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

Abstract

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 PM 2.5 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 PM 2.5 concentrations and policyrelevant 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. With 16.8 million people, the SoCAB is the second most populous urban area in the United States. A fossil-fuel-dependent transit and goods movement infrastructure along with a welldeveloped industrial presence within the 27,824-km 2 SoCAB generates significant emissions of oxides of nitrogen (NOx), volatile organic compounds (VOCs), directly emitted primary particulate matter (PM), and secondary PM precursors. Persistent highpressure systems, ample photochemistry, infrequent rainfall, and ventilation-inhibiting topography also contribute to severe airquality problems. The SoCAB currently does not attain federal air-quality standards for 8-h O 3 , 1-h O 3 , annual-averaged PM 2.5 , and 24-h PM 2.5 . Ozone levels within the SoCAB are often the highest in the nation (1).The hot and sunny conditions typically experienced within the SoCAB make urban surface modification a useful strategy to reduce urban temperatures. Meteorological impacts of roofing materials with enhanced solar reflectance (SR, synonymous with "albedo"), colloquially referred to as "cool roofs," are wellstudied and indicate several benefits in urban areas. The replacement of darker materials with high-reflectance surfaces within cities can help mitigate the urban heat...

Solar reflective pavements—A policy panacea to heat mitigation?

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Effects of urbanization on regional meteorology and air quality in Southern California

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“…They also do not resolve the wide spatial heterogeneity of urban land surface properties, with most studies assuming that urban properties are homogenous throughout the city. In addition, only a few studies investigate interactions between land surface changes and air quality for the Southern California region (Taha, 2015;Epstein et al, 2017;Zhang et al, 2018b), which is one of the most polluted areas in the United States (American Lung Association, 2012).…”

Effects of Urbanization on Regional Meteorology and Air Quality in Southern California

“…Higher (lower) temperatures favor phase-partitioning to the gas (particle) phase. Ventilation, which is the combined effect of vertical mixing and horizontal dispersion, can also influence pollutant concentrations (Epstein et al, 2017). Higher (lower) ventilation rates lead to lower (higher) pollutant concentrations especially in coastal cities like Los Angeles where upwind air under typical meteorological conditions is clean relative to urban air.…”

“…They also do not resolve the wide heterogeneity of urban land surface properties, with most studies assuming that urban properties are homogenous throughout the city. In addition, only few studies investigate interactions between land surface changes and air quality for the Southern California region (Taha, 2015;Epstein et al, 2017;Zhang et al, 2018b), which has among the worst air quality in the United States (American Lung Association, 2012).…”