Particulate Matter Soiling of Exterior Paints at a Rural Site

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Abstract
A soiling study was performed at an air monitoring site operated by the Research Triangle Institute in the relatively rural environmental conditions within Research Triangle Park, N.C. The study was designed to determine how various environmental factors contribute to the rate of soiling of white painted surfaces. Significant factors that were monitored were hourly rainfall and wind speed, and weekly data for dichotomous sampler measurements and total suspended matter concentrations. Gloss and flat white paints on hard-board were exposed vertically and horizontally, both protected and unprotected from rain for 16 wk. Measurements of exposed samples were taken at 2, 4, 8, and 16 wk. Reflectance was measured and scanning electron microscopy (SEM) stubs, which had been flush-mounted into the hardboard prior to painting, were removed at these times. Particle size distributions were determined by scanning electron microscopy. Major findings were: 1) reflectance change on sheltered surfaces was proportional to the fraction of the surface area covered by particles; 2) coarse mode particles contribute more than fine mode particles to soiling of both horizontal and vertical surfaces; 3) insoluble fine mode particles are not significantly washed off by rain; and 4) rain interacts with soluble particles to contribute to soiling by “staining” the surface.