Solar-reflective roofs remain cooler than absorptive roofs and thus conserve electricity otherwise needed for air conditioning. A currently controversial aspect of solar-reflective cool roofing is the extent to which an initially high solar reflectance decreases with time. We present experimental data on the spectral absorption of deposits that accumulate on roofs, and we attribute most of the absorption to carbon soot originally produced by combustion. The deposits absorb more at short wavelengths (e.g., in the blue) than in the red and infrared, imparting a slightly yellow tinge to formerly white surfaces. The initial rate of reflectance reduction by soot accumulation is consistent with known emission rates that are due to combustion. The long-term reflectance change appears to be determined by the ability of the soot to adhere to the roof, resisting washout by rain.

Applied Optics / Vol. 41, No. 12 / 20 April 2002