Open sources are those that emit particulate or gaseous air pollutants directly to the atmosphere without passing through a confined, well-defined flow stream at the point of discharge. Open sources are also referred to as non-ducted sources, fugitive sources, and non-point sources. Open source assessment initially focused on fugitive dust sources such as unpaved roadways, heavy construction activities, agricultural activities, open pit mines, and windblown dust. More recently, attention has turned to non-dust sources such as wildfires, controlled burning, and leaks of gases from industrial process equipment, often involving greenhouse gas emissions. Even though vehicle exhaust emissions can be characterized by dynamometer tests of tailpipe (ducted) emissions, the composite traffic source, consisting of a mix of vehicles of different types and ages, is best represented as an open source. 

Emissions from open sources are much more difficult to characterize than emissions from a stack or duct that that can be sampled upstream of the point of discharge. In addition, while ducted sources tend to emit at relatively consistent rates, open source emissions tend to vary strongly in time and space, which adds further to their complexity. As a result, it has not been feasible to develop a set of reliable tools for emission testing and even transport modeling of open source emissions, in spite of the obvious impacts of open sources.