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Air-quality implications of widespread adoption of cool roofs on ozone and particulate matter in southern California

Significance The South Coast Air Basin of California, a region of 16.8 million people, is among the most polluted air basins in the United States. A multidecadal effort to attain federal air-quality standards has led to significant progress, but much more work remains. Are recently implemented statewide building efficiency standards on rooftops counterproductive to these goals? With comprehensive regional models and intensive development of model input parameters, our research has identified the air-quality consequences that are expected to result from these efficiency standards. The results can inform policies to mitigate some air-quality penalties, while preserving the benefits of building efficiency standards. This work also sheds light on potential future policies aimed at reducing urban heating from pavement surfaces. The installation of roofing materials with increased solar reflectance (i.e., “cool roofs”) can mitigate the urban heat island effect and reduce energy use. In addition, meteorological changes, along with the possibility of enhanced UV reflection from these surfaces, can have complex impacts on ozone and PM2.5 concentrations. We aim to evaluate the air-quality impacts of widespread cool-roof installations prescribed by California’s Title 24 building energy efficiency standards within the heavily populated and polluted South Coast Air Basin (SoCAB). Development of a comprehensive rooftop area database and evaluation of spectral reflectance measurements of roofing materials allows us to project potential future changes in solar and UV reflectance for simulations using the Weather Research Forecast and Community Multiscale Air Quality (CMAQ) models. 2012 meteorological simulations indicate a decrease in daily maximum temperatures, daily maximum boundary layer heights, and ventilation coefficients throughout the SoCAB upon widespread installation of cool roofs. CMAQ simulations show significant increases in PM2.5 concentrations and policy-relevant design values. Changes in 8-h ozone concentrations depend on the potential change in UV reflectance, ranging from a decrease in population-weighted concentrations when UV reflectance remains unchanged to an increase when changes in UV reflectance are at an upper bound. However, 8-h policy-relevant ozone design values increase in all cases. Although the other benefits of cool roofs could outweigh small air-quality penalties, UV reflectance standards for cool roofing materials could mitigate these negative consequences. Results of this study motivate the careful consideration of future rooftop and pavement solar reflectance modification policies.