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Journal of Arid Land  2022, Vol. 14 Issue (12): 1395-1412    DOI: 10.1007/s40333-022-0036-6
Research article     
Concentrations, sources, and influential factors of water- soluble ions of atmospheric particles in Dunhuang Mogao Grottoes, a world heritage site in China
YANG Xiaoju1,2,3,4, WU Fasi1,2, XU Ruihong1,2, LI Na5, ZHANG Zhengmo1,2, XUE Ping1,2, WANG Wanfu1,2, ZHAO Xueyong3,*()
1National Research Center for Conservation of Ancient Wall Paintings and Earthen Sites, Dunhuang Academy, Dunhuang 736200, China
2Gansu Provincial Research Center for Conservation of Dunhang Cultural Heritage, Dunhuang 736200, China
3Naiman Desertification Research Station/Urat Desert-Grassland Research Station, Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences, Lanzhou 730000, China
4University of Chinese Academy of Sciences, Beijing 100049, China
5Chinese Academy of Cultural Heritage, Beijing 100049, China
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Atmospheric particle pollution is one of the major factors leading to degradation of ancient wall paintings, particularly heritage sites in arid and semi-arid regions. However, current systematic research on the changes, sources, and influential factors of atmospheric particulate matter and its water-soluble ion concentrations is not sufficient. Thus, the major water-soluble ion concentrations, sources, and influential factors of atmospheric particles PM2.5 and PM10 (particulate matter with an aerodynamic equivalent diameter ≤2.5 and 10.0 μm, respectively, in ambient air) were collected from Cave 16 and its ambient exterior environment in the Dunhuang Mogao Grottoes, China, between April 2015 and March 2016. Results showed that the concentrations of PM2.5 and PM10 inside and outside the cave were the highest in March 2016 and the lowest in December 2015. The higher particle concentration from March to May was related to the frequent occurrence of sand and dust events, and the lower particle concentration from June to September was associated with good diffusion conditions, increased precipitation, and an established cave shelterbelt. The concentration of particulate matter inside the cave was affected by the concentration of particles in the air outside the cave. Ca2+, NH+ 4, Na+, Cl-, and SO2- 4were the main components of the total ions of PM2.5 and PM10 both inside and outside the cave. The total ions inside the cave were frequently affected by the disturbance of tourists' activities during the peak tourist season from May to August. Under the influence of dust, the total concentrations of Cl-, SO2- 4, Na+, NH+ 4, and Ca2+ in particles of different sizes inside and outside the cave increased, and the concentrations of Cl-, SO2- 4, Na+, and Ca2+ decreased during precipitation period. Backward air mass trajectory analysis suggested that the pollutants were mainly from Xinjiang, China. The pollutant sources of air particulates are straw burning, secondary pollution sources, soil dust, dry spring rivers, and tourist activities.

Key wordsgrotto temple      atmospheric particulate matter pollution      water-soluble ion      water and salt transport      heritage preventive conservation     
Received: 26 June 2022      Published: 31 December 2022
Corresponding Authors: ZHAO Xueyong     E-mail:
Cite this article:

YANG Xiaoju, WU Fasi, XU Ruihong, LI Na, ZHANG Zhengmo, XUE Ping, WANG Wanfu, ZHAO Xueyong. Concentrations, sources, and influential factors of water- soluble ions of atmospheric particles in Dunhuang Mogao Grottoes, a world heritage site in China. Journal of Arid Land, 2022, 14(12): 1395-1412.

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Fig. 1 Location of the sampling sites and air particulate pollution at the Dunhuang Mogao Grottoes. (a), aerial photo of the study area; (b), sandstorm pollution outside the cave; (c), dust reduction on the surface of sculptures inside the cave; (d), orthophoto of the Dunhuang Mogao Grottoes; (e), air particulate matter pollution inside the cave; (f), sampling point outside the cave; (g), sampling point inside the cave.
Fig. 2 Pathways of particles entering the cave. (a), hand drawing of Cave 16 of the Mogao Grottoes, 2004 (the measurement unit in the figure is cm); (b), wooden cave door; (c), inside the cave.
Fig. 3 Monthly variation of PM2.5 and PM10 concentrations inside (a) and outside (b) the cave
Sampling point Air temperature Relative humidity Air pressure Wind speed Precipitation Number of tourists
PM2.5 outside -0.164 -0.046 0.197 0.217 -0.382 0.586*
PM10 outside -0.042 -0.123 0.107 0.299 -0.395 0.489
PM2.5 inside -0.191 -0.166 0.081 0.345 -0.271 0.615*
PM10 inside -0.160 -0.192 0.083 0.377 -0.281 0.598*
Table 1 Correlation coefficients of PM2.5 and PM10 inside and outside the cave with meteorological factors and number of tourists
Water-soluble ion Inside the cave Outside the cave
PM2.5 (µg/m3) PM10 (µg/m3) PM2.5/PM10 PM2.5 (µg/m3) PM10 (µg/m3) PM2.5/PM10
F- 0.0244±0.0222 0.0406±0.0292 0.5651±0.1571 0.0121±0.0065 0.0236±0.0146 0.5488±0.1378
Cl- 1.8148±1.1201 3.2967±1.9301 0.5640±0.1230 2.1414±1.8429 3.6763±2.5832 0.6117±0.1379
SO2- 4 1.0807±1.1201 2.0888±1.7851 0.4883±0.1424 1.5293±2.1851 2.6484±2.7977 0.5498±0.1490
NO- 3 0.4009±0.3349 0.7279±0.6338 0.5498±0.1041 0.5230±0.4101 0.8858±0.7251 0.6204±0.1027
Na+ 1.1287±0.8054 2.6107±1.7141 0.4496±0.1555 1.1014±0.7051 3.3736±3.8020 0.3935±0.1804
NH+ 4 1.8427±1.2874 2.9178±1.9891 0.6217±0.1770 1.6681±1.1033 2.7246±1.4897 0.5959±0.1633
K+ 0.4718±0.2525 0.9396±0.4702 0.5023±0.1133 0.4510±0.2507 0.9211±0.5518 0.4909±0.1009
Mg2+ 0.2946±0.2499 0.5676±0.3226 0.4759±0.1846 0.2397±0.1874 0.5367±0.3529 0.4288±0.1457
Ca2+ 3.9520±2.8727 9.1679±4.2838 0.4230±0.1907 4.0340±3.3046 10.5977±5.7177 0.3651±0.2167
TIC 11.0106±5.4744 22.3576±10.5183 - 11.7001±7.1713 25.3878±15.7937 -
NO- 3/SO2- 4 0.4206±0.2183 0.3471±0.1319 - 0.4420±0.2554 0.3617±0.1524 -
Table 2 Concentrations of water-soluble ions of PM2.5 and PM10 inside and outside the cave
Fig. 4 Proportion of water-soluble ions in PM2.5 (a) and PM10 (b) inside and outside (c and d) the cave
Fig. 5 Monthly average concentrations of water-soluble ions in PM2.5 (a) and PM10 (b) inside and outside (c and d) the cave
Fig. 6 Concentration of PM2.5 and PM10 inside and outside the cave of the Mogao Grottoes under sunny weather, sandstorm weather, and precipitation
Fig. 7 Concentrations of water-soluble ions in PM2.5 (a) and PM10 (b), relative proportion of soluble ions in PM2.5 (c) and PM10 (d) inside and outside the cave of the Mogao Grottoes under sunny weather, sandstorm weather, and precipitation
Fig. 8 Twenty-four-hour backward trajectory on 29 April, 2015. NOAA, National Oceanic and Atmospheric Administration; HYSPLIT, Hybrid Single-Particle Lagrangian Integrated Trajectory. GDAS, Global Data Assimilation System AGL, above ground level.
Fig. 9 Ion balance of PM2.5 (a and c) and PM10 (b and d) inside and outside the cave of the Mogao Grottoes
Content Load (inside the cave) Load (outside the cave)
F1 F2 F3 F1 F2 F3
Na+ 0.805 0.287 -0.348 0.329 0.896 0.094
NH+ 4 0.563 -0.305 -0.472 0.070 -0.133 0.945
K+ 0.865 0.008 0.010 0.382 0.606 0.639
Mg2+ -0.045 0.866 0.303 0.361 0.605 0.149
Ca2+ 0.144 0.962 -0.028 0.114 0.880 -0.359
F 0.113 0.306 0.764 0.823 0.118 0.153
Cl 0.705 0.283 0.260 0.771 0.498 0.060
NO- 3 0.894 -0.084 0.221 0.794 0.238 0.407
SO2- 4 0.787 0.001 0.157 0.863 0.315 -0.101
Eigenvalue 3.665 2.032 1.162 3.050 2.746 1.665
Contribution rate (%) 40.730 22.580 12.910 33.890 30.520 18.500
Table 3 Varimax rotated factor loading matrix for water-soluble ions in PM2.5 inside and outside the cave
Content Load (inside the cave) Load (outside the cave)
F1 F2 F3 F1 F2 F3
Na+ 0.511 0.727 0.135 0.878 0.292 0.183
NH+ 4 0.929 -0.057 0.020 0.071 0.846 0.415
K+ 0.673 0.603 0.146 0.729 0.536 0.224
Mg2+ 0.028 0.046 0.989 0.835 0.131 0.379
Ca2+ 0.060 0.350 0.911 0.902 0.009 0.268
F -0.034 0.753 0.219 0.292 0.347 0.856
Cl 0.397 0.830 0.129 0.853 0.435 0.045
NO- 3 0.868 0.319 0.012 0.485 0.743 0.096
SO2- 4 0.745 0.519 0.032 0.864 0.378 0.077
Eigenvalue 3.050 2.647 1.914 4.614 2.110 1.222
Contribution rate (%) 33.890 29.420 21.260 51.270 23.440 13.580
Table 4 Varimax rotated factor loading matrix for water-soluble ions in PM10 inside and outside the cave
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