Ush 102d

Sample cross-sections were primarily observed under optical microscopy in order to document the stratigraphic morphology of the paint layers. A dark field observation was performed, with a BX51 (Olympus Optical, Tokyo, Japan) microscope equipped with a digital scanner camera Olympus DP70. A 100-W halogen projection lamp and a Ushio Electric USH102D ultraviolet (UV) lamp (Ushio Inc, Tokyo, Japan) were employed for the acquisition of visible and fluorescent images, respectively.

Solid particles (80 mg) with different concentrations of catechol were poured into thin-wall quartz tubes (707-SQ-250M, Wilmad, USA). These particles were left in the dark for 3 days or subjected to light irradiation for 180 min in pure N₂ or O₂. The irradiation was provided by an arc light source (LOT-Oriel GmbH & CO. KG, Germany) equipped with a 100 W mercury lamp (USH-102D, Ushio, Japan). Two glass filters were used to restrict the emissions below 280 ± 10 nm or below 340 ± 20 nm (JB280 and JB340, Ceaulight, Beijing, China). The tubes were filled with a continuous flowing gas consisting of different N₂:O₂ ratios (100:0, 70:30, 50:50, 30:70, and 0:100). The gases (N₂ and O₂) were highly pure (≥99.999%), and the water content of the gases was less than 3 mg/m³. Because of the surface adsorbed O₂ and O₂ presence before N₂ inflow, it is not practical to completely remove O₂. Therefore, for the atomosphere condition of 100% N₂, it accutually refers to O₂-limited condition. As reference samples, solid particles were placed in the same atmosphere environment without UV/Vis light irradiation. The concentration of catechol was analyzed after 0, 10, 30, 60, 90, 120, 150, and 180 min of irradiation. All of the samples were run in duplicate.