Study on the transport mechanism of surface particulate matter based on the closed earth-air model

Particulate matter (PM) is a central concern in atmospheric environmental research and poses a serious threat to the long-term preservation of grotto heritage sites such as the Mogao Grottoes. Previous studies of PM in semi-enclosed grottoes have mainly focused on windblown sand intrusion, external meteorological variability, and human activities, while the internal driving role of earth-air vertical movement (EVM) has received little attention. Using the semi-enclosed basement of conservation department at the Mogao Grottoes as an ideal field site, we conducted a one-year in situ continuous monitoring campaign based on closed earth-air theoretical framework. Spearman correlation analysis and structural equation modeling (SEM) were applied to quantify the direct/indirect effects of EVM on PM and to clarify the mechanism by which EVM drives dynamic changes in surface PM. The results indicate that atmospheric pressure fluctuations drive EVM, which is a major natural factor influencing surface PM. Surface PM generally exhibited positive synchronous variations with earth-air flow (EF), CO₂ and ²²²Rn concentrations in earth-air, as well as with temperature and relative humidity (RH). However, earth-air RH and ²²²Rn showed weak negative correlations with PM₁₀₀, with correlation coefficients of -0.04 and -0.07. The SEM results further verify that CO₂ acts as a characteristic tracer for PM dynamics driven by EVM. It exerts positive driving effects on PM_2.5, PM₁ and PM₁₀₀ with corresponding path coefficients of 0.452, 0.470 and 0.386, respectively. Although ²²²Rn also imposes positive regulation on PM, its impact intensity is far weaker than that of CO₂. By contrast, RH produces prominent inhibitory effects on PM, while AP and temperature only exert slight negative influences. PM_2.5 and PM₁ were strongly affected by EVM, whereas PM₁₀₀ was only weakly affected. This study provides a novel theoretical perspective on matter exchange at the land-atmosphere interface for elucidating the transport mechanisms of surface PM.