Dust, soot, organics, and other particulates accumulate on outdoor module surfaces over time, absorbing and scattering some fraction of plane-of-array irradiance before it enters the PV module. This loss mechanism is called soiling and its magnitude depends on the nature of the accumulated particulate material and how much particulate has accumulated since the last module cleaning. Some types of soiling can be cleaned naturally by precipitation while others require manual washing to remove. The complement of fractional soiling loss is called the soiling ratio $$SR$$ and represents the fraction of light that is transmitted through (not absorbed or scattered by) the soiling layer.
Differences in proximity to dusty agricultural fields, smoggy roadways, industrial sites, and other particulate sources, along with variation in humidity and particulate size, can cause systems within a region, or even subarrays within a single system, to experience very different soiling accumulation rates and thus soiling losses. Inaccurate soiling assumptions are a significant contributor to overall model uncertainty in locations subject to heavy soiling.