A study of the ozone oxidation mechanism of the organic pigment safflower yellow in grotto temple murals

As precious historical and cultural heritage, grotto temple murals are increasingly threatened by severe pigment fading under long-term multi-environmental stress, with ozone(O₃)-induced oxidative degradation of organic pigments being a critical unresolved issue. Herein, high-performance liquid chromatography (HPLC) was used to analyze mural samples from Tiantishan Grottoes, the Potala Palace, and Maijishan Grottoes, successfully identifying the natural organic pigment safflower yellow (SY). Comparative analysis between laboratory-simulated and actual mural samples confirmed that the SY was detected in the murals as a mixture of native SY and its O₃ oxidation degradation products. The mass contents of SY in samples TTS-0624, BDLG and MJS-156 were 470, 95 and 315 μg/g, respectively. The oxidation pathways and products are mutually corroborated by the results of ultraviolet spectroscopy (UV), Fourier transform infrared spectroscopy (FTIR), HPLC, and high-performance liquid chromatography-mass spectrometry (HPLC-MS). These results reveal the degradation mechanism and influence of ambient ozone on organic pigments in murals, providing important references for the study of gas-phase evolution processes of natural organic pigments under the background of increasingly severe atmospheric O₃ pollution. They also offer theoretical support and technical basis for the targeted protection of precious cultural heritage such as ancient Chinese grotto temple murals.