Erosion of limestone building surfaces caused by wind-driven rain: 1. Field measurements

Complex patterns of discoloration are often seen on the surfaces of stone buildings in urban areas. These patterns reflect interactions between atmospheric pollutants, the surface layers of stone, and wind-driven rain that can erode the surface. This first paper in a two-paper series presents field measurements of wind-driven rain on a tall limestone building. The volume of driving rain on the building wall was measured at 16 locations over a 21-month period, and meteorological data were recorded for the same period. Analysis of data from 94 rain events suggests that wind-driven rain is strongly affected by rainfall intensity, wind speed, wind direction and measurement location. The five locations with driving rain volumes <4l over this period are characterized by heavily soiled walls, while the two locations with driving rain volumes >8l are characterized by white, eroded walls. The remaining nine locations have driving rain volumes in the range 4-8l and varying amounts of soiling, with no clear relationship between these two variables. It is hypothesized that variation in raindrop momentum, which was not measured, is partially responsible for surface erosion and thus removal of soiling in this last category.