Rooftop garden in planters, Toronto, Ontario
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A rooftop garden, or green roof, improves air quality in three ways, one direct and two indirect. The direct method is the removal of dirt and pollution directly from the air. According to the Canadian Mortgage and Housing Corporation report Greenbacks from Green Roofs, airborne particulates and some other types of chemical compounds tend to get trapped in the leaves, branches and stem surface areas of plants. Plants are also known to absorb some gaseous pollutants and sequester them in their leaves.
Some of our information has to be gleaned from work with other plants. Research in Germany has shown that treed urban streets have only 10 to 15 per cent of the total dust particles found on similar streets without trees. In Frankfurt a street without trees had an air pollution count of 10 000 to 20 000 dirt particles per litre of air while a treed street in the same neighborhood had an air pollution count of only 3000 dirt particles per litre of air.
Using similar figures, it is assumed that a rooftop garden with 2000 m2 of unmowed grass could remove 4000 kg of dirt out of the air (2 kg / m2 of roof). This estimate is probably high since the lower portion of the grass layer is not in direct contact with moving air. However, even if the figures were reduced by 90 per cent, the rooftop garden would still take out remove 0.2 kg of particles per m2 of roof every year.
Indirectly, a rooftop garden could reduce the energy consumption of small buildings for heating and air conditioning, thus reducing the pollutants emitted if the energy was generated from fossil fuels. The benefits would be largest on a one-story building, but even on a three-story building, energy consumption for heating and cooling could be reduced by five to ten per cent. If rooftop gardens were widespread, their presence could reduce a city's urban heat island, the increase in urban temperatures that is caused by a lack of vegetation. Vegetation causes a portion of incoming solar energy to be used for evapotranspiration, energy which otherwise would be absorbed by non-vegetated surfaces and reradiated as heat. Smog forms more rapidly under higher temperatures, and even a reduction in urban temperatures even 2 to 4°C will have a significant impact on this pollutant. At this point, research is underway to provide estimates of how many roofs would require gardens to achieve these reductions.
Related Links:
EnviroZine article: Rooftop Gardening
Greenbacks from Green Roofs (.pdf format, 368KB)
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