Deposition of Urban Pollution on the Ara Pacis, Rome.

The deposition of urban pollution, mainly due to traffic, has been studied at the Ara Pacis, Rome. This monument was protected by a glass building against environmental factors, especially rainfall, wind and severe cold. The building acts as a green-house and facilitates dry deposition. The pollution (especially soot) derives mainly from the arterial road which is on the western side of the monument. Field tests were carried out in spring, summer and autumn in order to measure the exchange of heat and moisture between air and monument, the thermal shocks associated with shadow and successive lighting. The field tests consisted of monitoring thermal and hygrometric parameters, ventilation, solar radiation and particle size distribution. The particle concentration varies daily and seasonally with emission and atmospheric stability, and has a maximum in the early morning. As the ratio of the areas of the windows to the cross section of the building is very small, the air coming from outside very soon looses its turbulence, and in the absence of eddies there is no resuspension of particles and the sedimentation occurs undisturbed, without any counteracting mechanism. The result is a blackening of the upper part of all the protruding relieves. The thermophoretic cycles appear to be sometimes responsible for increasing the deposition, and sometimes counteracting it, i.e. when the marble is hotter than the air.