Description: A number of buildings in New York emit large black pollution plumes (as shown on the left-hand side of Figure \ref{643439}). Often these are from large (and old) boilers used for building heating.
Features / Attributes: A typical example appears in around 3-10 consecutive UO images. The background subtracted images show the plume highly concentrated at the point of emission, and expanding and diffusing as the plume progresses and dissipates into the air. 
Source of contamination: Detection of black smoke plumes is highly variable across days inspected, due to weather and visibility considerations such as snow and fog. Wind is also a large determinant in visibility as high wind speeds cause faster dissipation.
Issues / challenges: Another challenge with these black smoke plumes as they rise into the air above the city skyline is they appear very similarly to clouds in the sky. The temporal context is very useful in disambiguating them, as the object trajectory is distinct from clouds.

Plume (steam vents)