Even the Terracotta Warriors, a must-see attraction included in China travel packages , are feeling the effects of China's choking air pollution.
Chinese scientists have found that the indoor environment where the Terracotta Warriors are housed could cause them to deteriorate, prompting some scientists to raise the idea of using air curtain technology to help control the environment in the pits.
In early February, Gu Zhaolin, a professor at Xi'an Jiaotong University, and his team published a paper in Environmental Science and Technology, a prominent journal under the American Chemical Society, which showed the results of a research project sponsored by the Ministry of Science and Technology to develop a system to maintain a comfortable environment for the Terracotta Warriors.
An air curtain looks similar to household air conditioners. The machines eject a powerful airflow, which forms a sheer air layer that separates the environment on the two sides.
"When you are shopping in the supermarket, you might have seen those freezers without a cover. The reason that the temperature in the freezers is lower than the outside is that they have the air curtain, which you cannot see but does exist, to prevent the exchange of air and heat," Gu said.
"The air curtain is just like other barriers, for example a glass wall. It can prevent more than 90 percent of the pollutants in the air from entering the pits," he said.
Installing an air curtain inside the pits could protect the Terracotta Warriors and other pottery relics from further damage, Gu said.
Major environmental factors affecting the relics include temperature, humidity, air pollutants, lighting, dust and mold.
Ma Tao, deputy director of the Shaanxi Provincial Institute for the Conservation of Cultural Heritage, said that the pollutants from visitors and atmospheric pollution could penetrate the enclosed hall and cause the relics to deteriorate. Ma is a co-author of the paper published in Environmental Science and Technology.
"For example, automobile exhaust includes pollutants such as hydrocarbon compounds, nitrogen oxides and sulfur dioxide, and when it penetrates the Terracotta Warriors, it decomposes the pigments on the surface, then it deposits and continues to erode the pottery," Ma said.
Earlier studies on the Terracotta Warriors on display show that humidity and dust are the most obvious factors affecting the ancient relics, Ma said.
Most of the more than 1,500 Terracotta Warriors that have been excavated have lost their color, and being exposed to the air will worsen the deterioration, he said.
The Emperor Qin Shihuang's Mausoleum Museum has been monitoring the environment in the pits since the 1980s, and has published successive measures for environmental control, including improving museum greening, adapting windows for proper lighting and banning vehicles from its gardens.
A team of scientists is also studying the influence of particulate matter on the Terracotta Warriors.
Zhou Tie, chief engineer of the Emperor Qin's Terracotta Warriors and Horses Museum, said environmental control at the museum must be further strengthened.
"Air curtain technology, which is a kind of modern science and technology, has found a good application in other fields. Applying this technology in the museum is a good idea, which can effectively isolate the outside contaminants from the heritage items inside," Zhou said.
However, Zhou said, as there is no successful precedent applying the system to a site museum - which involves water, earth and air in multiple fields with many environmental factors - the technology still needs long-term experimentation.
Air curtains have been used in museums outside China to fix the indoor environment, instead of preserving relics.
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