2.1 Thermal Performance of Cool Pavements in Los Angeles Residential Neighborhoods: A Pedestrian Perspective

Monday, 13 January 2020: 10:30 AM
104B (Boston Convention and Exhibition Center)
Ariane Middel, Arizona State Univ., Tempe, AZ; Arizona State Univ., Tempe, AZ; and V. K. Turner, F. A. Schneider, Y. Zhang, and M. Stiller

Coating asphalt roads with highly reflective paint to create cool pavements has recently gained attention to mitigate the Urban Heat Island, reduce building energy use in cities, and address public health concerns. We present the first human thermal exposure observations of “CoolSeal” highly-reflective asphalt used by the Los Angeles Streets Department in residential neighborhoods (Sun Valley and Pacoima) to investigate the thermal performance of cool pavement from a pedestrian perspective. We conducted hourly microclimate transects with MaRTy, a mobile biometeorological instrument platform that measures Mean Radiant Temperature (MRT), air temperature (Ta), surface temperature (Ts), horizontal wind speed (v), and relative humidity (RH) at pedestrian height (1.1 to 1.5 m). MaRTy sampled 8 sites in each neighborhood: four sites on cool pavement and four reference sites on regular asphalt. Each site was a location-triple with observations in the center of the street and on the two adjacent concrete sidewalks. Transects were performed between 11:00 am and 9:00 pm on July 30, 2019, a typical summer day with low wind speeds and maximum air temperature of 31.0 °C.
The 2-months old cool pavement in Sun Valley had an average pyranometer-derived albedo of 0.18, the 2-weeks old cool pavement in Pacoima exhibited an albedo of 0.25, and the regular asphalt had an albedo of 0.06 to 0.08. The cool pavement Ts in both neighborhoods was consistently lower than Ts of regular asphalt: up to 6.0°C during midday, 4.0°C in the early evening, and 1.7°C after sunset. Cool pavement Ts was comparable to the adjacent concrete sidewalk Ts during the day but was 2.0°C cooler than concrete after sunset. The coated asphalt reflected up to 130 Wm-2 (Sun Valley) and 168 Wm-2 (Pacoima) more shortwave radiation than regular asphalt at solar noon at an hourly daytime average of 118 Wm-2 and 144 Wm-2, respectively. In the early evening, MaRTy observed 20 to 30 Wm-2 of added reflected shortwave radiation on sidewalks at the cool pavement sites. Conversely, cool pavements emitted 33 to 47 Wm-2 less longwave radiation than regular asphalt before 4:00 pm. Due to a net radiation gain, cool pavement MRT was 4.0°C higher during midday and 2.0°C higher in the afternoon. Although cool pavement Ta was decreased by 0.5°C in the afternoon, after-sunset cooling was negligible with 0.1 to 0.2 °C, probably due to the limited extent of the treated area.
Findings illustrate the benefits and disadvantages of cool pavement with respect to various thermal performance metrics and municipal goals. Before implementation, a cost-benefit analysis should be conducted that carefully considers UHI mitigation benefits (reduction of Ta and building energy use), human thermal exposure changes (decreased thermal comfort), added safety due to better visibility at night, implementation costs, and other benefits and disadvantages in the context of space use and mitigation priorities.
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