Multi O- and S-isotopes as tracers of black crusts formation under volcanic and non-volcanic atmospheric conditions in Sicily (Italy)

The deterioration of monument or building stone materials is mostly due to the growth of black crusts that cause blackening and disaggregation of the exposed surface. This study reports on new oxygen (δ¹⁷O, δ¹⁸O and Δ¹⁷O) and sulphur (δ³³S, δ³⁴S, δ³⁶S, Δ³³S and Δ³⁶S) isotopic analyses of black crust sulphates formed on building stones in Sicily (Southern Italy). The measurements are used to identify the possible influence of volcanic emissions on black crust formation. Black crusts were mostly sampled on carbonate stone substrate in different locations subject to various sulphur emission sources (marine, anthropogenic and volcanic). Unlike atmospheric sulphate aerosols that mostly exhibit Δ³³S > 0‰, here most of the analysed black crust sulphates show negative Δ³³S. This confirms that black crust sulphates do not result from deposition of sulphate aerosols or of rainwater but mostly from the oxidation of dry deposited SO₂ onto the stone substrate. The δ³⁴S and δ¹⁸O values indicate that most of black crust sulphate originates from anthropogenic activities. Δ¹⁷O values are found to be related to the sampling location. The largest ¹⁷O-anomalies (up to ~4‰) are measured in black crust from areas highly influenced by volcanic emissions, which demonstrates the strong involvement of ozone in the formation of black crusts in volcanically influenced environments.