Abstract
The effects of the dry and wet acid deposition from both combustion and greenhouse gases (mainly CO~2~, SO~x~ and NO~x~) can be observed in the stones, mortars, bricks and decorative materials used in Built Heritage elements. Those deposition phenomena have greater effects in urban atmospheres, especially in medium or highly polluted ones. Most of the products formed as a consequence of decaying are alkaline and alkaline‐earth oxoanions, which show, in general, a medium or high Raman scattering. The coupling of the experimental evidence (the detection of some metal–oxoanionic compounds by Raman spectroscopy) with the knowledge of chemical equilibrium is presented in this work as a tool to diagnose the impacts of CO~2~, SO~x~ and NO~x~ on the properties of Built Heritage. The compounds identified by Raman spectroscopy on different materials sampled in buildings of the Metropolitan Bilbao area (North Spain) included nitrocalcite, nitratine, nitromagnesite, nitrobarite as well as gypsum and soot as part of the black crust on carbonate‐based materials, whereas calcite, natron, nitratine and mirabillite were recognised as composition of the white efflorescence in non‐carbonate materials and mortars. An overall degradation pathway is proposed to explain the formation of each decayed compound as a function of the original characteristic material. Copyright © 2008 John Wiley & Sons, Ltd.