London Air Quality - Pollution

whats this about?

A diesel particulate filter (or DPF) is a device designed to remove diesel particulate matter or soot from the exhaust gas of a diesel engine. Wall-flow diesel particulate filters usually remove 85% or more of the soot, and under certain conditions can attain soot removal efficiencies approaching 100%
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http://en.wikipedia.org/wiki/Diesel_particulate_filter