JOURNAL OF RAMAN SPECTROSCOPY J. Raman Spectrosc. 2008; 39: 1042–1049 Published online 17 March 2008 in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/jrs.1937 Raman spectroscopy as a tool to diagnose the impacts of combustion and greenhouse acid gases on properties of Built Heritage A. Sarmiento, 1 M. Maguregui, 1 I. Martinez-Arkarazo, 1* M. Angulo, 1 K. Castro, 1 M. A. Olaz ´ abal, 1 L. A. Fern ´ andez, 1 M. D. Rodr´ ıguez-Laso, 2 A. M. Mujika, 3 J. G ´ omez 3 and J. M. Madariaga 1 1 Department of Analytical Chemistry, University of the Basque Country, (UPV/EHU), 48080 Bilbao, Spain 2 Department of Painting, University of the Basque Country, (UPV/EHU), 48080 Bilbao, Spain 3 Department of Sculpture, University of the Basque Country, (UPV/EHU), 48080 Bilbao, Spain Received 13 November 2007; Accepted 9 January 2008 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. KEYWORDS: acid gases; Built Heritage; diagnosis; impacts; Raman spectroscopy INTRODUCTION Dry and wet acid deposition from both combustion and greenhouse acid gases (mainly CO 2 , SO x and NO x ) are two phenomena responsible in part for the damage of mineral materials and stones. The effects of the acid attack can be observed in the stones, mortars, bricks and decorative materials used in Built Heritage. Some works dated 10 years ago referred, for instance, the presence of calcium (and magnesium) salts of sulfate, sulfite, nitrate and nitrite 1 as decayed compounds in such materials. These salts are nowadays well recognised as indicators of decaying. The cited deposition phenomena have greater effects in urban atmospheres, especially in medium or highly polluted ones. However, in areas considered as non-polluted, similar L Correspondence to: I. Martinez-Arkarazo, Department of Analytical Chemistry, University of the Basque Country, (UPV/EHU), 48080 Bilbao, Spain. E-mail: irantzu.martinez@ehu.es damages are said to be observed in the long term owing to the presence of greenhouse acid gases in the atmosphere. 2,3 Most of the different analytical approaches to define the decayed products make use of surface analysis techniques (proton induced X-ray emission (PIXE), scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) or X-ray diffraction (XRD) 3 ). Wet analytical methods, such as the Italian NORMAL 13/83, 4 are currently used as well, but with those methods only total concentrations of anions and cations can be determined, i.e. ion chromatography applied to aqueous extracts obtained from solid samples after performing a test to extract soluble salts. 5 The products formed as a consequence of the acid attack of greenhouse and combustion acid gases on materials of Built Heritage are metallic oxoanions, 6 all of which prac- tically show medium or high Raman scattering. Therefore, Raman spectroscopy can be one of the best analytical tools to detect such compounds because it can (1) differentiate Copyright  2008 John Wiley & Sons, Ltd.