Grotto Microenvironment Shapes Aerosol Composition: Implications for Long-Term Preservation of Stone Carvings

Air pollution-induced deterioration is recognized as one of the main causes of stone heritage degradation. However, the differences between indoor and outdoor airborne particles and their implications remain poorly understood due to the scarce of observational data. We compared the physical and chemical characteristics of individual particles collected inside and outside Cave 5 of the Yungang Grottoes through a quantitative electron probe microanalysis. Five groups were identified, with reactive mineral dust (RMD) being dominant: 51.7% inside versus 46.0% outside. Moreover, the average atomic concentration of S was also higher: 3.80 ± 1.49% vs 2.66 ± 0.77%. Their primary form was S-containing Ca-AlSiOx, which was more readily formed via heterogeneous reactions with SO₂ under specific microenvironment. This suggests that the interior microenvironment, with more stable temperatures and higher relative humidity, effectively promotes chemical reactions occurring on the particles, which may lead to distinct weathering impacts on grotto heritage inside the cave.