Black crust as a passive sampler of urban pollution on the heritage buildings in Delhi, India: An analytical and modelling approach

Delhi is one of the most polluted megacities in the world. Since the ancient era, Delhi has been known for its rich heritage sites like the Red Fort, Humayun’s Tomb, and Qutub Minar, which are UNESCO World Heritage Sites. In this study, we investigate urban pollution-linked alterations at the heritage buildings (HBs) of Delhi, India, through a comparative characterization of deposited black crust (BC) and the underlying red sandstone (RS) collected from the exposed surfaces of the HBs. The BC on HBs can act as an integrative passive sampler of urban pollution, recording particulate matter, reactive gases (SO_x/NO_x), and associated heavy metal deposition. To achieve this objective, we applied a multi-analytical workflow combining X-ray diffraction (XRD), X-ray fluorescence (XRF), scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM–EDX), Fourier transform infrared spectroscopy (FTIR), carbonaceous analysis, and inductively coupled plasma mass spectrometry (ICP-MS) resolved phase assemblages, morphology, major-elemental composition profiles, and signatures of trace elements. The outcome suggests that the urban pollution sources, including vehicular emissions, road/construction/soil dust, industrial activity, and biomass burning, were identified as fingerprints of calcium (Ca) and sulfur (S) in the formed BC at the RS substrate. In this scenario, BC was enriched with Ca and S, which may cause sulfation phenomena to occur at the RS substrate as it contains low intrinsic Ca. Therefore, gypsum was identified as a dominant deteriorating agent, along with weddellite, bassanite, while carbon and heavy metals were also embedded in BC relative to the RS substrate. Additionally, in this work, a modeling approach is also utilized to assess the pollution dispersion impacts on the built HBs and linkage with BC deposition, which was employed using coupled WRF (Weather Research and Forecasting) and AERMOD (the American Meteorological Society/Environmental Protection Agency Regulatory Model) technique. Hence, considering these analytical and modelling approaches contributes to applying the site-specific conservation and preservation interventions in similar urban polluted environments.