Crusts formed on different building materials from the cathedral of Seville, Spain

The crusts are present on façades of the Seville Cathedral, but always in areas that are sheltered from rain. In areas oriented to the sun receiving rainwater the crusts are very thin or are not present (Alcalde et al. 2003). The air of urban areas is contaminated mainly by combustion produced by traffic (SaizJimenez et al. 2004, Bonazza et al. 2004).