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Hicks (1982) describes the mechanisms of deposition of both wet and dry aerosol particles on sandstones. Wet deposition mainly takes the form of intermittent doses of pollutants, most of which are in dilute solution. The aerosols are formed by polluting material becoming incorporated into rain as it forms in cloud or by wash out during precipitation. Coarse particles due to their inertia, and fine particles, due to diffusion onto the surfaces of water droplets, are more easily incorporated into this deposition phase (Verhoef,1988). Dry deposition is a slower but more continuous process. It is greater on surfaces where condensation is taking place or where surfaces are wet. In these situations particles adhere to the moist surface of the stone. The particles remain attached to the surface when the moisture has evaporated.The rate of deposition of dry particles is closely related to air quality in the immediate vicinity of the stone. During the day, deposition tends to be greater on cooler surfaces. The mechanisms of deposition of aerosols on stone surfaces are very complicated and are influenced by the physical and chemical nature of the stone.

Soot Light absorbing particles, particularly soot, are important in terms of facade soiling. The level of soot present in the atmosphere tends to correlate with the degree of facade soiling, however it is probable that the attachment of soot particles has a low efficiency under wet conditions (Verhoef, 1988). Soot can fill the pore spaces of many sandstones (Schaffer, 1932).Heavy deposits of soot are commonly found on areas of sloping facades (for example on window ledges). The soiling patterns found on facades are typically the result of the eroding effect, water run-off and soot deposits on the surface of the stone (Verhoef, 1988).

2.3 Atmospheric factors influencing soiling Water Mist in the air causes coagulation of particles and hence their sedimentation. Rain and other forms of atmospheric water can capture particles and atmospheric pollutants and precipitate them. The concentration of these pollutants is increased in foggy weather, when coagulation of the particles takes place. Surface condensation on the face of stone contributes to the soiling of the surface. Winter conditions in Scotland typically produce an ambient relative humidity in excess of 80%, and promote condensation on the stone surface. It may therefore be surmised that the geographical location of the building and its micro-climate will exert a considerable influence on the rate of soiling and decay of the stone.

Temperature The high thermal capacity of stone on external surfaces has the effect of creating a temperature gradient within the boundary layer of air in contact with the stone. The surface temperature of the stone may be significantlybelow that of the ambient air. As there is more molecular agitation in hot air than in cold air, there is a tendency for dust to be condensed onto the colder surface (Verhoef, 1988).

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Guide for Practitioners - Stonecleaning PLU 7545  

The original practitioners guide based on stonecleaning research includes background on sandstone characteristics, the effects of soiling ag...

Guide for Practitioners - Stonecleaning PLU 7545  

The original practitioners guide based on stonecleaning research includes background on sandstone characteristics, the effects of soiling ag...

Profile for hspubs
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