A New ARChive of urban atmosphere evolution during the last centuries revealed by in-situ analysis of Black crusts from HISTorical monuments – ANARCHIST

Knowledge of past and present air pollution is essential for understanding human-environment interactions and assessing their impact on health, ecosystems and cultural heritage. However, direct measurements of air quality are recent and sporadic. To go back into the past, pollution proxies need to be found. This is why, in the ANARCHIST project, we propose to study the black crusts formed on monuments - characteristic of the urban environment and representative of local pollution - as archives of recent and ancient pollution. Sheltered from the rains, black crusts are formed of gypsum strata that have trapped particulate matter from the atmosphere. As a result, the gypsum matrix and its contents can potentially bear witness of chemical reactions and sources of pollution over time. To reveal the potential of black crusts, we will have at our disposal a set of unique samples exposed to the Parisian atmosphere from the 13th century to the present day (statues of the heads of the Kings of Judah, Père Lachaise cemetery). In order to analyze them, the project will develop innovative protocols for characterizing various tracers at the micrometric scale: metallic elements and 14C to trace sources; lead isotopes to trace sources and date samples; S, O and N isotopes to trace the oxidizing character of the atmosphere. The results will then be compared with a historical study (carried out in parallel using different type of archives). Once the methodology has been validated, we will have a unique archive for tracing pollution sources and the evolution of their contribution over time, and for a better understanding of atmospheric chemical processes. This archive could then be used to obtain data on past pollution at numerous sites, to assess the validity of chemistry-transport models in the past, to gain a better understanding of our industrial history, or to evaluate exposure to pollution on a fine spatial scale.