Formate, acetate and oxalate in black crusts on historic monuments

Much basic knowledge is still required as to the effects of multipollutants on historic monuments and buildings. Since black crust formation on monuments is due to the deposition of airborne pollutants on areas sheltered from wash out, the discrimination and measurement of their carbon components is fundamental for the partitioning of air pollution sources and the identification of the damage mechanisms involved. Such knowledge is an essential prerequisite for the maintenance of modern buildings, as well as the protection of ancient masonry. The total carbon (Ct) in damage layers can be considered as being composed of three main fractions: carbonate carbon (Cc), organic carbon (Co) and elemental carbon (Ce). Organic carbon has both anthropic and natural origins. The organic fraction of Ct includes the short carbon chain acids, i.e. formic, acetic and oxalic acids, whose anions are constantly detected in black crusts, alongside the anions that are typical tracers of anthropic pollution. In the atmosphere, these acids may play an important role in determining the acidity of precipitation and the dry deposition flux of acids in urbanised regions. In damage layers, they are commonly thought to originate from micro-organism metabolisms and protective treatments on monument surfaces. Primary and secondary atmospheric pollutants are advanced as important sources of formate, acetate and oxalate anions in the black crust formation.