Sponsors: National Aeronautics and Space Administration (NASA) NASA Goddard Space Flight Center (GSFC) NASA Goddard Institute for Space Studies (GISS) NASA New York City Research Initiative (NYCRI) NSF Research Experience for Undergraduates (REU) Contributors: Stuart Gaffin, Ph.D., Cynthia Rosenzweig, Ph.D. Alan Roditi, High School Teacher Brittany Hsu, Undergraduate Student Emma Hartung, High School Student White Roofs to the Rescue: Combatting the Urban Heat Island Effect Emma Hartung, Brittany Hsu, and Alan Roditi AbstractBackground Surface Temperatures Emissivity With global temperatures projected to rise in the upcoming decades due to climate change, scientists are investigating increasing global albedo as a cooling strategy. Cooling strategies are especially necessary in overcrowded urban areas due to the Urban Heat Island effect (UHI), which is the tendency for cities to absorb and retain more heat than surrounding suburban areas. A major cause of UHI is the low albedo, or reflectivity, of urban surfaces. Albedo is measured from 0 to 1, and urban areas tend to have albedos 2-5% lower than those of the surrounding areas (Jin et al., 2005). Additionally, emissivity, the ability to release absorbed heat, also measured from 0 to 1, tends to be 1-2% lower in urban areas (Jin et al., 2005). Cool, reflective white roofs can potentially mitigate UHI, decrease summer temps, lower cooling costs by over 20%, and improve air quality (Akbari, 2009). NYC’s Mayor Michael Bloomberg and US Secretary of Energy Steven Chu are taking an active interest in these initiatives. In 2010, Gaffin et al. conducted a study on the Con Ed white roof in NYC and found that summertime peak white roof temperatures averaged ~40°F (22°C) cooler than black roofs. New York City recently implemented an innovative urban heat island mitigation program that involves painting black roofs white with elastomeric acrylic paint. The city is currently applying this technology to one million square feet of rooftops each summer, and similar efforts are beginning in other cities worldwide. We analyzed the first field data from this New York City cool roofs initiative. Our results showed that the white roof’s surface temperatures were 7.8°C (14.0°F) cooler on average than those of the black roof, and that its high temperatures were 24.3°C (43.7°F) cooler on average. The data show a strong positive correlation between the white surface’s cooling effect and incident radiation, indicating that white roofs are most effective during the hottest part of the day and demonstrating the importance of albedo. The albedo tended to be highest in the early morning and evening and lower around midday, which is consistent with previous studies. When the white roof was recoated, its average albedo increased significantly from.35 to.63, demonstrating that white roofs must be repainted if they are to remain effective. Our emissivity results averaged.97 after the recoating, indicating that the white roof releases absorbed heat efficiently. Experimental Setup White roof temps were significantly lower, especially during the hottest part of the day. White paint is high-albedo and high-emissivity. Maintenance is critical to sustaining a high albedo. Professional membranes may stay effective longer. Future studies include: Investigating emissivity results Comparing white professional membranes and white painted roof performance and longevity Albedo Emissivity values averaged.97 after the recoating, indicating that the roof releases absorbed heat efficiently. In the middle of the day, as roof surface temperatures were rising, the roof’s emissivity dipped, rising again as the surface temperatures fell toward the end of the day. Infrared thermosensors record black and white surface temperatures Pyranometers (incident radiation and reflected radiation) calculate white roof albedo Contact probe records white roof temperature and subs into equation for emissivity: E Actual = 0.95(T IR ÷T Surf ) 4 Averages of instantaneous values obtained by each instrument were logged at 15 minute, 1 hour, and daily intervals by the Campbell Scientific CR1000 Datalogger. Strong positive correlation between incident radiation and the white roof’s cooling effect. The white roof is most effective when incident radiation levels (and thus temperatures) are highest. Illustrates importance of albedo to white roofs’ advantage. White roof temperatures were significantly lower than black roof temperatures, averaging 7.8°C (14.0°F) cooler post-recoating. White roof peak temperatures averaged 24.3°C (43.7°F) cooler than black roof peaks, similar to the Con Ed roof’s summer cooling last year. Minimal temperature difference at lows. Mean albedo dropped to 0.35 after two years. Recoating raised the albedo to 0.63 on average. Lower than expected, but still highly reflective. Painted white roofs require frequent maintenance. Albedo followed a U-shaped trend consistent with previous literature. U-shaped trend is due to surfaces’ tendency to reflect light more effectively when it hits them at low angles. Conclusion and Future WorkReferences Akbari H., Levinson R., Rainer L. (2005). Monitoring the energy-use effects of cool roofs on California commercial buildings. Energy and Buildings, vol. 37, Akbari H., Menon S., and Rosenfeld A. (2008) Global cooling: increasing world-wide urban albedos to offset carbon dioxide. Climatic Change, vol. 95, issue 3-4. Gaffin S. R., Rosenzweig C., Eichenbaum-Pikser J., Khanbilvardi R., and Susca T. (2010). A Temperature and Seasonal Energy Analysis of Green, White, and Black Roofs. Columbia University, Center for Climate Systems Research. New York. 19 pages. Jin M., Dickinson RE., and Zhang D., (2005), The footprint of urban areas on global climates as characterized by MODIS, American Meteorological Society, Ridgwell A., Singarayer J. S., Hetherington A. M., and Valdes P. J. (2009). Tackling regional climate change by leaf albedo bio- geoengineering. Current Biology, vol. 19, issue 2, Woodward I. F., Bardgett R. D., Raven J. A., and Hetherington A. M. (2009). Biological approaches to global environment change mitigation and remediation. Current Biology, vol. 19, issue 14, The white roof at MoMA Queens was painted in 2009 and recoated on May 25, Albedo was always > 0.6, except for the anomaly at 7:00 AM EST. 7:00 AM drop and 5:30 PM spike was likely due to shadowing. Recoating The MoMA Queens white roof, pictured before (above left) and after (above right) the roof recoating on 5/25/11.