Microclimatic factors affecting the Trajan Column

Abstract

Because of meteorological and microclimatic factors (e.g. solar radiation, gradients of temperature and moisture, air speed and turbulence) the Trajan Column is subject to deposition processes on a diurnal cycle, which alternatively increase and decrease the deposition rate of airborne pollutants. The diurnal trend of the pollutant concentration in Rome is bi-modal and the two maxima are associated with different deposition velocities, so that a simple correlation between stone deterioration and pollutant concentration averages appears to be meaningless. Field tests were carried out on the monument to investigate the diurnal and seasonal trend of factors that affect dry deposition in relation to the meteorological variables and pollution levels, i.e. phoretic forces due to temperature gradients (thermophoresis), condensation and evaporation (diffusiophoresis and Stefan flow), inertial impaction, electrostatic capture and gravitational settling. In particular, when the marble is warmer than the surrounding air, a layer of warm air rises continually along the warm part of the column, increases the upward drag of the falling particles and results in a net decrease of the gravitational settling of the coarse particles (i.e. with diameter d > 1 μm); when the marble is colder, the downward drag increases the sedimentation rate. The wind field around the monument was measured in order to investigate the diffusion potential. Turbulence generated by the shape of the column and its roughness clearly dominates the natural turbulence of the free flow. Rainwater has been recognized as being the cause of severe dissolution except in zones where whewellite is present. This is seen by comparing the deterioration pattern with the rainfall rose, and by following the run-off path which is determined by the geometry of the bas-reliefs. Dissolution is due to the chemical activity of the dry and wet depositions; it is also dependent on the physical regime of the run-off, i.e. laminar or turbulent. In the past, in addition to the physico-chemical weathering, biological deterioration occurred where meteoric water remained longer. Micrometeorological field test were carried out on the Trajan Column to investigate the diurnal and seasonal evolution of the main temperature and humidity parameters to know the microclimate of the column. Four seasonal tests have been made, each of 6 days under different weather conditions. The measurements have been made on horizontal and vertical profiles at different distances from the surface to study the heat and moisture exchanges between marble and atmosphere. The wind loads and their effects on the mechanical structure resulted in being negligible.