Selection of relevant black crusts samples as ancient air pollution archives

Since the Industrial revolution and due to increasing anthropogenic emissions, the composition of the atmosphere has been modified, leading to climate change and pollution. The impacts of pollution were depicted through paintings and writing from the beginning of the 19^th century, but pollution measurements are relatively recent. In order to obtain ancient air pollution data, proxies in urban area need to be found.

Black crusts formed on limestone and marble monuments in urban area seem to be a good candidate as local proxy. Mainly composed of gypsum (CaSO₄.2H₂O), they are a chemical alteration pattern resulting from the reaction of the dissolution of the calcite (CaCO₃) of the stone and of sulfation by sulphur dioxide (SO₂) from the atmosphere. Particulate matter accumulates in the newly formed gypsum layer in sheltered area from the rain, thus giving the black crust a passive sampler potential.

To use black crusts as past air pollution archives, samples were collected at Père Lachaise cemetery (Paris) on ancient, dated (1820-1887) and unrestored limestone or marble tombs. Different types of analyses were performed to study sample morphology (by Optical Microscope), particulate matter (by Scanning Electron Microscopy) and chemical composition (especially major elements and trace metals by ICP-AES, LA-ICP-MS). Results underline two important features to use black crusts as past air pollution archives. First, the low variability of chemical composition of black crusts from Père Lachaise cemetery highlights that the black crusts are representative of the site and register the background pollution. Then, the morphology (laminar vs. dendritic) of black crusts is a key parameter to sample black crusts as the stratigraphy is much better preserved in laminar black crusts.