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Through a variety of physical, biological and chemical treatment processes that filter pollutants and reduce the volume of precipitation runoff, green roofs reduce the amount of pollution delivered to the local drainage system and, ultimately, to receiving waters. One pollutant that green roofs help control, for example, is nitrogen. 

While nitrogen gas occurs naturally as a major component of the atmosphere, nitrogen compounds from automobile exhaust, agricultural fertilizers and industrial activities can create a significant pollution problem.  Airborne nitrogen compounds can fall to the ground in dust, raindrops, or simply due to gravity.  When these compounds are carried away with stormwater runoff, they contribute to eutrophication problems in surface water.   Green roofs, however, can help make nitrogen pollution less of a water quality problem.

Green roofs contribute to improved water quality not only by retaining and filtering the rainwater through the soil and root uptake zone, but also through:

·      The vegetation, which slows down the water through friction and root absorption.

·      The foliage in particular, which collects dust, transpires moisture and provides shade. 

·      The binding of potential pollutants to clay and organic matter in the roof top soil matrix (Dramstad, et al, 1996). 

In addition, the temperature of the water not retained on the rooftop is moderated before draining downstream.

(All of the above comes from Low Impact Development, Urban Design Tools- Green Roofs, see http://www.lid-stormwater.net/greenroofs_benefits.htm)

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