A model is developed which can be used to predict the change in reflectance from a surface as a function of time. Reflectance change is a measure of soiling caused by the deposition of particles on a surface. The major inputs to the model are the parameters to a bimodal distribution of ambient particle concentration, surface roughness, friction velocity, particle and substrate reflectance, and whether the surface is vertical or horizontal. A new model for deposition velocity was developed for rough surfaces using Reynold's number - friction factor data and available particle deposition data. The model is used to demonstrate the effects of changing input parameters on rates of soiling of surfaces.