Grotto microenvironment shapes aerosol composition: Implications for long-term preservation of stone carvings

Microenvironment inside grottoes plays an important role in affecting aerosol-induced deterioration on stone heritage. To explore the effects of microenvironment on composition and aging status of airborne particles, we compared physicochemical characteristics of 5299 individual particles in 36 samples collected in January, 2024 inside and outside Cave 5 of Yungang Grottoes through a quantitative electron probe microanalysis. Five groups were identified, with reacted mineral dust (RMD) being dominant, followed by unreacted mineral dust, carbonaceous particles, sodium salts, and others. The abundances of RMD inside the cave outnumbered those outside (51.7% vs 46.0%). Many sulfur-containing Ca-AlSiOx particles, likely formed via heterogeneous reactions of aluminosilicates with atmospheric SO₂, were observed, with markedly higher sulfur levels in indoor particles. This suggests that interior microenvironment featuring stable temperature and higher relative humidity inside the cave effectively promote chemical reactions on airborne particles, which may lead to distinct weathering impacts on grotto heritage.