How do green roofs and urban vegetation contribute to reducing air and water pollution?

• A. They have no effect on pollutant concentrations.
• B. They only provide aesthetic value.
• C. They primarily raise water demand and increase runoff.
• D. They reduce stormwater runoff, filter particulates, mitigate heat islands, and can modestly reduce local pollutant concentrations.

Answer: (D)

Green roofs and urban vegetation address pollution through several interconnected effects. For water, plants and soil on roofs intercept rainfall, store and absorb water, and release it slowly through evapotranspiration, which lowers stormwater runoff and the amount of polluted water reaching rivers and sewers. For air, leaves and soil surfaces trap and remove particulates from the air, while the increased surface roughness and shading slow winds and help particulates settle out. Vegetation also cools urban areas, reducing heat island intensity; this cooler climate lowers the production of ground-level ozone and other pollutants and can cut energy demand (which in turn lowers emissions that cause air pollution). Taken together, these mechanisms lead to reduced runoff, cleaner air particles, and modestly lower local pollutant concentrations.