Despite popular belief that green roofs – roofs that are planted with vegetation – improve indoor air quality by filtering the air coming into the buildings from the outside, a recent study from Portland State University showed only trivial results for one pollutant, ozone.
The researchers from PSU’s departments of Mechanical and Materials Engineering, Biology and the university’s Honors College, set up measuring devices on the roof of a big-box retail store in North Portland that was split between a green roof and a more conventional white membrane roof.
They measured the air coming into the building from outdoor intake vents, and found that the air coming in from the green roof area had only modestly lower ozone levels than the air coming in from the unplanted area. They found that the vegetation trapped and filtered some of the ozone in the outdoor air, but not enough to support claims of improved air quality, according to Elliott Gall, the project’s lead researcher.
The trapping effect is a process known as dry deposition, in which airborne particles collect or deposit themselves on solid surfaces. It’s a natural process that is key to removing pollutants from the atmosphere.
“It is possible that some other roof designs -- for example intensive rooftops with higher plant densities and taller plants -- may increase the removal of ozone,” Gall said. “These studies are necessary to understand if green roofs actually improve local air quality, or if this is a myth.”
Despite the findings showing only minimal improvement to indoor air quality, green roofs are still recognized as having other positive environmental benefits, including reducing carbon dioxide, decreasing storm water runoff and cutting down on urban heat, according to PSU researchers.
The study was conducted over a two-day period. The authors said the findings warrant a longer-term study – one that could include measuring other pollutants as well as ozone.
The study was published in the March 15 edition of Building and Environment.