戈壁近地表粉尘释放的环境动力机制研究—以敦煌莫高窟及周边地区为例

Gobi dust has always been a major environmental problem that threats to the protection of cultural relics in the Mogao Grottoes, Dunhuang. Dust weakens the artistic effect of the wall paintings by making them discolor, fade, crisp and even fall out. Quantifying the contribution of dust emission from varied types of gobi is the key for the protection of the wall paintings in Mogao Grottoes, while the quantification cannot be realized right now. The main reason is that the parametric study on gobi surface properties is relatively weak, lacking the dust emission mode reflecting the characteristics of environmental dynamics and related field observation. This project carries out studies on environmental dynamics of dust emission over near-surface gobi by field observation and wind tunnel simulation, which can evaluate quantitatively the effect of soil shear strength of different types of gobi on dust emission, establish the mathematical relationship between ground-air temperature difference and vertical speed and quantify the response of vertical speeds to dust emission rates of different types of gobi, thus revealing the environmental dynamism of dust emission over near-surface gobi. Besides, dust emission modes with environmental dynamic features can be developed, and it can be applicable to accurately quantify dust emission intensity for different types of near-surface gobi in Mogao Grottoes and its surrounding region and thus to evaluate scientifically the contribution of dust emitted from different types of gobi under different wind speeds to the grottoes. Therefore, this project can not only provide the scientific theory basis for the control of dust hazards in the Mogao Grottoes, but it is of guiding significance for the prevention of dust hazards for other cultural relics in the gobi region along the “One Belt And One Road”.

戈壁粉尘一直是困扰敦煌莫高窟文物保护的主要环境问题之一。粉尘使壁画变色、褪色、酥脆，甚至脱落。莫高窟文物保护亟待解决的一个关键科学问题是量化不同类型戈壁粉尘源对洞窟粉尘的贡献。目前还不能准确计算戈壁近地表粉尘释放通量，造成这种现状的原因是戈壁地表性质参数化研究薄弱，缺乏实地观测以及具有环境动力性质的戈壁粉尘释放模式。本项目通过野外观测以及风洞实验对敦煌莫高窟及周边地区不同类型戈壁粉尘释放过程的环境动力机制进行研究，定量评价不同类型戈壁土壤抗剪强度对沙尘释放的影响，建立不同类型戈壁地气温差与垂直风速的数理关系，揭示戈壁地表粉尘释放的环境动力机制，构建具有环境动力性质的戈壁近地表粉尘释放模式，评估各种戈壁类型对莫高窟洞窟粉尘的“贡献”，不仅为莫高窟粉尘危害的防治提供科学理论依据，而且对“一带一路”沿线其他戈壁地区文物的粉尘危害防治具有指导意义。

戈壁地表粉尘含量是决定粉尘释放的一个重要因素，其贡献率要大于剪切风速。在相同剪切风速下，粉尘释放通量随戈壁地表粉尘含量（＜63µm）的增加呈指数增大。水平输沙通量是戈壁地表起尘动力机制的另一个关键物理量，也是建立粉尘释放模型的一个重要参数。地气温差导致的粉尘气动抬升对于戈壁粉尘释放过程中的作用不容忽视，粉尘浓度随戈壁地表温度的升高呈线性增大，尤其在较高风速条件（＞12m.s-1）下粉尘气动抬升作用更加明显，在27-57.7℃变化范围内14m.s-1的粉尘浓度是12m.s-1的3.2倍。建立了戈壁地表粉尘含量（C）、水平输沙通量（Q）及气动夹带作用（a）与粉尘释放通量（F）的数理关系：F"=a"*〖"1.05" 〗^c*Q。粉尘释放通量观测值与模拟值二者相关性较好，决定系数R2>0.90。跃移沙粒的冲击及反弹能量的转化是粉尘释放的一个重要动力过程。跃移沙粒的反弹能量与粉尘释放通量相关性最强，冲击能量次之，5cm高度撞击能量相关性最小。莫高窟周边沙砾质戈壁是威胁洞窟文物的主要尘源地。初步估算表明：莫高窟窟顶重点保护区内PM10年释放总量约为87266.74kg，其中沙砾质戈壁单位面积PM10年释放总量为7.07×10-3 kg·m-2·a-1，沙漠单位面积PM10年释放总量为5.23×10-3 kg·m-2·a-1；莫高窟窟区年降尘总量（TSP）约为2489.58kg，其中PM10降尘量约为1867.18kg，与1992年莫高窟综合防护体系未建成时研究的降尘结果相比减少90%以上，表明莫高窟顶风沙综合防护体系对抑制粉尘释放及传输具有重要作用。本项研究为甘肃省生态保护及重大水利工程“引哈济党”提供了关键科学依据，科学翔实的调研报告助推引哈济党工程早日立项建设。