Cool roof materials increasing air pollution
Scientists have discovered that widespread installation of particular ‘cool roof’ materials in Southern California could increase ozone and fine particulate pollution levels.
The study, carried out by researchers at the South Coast Air Quality Management District and the University of Southern California, states that the predicted rise in ozone level can be reduced or avoided completely by implementing more comprehensive performance standards for cool-roofing materials.
Marginal increases in fine particulate levels are predicted to occur regardless of whether or not enhanced standards are put in place.
SCAQMD’s executive officer, Wayne Nastri, said: “We don’t want to discourage the use of cool roofing technology, but the future, widespread use of certain kinds of cool roofing materials could slightly increase air pollution levels.
“Our study shows what must be done to allow us to cool our cities whilst also avoiding increasing ozone levels as an unintended consequence.”
The study - ‘Air Quality Implications of Widespread Adoption of Cool Roofs on Ozone and Particulate Matter in Southern California’ - was carried out by a team of SCAQMD scientists and published in the Proceedings of the National Academy of Sciences.
Sophisticated meteorology and air quality computer models, measurements of cool roofing materials and detailed databases of the region’s rooftops were used to predict the impact on air quality of the increasing use of cool roof materials - which are light-coloured and reflective - that are predicted to result from existing statewide energy efficiency standards.
Previous scientific studies have found that replacing darker roofs and building materials with highly reflective materials has the ability to reduce peak daytime temperatures and mitigate the “urban heat island effect” - where cities can be significantly warmer than less-urbanised areas.
However, these cool-roofing materials reflect more UV light than their traditional counterparts, making the potential for ozone formation more likely. UV rays fuel smog formation on their way down to Earth and when the cool roofs bounce the rays back up into the atmosphere, a ‘double shot’ of ozone formation is created.
This ozone increase can be avoided if a comprehensive roofing standard is adopted to prevent 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.