Atmospheric Pollution is deleterious to the built environment. Carbon particles combined with sulphur dioxide, nitrogen and other pollutants can overlay and solidify on the surface of building materials. Those deposits are removed using techniques used in restoration rather than cleaning. Broadly There are 3 techniques appropriate to the various task : The application of poultices, wet or dry blasting of inert material such as calcium carbonate.
Other marks such as chewing gum, paint ,bitumen and occasionally metabolites are removed using a liquid spray of superheated water. The temperature at the nozzle is 150oC.
Efflorescence can sometimes be brushed but if hard, wet blasting is often the safest solution.