Fate of atmospheric particles within the Buddhist cave temples at Yungang, China

Mech. Eng., Dep. Environ. Quality Lab., California Inst. Technol., Pasadena, CA 91125, USA

Environmental Science & Technology vol. 30, no. 12, pp. 3425-3434, Dec 1996 ISSN 0013-936X

The Yungang Grottoes are a collection of man-made cave temples dating from the 5th century A.D. that now are situated in the middle of one of China's largest coal mining regions. Air pollutant particles enter these caves and deposit onto the more than 50 000 stone carvings contained within the caves, leading to rapid soiling of the sculptures. In order to study this problem, computer-based models have been combined that simulate the air flow into the caves and particle deposition within the caves. The evolution of the airborne particle concentration and size distribution is tracked as outdoor air is drawn into the caves by a natural convection flow that is driven by the temperature difference between the outdoor air and the interior walls of the caves. Particle deposition rates are computed from the boundary layer flows along the surfaces within the caves. Predicted coarse airborne particle (diameter > 2.3 mu m) size distribution and coarse particle deposition fluxes to horizontal surfaces within caves 6 and 9 at Yungang compare closely to experimental observations made during the period April 15-16, 1991. It is found that horizontal surfaces within caves 6 and 9 at Yungang would become completely covered by a full monolayer of particles in only 0.3-0.5 yr under the April conditions studied here and will be soiled even more rapidly under annual average conditions. The model developed here can be used in the future to compute the effects of particle filtration systems and/or altered ventilation rates on soiling within the grottoes.