Green roofs act as passive filters of airborne particulate matter. This study estimated that 0.21 metric tonnes of PM10 a year could be removed if all flat roofs in an area of Manchester had extensive green roofs. The two grasses studied were more effective than plantain and sedum at capturing small particulate matter.
Speak, Andrew. F., J. J. Rothwell, S. J. Lindley, and C. L. Smith. (2012). Urban particulate pollution reduction by four species of green roof vegetation in a UK city. Atmospheric Environment, 61, 283-293.