakmul, Campeche, considered by UNESCO as a Mixed (cultural and natural) World Heritage Site, said the researcher.

In Mexico City, the possible damage is present in buildings such as the Palace of Fine Arts, which is built with marble, another form of calcium carbonate. Several buildings in the Historic Center are covered with tezontle mosaics, a resistant, light, and porous product. Being a kind of sponge with holes, it is easy for air and rain to penetrate inside.

Luis Miguel Urbina's doctoral project is currently underway to evaluate the impact of atmospheric deposition on the building material of the San Juan de Ulúa fortress, as well as on the Templo Mayor and Ciudad Universitaria, the latter located in the capital of the Mexican Republic.

It is also necessary to study in greater depth how susceptible volcanic stone, such as that found in the Templo Mayor, is to atmospheric deposition, since most of the research at the world level, for example in Europe, is focused on material made of calcium carbonate, described Sosa Echeverría. UNAM institutes (Physics, Aesthetics, Chemistry, Atmospheric Sciences, and Climate Change, as well as other institutions of the country) were linked through the "Network of Sciences Applied to the Research and Conservation of Cultural Heritage", of Conacyt.

In addition to monitoring stations and studies to diagnose the degree of affectation by atmospheric deposition, in Mexico, it is necessary to take measures to reduce, prevent and control the impact of pollution on various archaeological sites. There are in situ strategies whereby fiberglass structures are installed on-site, for example, for tourists to observe outside. Likewise, the original pieces are kept in museums or laboratories to protect them from rain, dust, solar radiation, etcetera. In addition to the restriction of areas, natural or artificial coatings are applied, among other measures.

To reduce emissions of acid rain precursors, it is possible to change or improve fuels, control systems in industries or services, and the improvement of vehicle technologies.

Research on atmospheric deposition in some regions of Mexico is carried out with the collaboration of national and international institutions, among them: the Mexico City Atmospheric Monitoring System; the universities of Veracruz, Campeche, and Quintana Roo; the National Institute of Anthropology and History, the National Atmospheric Deposition Program of the United States, and the World Meteorological Organization.