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Which consequence does the urban boundary layer have for pollution?

  • A.No consequence
  • B.Complete removal of the pollutants, which are dispersed by the enhanced turbulence generated by the buildings
  • C.Better vertical mixing but higher emission density
  • D.Worse vertical mixing, since the buildings obstruct the flow and prevent the pollutants from being carried away

Answer: C

The city mixes more effectively than the surrounding countryside at night, which helps, but concentrates emissions into a small area, which more than offsets the advantage. The net result is higher concentrations. Street canyons can also trap pollutants at the very smallest scales despite the enhanced mixing above roof level.

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