Building stone surfaces exposed to the urban environment are subjected to rapid and differentiated deterioration, due to various chemical and physical attacks. Apart from a chemical deterioration below the sulfated crusts, which mainly grow outwards at the unaltered gypsum/carbonatic rock interface, another physical deterioration was observed. The latter is, at times, even more important than the rapid chemical attack typical of the zones which are subjected to heavy washing by rainwater. The formation and growth of three different kinds of deterioration layers are discussed with reference to the main kinds of deterioration characterized by white, grey, and black external appearance. Analyses of airborne particulate matter and of deterioration layers of urban monuments led to the conclusion that not all the gypsum of crusts which can be observed on marble monuments is derived from the transformation of the underlying rock, but it is also partially transported by aerosols.