Transition metals and water-soluble ions in deposits on a building and their potential catalysis of stone decay J.J. McAlister a, * , B.J. Smith a , A. To ¨ ro ¨k b a School of Geography, Archaeology and Palaeoecology, Queen’s University, University Road, Belfast BT7 1NN, United Kingdom b Department of Construction Materials and Engineering Geology, Budapest University of Technology and Economics, H-1521 Budapest, Hungary article info Article history: Received 27 February 2008 Received in revised form 19 May 2008 Accepted 23 May 2008 Keywords: Dust Transition metals Selective extraction Surface reactions Stone decay Budapest abstract Atmospheric particulates (dust) deposited on buildings are a complex chemical and mineralogical mixture including transition metal oxide matrices that act as a significant medium for further surface reactions and provide efficient sinks for pollutants, especially in urban environments. Once deposited, their transformation by reaction with specific and often highly localised environmental conditions across building facades is related to their degree of exposure to rain-wash. These transformations are central to the soiling of buildings and the availability of salts that lead to stone decay. To investigate these rela- tionships, samples were collected at high and low elevations and under highly and moderately sheltered conditions from a building located on a busy arterial route in Budapest. Selective extraction analysis highlights the mobility/availability of Fe, Mn, Zn, Cu, Cr, Pb and Ni, plus water-soluble Ca 2þ , Mg 2þ , Na þ ,K þ , Cl  , SO 4 2 and NO 3  and their potential to take part in surface reactions that could enhance stone decay. Concentrations of water-soluble Fe, Mn and Zn in sheltered dust reach 126 mg kg 1 , 80 mg kg 1 and 220 mg kg 1 respectively and under acidic environmental conditions and high humidity, similar levels of Mn and significantly higher concentrations of Fe, Zn, Cu and Pb may be released from the exchangeable/carbonate phase making these metals potentially available to catalyse surface reactions. Sulphate and nitrate coatings plus sufficient moisture increase metal solubility and active sites may be regenerated allowing mobile transition metals to become available and possibly catalyse further surface reactions. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Weathering of buildings by atmospheric pollution is a major problem in urban environments. Buildings are subjected to high levels of particle deposition by both wet and dry mechanisms. There are a number of phoretic effects that cause dry deposition and these include, condensation, thermal collision of air molecules (Brownian motion), temperature effects (Thermophoresis), gravitationalsettling and electrostatic forces (Electrophoresis) (Camuffo, 1998). Dry deposition tends to be slow and continuous, whereas wet deposition can deliver sudden and infrequent concen- trations of pollutants in dilute solution (Colin, 1998). Wetting of deposited dust and underlying stone can mobi- lise any soluble salts present, especially sulphates, nitrates and chlorides and later evaporation can cause these salts to crystallise. This is the basis of a well-researched stone decay process (Brimblecombe and Camuffo, 2003; Saiz-Jimenez, 2004). Included in this is the development of so-called ‘‘black crusts’’ composing interlocking crystals of gypsum that bind in a wide variety of combustion particles. These crusts and the wisdom of their removal have long been a dominant topic of debate for stone conservators in areas such as Western Europe. In post-communist and Eastern Europe, however, this debate has taken significance in terms of both the number of highly polluted buildings and ongoing * Corresponding author. Tel.: þ44 289 0973393; fax: þ44 289 0973212. E-mail address: j.mcalister@qub.ac.uk (J.J. McAlister). Contents lists available at ScienceDirect Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv 1352-2310/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2008.05.067 Atmospheric Environment 42 (2008) 7657–7668