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

Atmospheric particulate matter represents a critical environmental issue that endangers public health and the long-term preservation of cultural heritage sites, such as grotto temples. While the source apportionment, spatiotemporal evolution, and driving mechanisms of particulate matter in open atmospheric environments has been well developed. Nevertheless, there is still a lack of systematic and in-depth understanding of the intrinsic driving mechanism of earth-air vertical movement on particulate matter transport within semi-enclosed grotto spaces. This study took the semi-enclosed basement of the conservation department of Dunhuang Mogao Grottoes as the research object. Based on the closed earth-air theoretical model, one-year in-situ continuous monitoring was carried out to synchronously obtain data on earth-air parameters, meteorological indicators and multi-size particulate matter concentrations, so as to quantitatively reveal the driving mechanism of earth-air vertical movement on the dynamic variation of particulate matter. The results show that atmospheric pressure fluctuations constitute the key driving force of earth-air vertical movement. Overall, particulate matter concentrations exhibit a significant positive correlation with earth-air flow, CO2, 222Rn, temperature and relative humidity. Only earth-air relative humidity and ²²²Rn are negatively correlated with PM100, and earth-air CO2 is identified as the key factor most strongly associated with particulate matter. Structural equation modeling verified that earth-air vertical movement serves as the key natural driving force for the dynamic variations of surface particulate matter in semi-enclosed spaces. Earth-air CO2 can promote the hygroscopic growth of particulate matter and the formation of secondary aerosols through the dual effects of physical transport and chemical coupling, exerting a significant positive driving effect on particulate matter of all particle sizes. Earth-air relative humidity imposes an inhibitory effect on the dynamic variation of particulate matter, while 222Rn has almost no notable impact on particulate matter. This finding breaks the cognitive limitation of traditional studies that merely focus on external pollution sources. It verifies that earth-air vertical movement acts as a crucial natural driving force for the transport of surface particulate matter, providing an entirely new scientific basis for the precise prevention and control of particulate pollution as well as the preventive conservation of grotto temple cultural heritage sites.