An Analytical Approach to Characterize the Black Crust to Assess the Air Pollution Impact on Humayun’s Tomb, Delhi, India

Delhi has been a culturally rich city since ancient times, and hosts numerous heritage buildings, several of which are inscribed on the UNESCO World Heritage List. As one of the most polluted hotspots, the megacity presents elevated risks of pollutant-driven stone alteration. In this study, Humayun’s Tomb is analyzed for the pollutant-driven decay processes by characterizing the rooftop black crust (BC) formed under varying exposure conditions. A multi-analytical protocol, including XRF, XRD, FTIR, SEM-EDX, CHNS analyzer, and ICP-MS, was adopted. Outcomes consistent with the dominance of pollution-induced sulfur and calcium led to the formation of the BC matrix along with the carbonaceous framework. Gypsum was most abundant at the more wind-exposed, rain-sheltered rooftop locations and comparatively weaker at more protected areas, revealing a clear exposure and orientation related gradient in crust development. Trace-element profiles further suggest regional air pollution markers, consistent with Delhi’s major emission sources, including vehicular traffic, road/soil dust, construction dust, biomass burning, and industrial activities. As the first integrated analytical investigation at Humayun’s Tomb, this study provides a multi-analytical evidence base to inform preventive conservation strategies and the prioritization of cleaning and monitoring actions at this site and at comparable monuments in similarly polluted urban environments. Identify the pollutant variability site based on orientation and environmental exposure at the Tomb.External Ca sources drive sulfation on SiO2 and Al2O3 rich sandstone.Black crust identified as a passive air pollution sampler at the Tomb.Ca―S―O embedded with carbon across BC suggests soot accumulation during BC deposition. Identify the pollutant variability site based on orientation and environmental exposure at the Tomb. External Ca sources drive sulfation on SiO2 and Al2O3 rich sandstone. Black crust identified as a passive air pollution sampler at the Tomb. Ca―S―O embedded with carbon across BC suggests soot accumulation during BC deposition.