A popular solution to combat the urban heat island effect may have an unforeseen consequence: increasing air pollution.
“Cool roofs” are required on most new non-residential buildings in California, and cities like Pasadena and Los Angeles have cool-roof ordinances that go even further.
Cool roofs work by bouncing solar radiation back into the atmosphere, absorbing less heat. Usually white or light-colored, they can reflect 60 percent more sunlight and stay up to 55 degrees cooler than a typical roof, according to the Lawrence Berkeley National Lab.
That got air quality scientist Scott Epstein wondering: what might the impact be of widespread adoption of cool roofs on air pollution in greater Los Angeles?
Epstein, who works for the South Coast Air Quality Management District, had read about how winter-time air pollution in rural Utah was worse than in the summer because snow reflects more ultra-violet (UV) light back into the atmosphere, triggering the chemical reaction that forms ozone or smog. In addition, cooler winter temperatures can lead to inversions that trap pollution near the ground.
“After thinking about this study, we wondered if cool roofs could have a similar effect in greater Southern California,” he said, because they act like snow, reflecting more light and lowering surface temperatures.
The answer, he found, is yes – at least for the most reflective cool roofs.
Southern California would violate federal particulate matter standards for an additional two days a year if cool roofs are widely adopted throughout the region. Smog, or ozone, would also worsen, especially in Riverside and San Bernardino Counties.
The air district chose to investigate the air quality impacts of cool roofs because the region is so far out of compliance with federal air standards, said spokesman Sam Atwood. The agency must cut concentrations of smog-forming nitrogen dioxide 55 percent by 2031 in order to meet the latest federal standard.
“We have such a mammoth task, we have to do everything that helps,” Atwood said. “And we have to try to prevent anything that would hinder us from reaching our goal.”
But Ronnen Levinson, head of the Heat Island Group at Lawrence Berkeley National Lab, said AQMD may have overemphasized how reflective cool roofs are of UV light, thereby overestimating their contribution to ozone formation.
“The biggest consequence (of installing a cool roof) is you’re going to save energy in your building,” he said. “All the other effects are smaller.”
Epstein agreed that the benefits of cool roofs – cheaper energy bills, lower energy consumption, fewer people suffering from heat-related illnesses – likely outweigh the costs documented in his study.
“We want to be really careful to frame this as not to discourage this type of technology,” he said.
Because some roofing materials are less reflective of UV-light than others, some cool roofs could contribute less to ozone formation, according to the study. AQMD is working with the Cool Roof Rating Council to try to ensure that new roofs have the temperature-reducing benefits without contributing to pollution.
“These materials are out there today. It’s not something that has to be invented,” Atwood said. “It’s just a matter of being mindful in creating a new standard so that we get the benefits of cool roofs and preventing any unintended increase in ozone.”
This post was updated on August 10 to clarify Ronnen Levinson's comment.