Quantity and spatial distribution of earth-air activity in Cave 108 of the Dunhuang Mogao Grottoes

Monitoring of earth-air humidity in soil has revealed extensive rising/falling activity on the land. Alternation of dry and wet air is the main root cause deterioration in cave wall paintings. However, the quantity and spatial distribution of earth-air activity in caves remain unclear. This study used air flowmeters to monitor the earth-air outflow at 136 points on the west, south, and north walls of Cave 108 in the Dunhuang Mogao Grottoes, and to determine its spatial distribution. Results showed that under the influence of atmospheric fluctuation, there is frequent air exchange between the atmosphere and the earth-air, and that the daily outflow of earth-air from Cave 108 is 50.893 m(3)/d. Earth-air flux is negatively correlated with atmospheric pressure, and the fluctuation of atmospheric pressure is the main driving force of earth-air activity. The average outflow of earth-air from the ground of Cave 108 is 0.109 L/(min center dot m(2)). Apart from that, the overall spatial distribution of earth-air quantity showed the following pattern: ground > west wall > north wall > south wall. What's more, the outflow of earth-air is inversely proportional to the wall height, while proportional to the depth of the cave embedded in the cliff. In such case, sealing the cave floor with concrete will heighten earth-air flow on the walls, thereby quickening the deterioration process of wall paintings. This study unveiled the deterioration mechanism of wall paintings in the context of earth-air conditions, thus establishing a foundation for the scientific protection of wall paintings.