Review of plants to mitigate particulate matter, ozone as well as nitrogen dioxide air pollutants and applicable recommendations for green roofs in Montreal, Quebec.

In urbanized regions with expansive impervious surfaces and often low vegetation cover, air pollution due to motor vehicles and other combustion sources, is a problem. The poor air quality days in Montreal, Quebec are mainly due to fine particulate matter and ozone. Businesses using wood ovens are a source of particulates. Careful vegetation selection and increased green roof usage can improve air quality. This paper reviews different green roofs and the capability of plants in particulate matter (PM), ozone (O) as well as nitrogen dioxide (NO) level reductions. Both the recommended green roof category and plants to reduce these pollutants in Montreal's zone 5 hardiness region are provided. Green roofs with larger vegetation including shrubs and trees, or intensive green roofs, remove air pollutants to a greater extent and are advisable to implement on existing, retrofitted or new buildings. PM is most effectively captured by pines. The small Pinus strobus 'Nana', Pinus mugho var. pumilio, Pinus mugho 'Slowmound' and Pinus pumila 'Dwarf Blue' are good candidates for intensive green roofs. Drought tolerant, deciduous broadleaved trees with low biogenic volatile organic compound emissions including Japanese Maple or Acer palmatum 'Shaina' and 'Mikawa-Yatsubusa' are options to reduce O levels. Magnolias are tolerant to NO and it is important in their metabolic pathways. The small cold-tolerant Magnolia 'Genie' is a good option to remove NO in urban settings and to indirectly reduce O formation. Given the emissions by Montreal businesses' wood ovens, calculations performed based on their respective complex roof areas obtained via Google Earth Pro indicates 88% Pinus mugho var. pumilio roof coverage can annually remove 92.37 kg of PM of which 35.10 kg is PM. The removal rates are 4.00 g/m and 1.52 g/m for PM and PM, respectively. This paper provides insight to addressing air pollution through urban rooftop greening.