The Historic Built Environment As a Long-Term Geochemical Archive: Telling the Time on the Urban "Pollution Clock"

This study introduces a novel methodology for utilizinghistoricbuilt environments as reliable long-term geochemical archives, addressinga gap in the reconstruction of past anthropogenic pollution levelsin urban settings. For the first time, we employ high-resolution laserablation mass spectrometry for lead isotope (Pb-206/Pb-207 and Pb-208/Pb-206) analysis on 350-year-oldblack crust stratigraphies found on historic built structures, providinginsights into past air pollution signatures. Our findings reveal agradual shift in the crust stratigraphy toward lower Pb-206/Pb-207 and higher Pb-208/(206)Pbisotope ratios from the older to the younger layers, indicating changesin lead sources over time. Mass balance analysis of the isotope datashows black crust layers formed since 1669 primarily contain over90% Pb from coal burning, while other lead sources from a set of modernpollution including but not limited to leaded gasoline (introducedafter 1920) become dominant (up to 60%) from 1875 onward. In contrastto global archives such as ice cores that provide integrated signalsof long-distance pollution, our study contributes to a deeper understandingof localized pollution levels, specifically in urban settings. Ourapproach complements multiple sources of evidence, enhancing our understandingof air pollution dynamics and trends, and the impact of human activitieson urban environments. This papercontributes a novel approach to developing amore reliable systematic methodology for utilizing black crusts aslong-term geochemical archives with a finer-scale resolution for urbanpollution.