Organic anions in damage layers on monuments and buildings

This article is focused on small (C1–C2) organic anions present in the damage layers on historic monuments and buildings. Formate, acetate and oxalate are consistently found in black crusts, where atmospheric deposition accumulates along with the products of the chemical transformation of stone and mortars. While sulphation processes affecting building materials have been extensively studied, the importance of carbon compounds in black crusts is only recently being realised. Recent data show carbon to be the second most important airborne element after sulphur in damage layers on building exteriors. Total carbon is composed of carbonate-, organic-, and elemental carbon. The organic fraction includes formate, acetate and oxalate; these are always detected in black crusts. Their origin, role and measurement in the atmosphere and in the museum environment have been the subject of many studies, but little has been reported concerning their presence in building exterior damage layers. This paper presents data on these anions in damage layers on stones and mortars sampled on monuments and buildings at different urban, suburban and rural European sites. Oxalate encountered in black crusts likely originates from the metabolism of micro-organisms and protective treatments on surfaces. Primary and secondary atmospheric pollutants are likely the main sources of formate and acetate anions.