Californian energy efficiency standards are set to see cool roof technology take off, but scientists have raised concerns that widespread use could lead to increased levels of ozone and fine particulate pollution.
Researchers from the University of Southern California and South Coast Air Quality Management District found that many cool roofing materials reflected more UV light than traditional roofing. UV light when interacting with pollutants in the atmosphere creates ozone, so UV reflected back into the atmosphere could lead to a “double shot” of ozone-forming UV, they warned.
“Cool roofs have many benefits including reduced energy use for cooling and mitigation of the significant health impacts of heat waves,” SCAQMD executive officer Wayne Nastri said.
He said while widespread use of certain cool roof materials could “slightly” increase air pollution, the scientists “in no way” wanted to discourage the technology, which has been named as key to reducing the urban heat island effect.
“This study shows what needs to be done to help cool our cities and avoid increasing ozone levels as an unintended consequence.”
The scientists said ozone increase could be avoided if a “comprehensive roofing standard” was adopted that prevented the overall UV reflectance of newly installed cool roofs from increasing.
“Cool roofing materials are available today that reflect the same amount or even less UV than traditional roofing materials,” they said.
Study co-lead Dr George Ban-Weiss said the study highlighted the importance of considering co-benefits and unintended consequences of sustainability solutions on other environmental systems.
“Whether air pollution improves or worsens from cool roof installations depends on a host of competing chemical and meteorological factors,” he said.
“Given that our study focuses on the Los Angeles basin, future research is needed to investigate how these competing processes dictate air pollution impacts in cities around California and beyond.”
While ozone could be mitigated by putting standards in place, the study found that PM2.5 levels would increase slightly regardless, due to “overall cooler surface temperatures resulting in weaker sea breezes and lower inversion layers”.
Though perhaps this would be a small price to pay for a cooler city. And perhaps with the growth of electric vehicles, the pollution caused by cars (the real culprit) that then interacts with sunlight to create smog could be reduced in future.
The study, Air-quality implications of widespread adoption of cool roofs on ozone and particulate matter in southern California, is published in PNAS.