Measuring the impacts of a real-world neighborhood-scale cool pavement deployment on albedo and temperatures in Los Angeles

Abstract

Climate change is expected to exacerbate the urban heat island effect in cities worldwide, increasing the risk of heat-related morbidity and mortality. Solar reflective “cool pavement” is one of several mitigation strategies that may counteract the negative effects of the urban heat island effect. An increase in pavement albedo results in less heat absorption, which results in reduced surface temperatures (Tsurface). Near surface air temperatures (Tair) could also be reduced if cool pavements are deployed at sufficiently large spatial scales, though this has never been confirmed by field measurements. This field study is the first to conduct controlled measurements of the impacts of neighborhood-scale cool pavement installations. We measured the impacts of cool pavement on albedo, Tsurface, and Tair. In addition, pavement albedo was monitored after installation to assess its degradation over time. The field site (~0.64 km2) was located in Covina, California; ~30 km east of Downtown Los Angeles. We found that an average pavement albedo increase of 0.18 (from 0.08 to 0.26) corresponded to maximum neighborhood averaged Tsurface and Tair reductions of 5 °C and 0.2 °C, respectively. Maximum Tsurface reductions were observed in the afternoon, while minimum reductions of 0.9 °C were observed in the morning. Tair reductions were detected at 12:00 local standard time (LST), and from 20:00 LST to 22:59 LST, suggesting that cool pavement decreases Tair during the daytime as well as in the evening. An average albedo reduction of 30% corresponded to a ~1 °C reduction in the Tsurface cooling efficacy. Although we present here the first measured Tair reductions due to cool pavement, we emphasize that the tradeoffs between Tair reductions and reflected shortwave radiation increases are still unclear and warrant further investigation in order to holistically assess the efficacy of cool pavements, especially with regards to pedestrian thermal comfort.

Evidence-based Guidance on Reflective Pavement for Urban Heat Mitigation: A Case Study in Phoenix, Arizona

“…Over the seven-month period, monthly measurements indicate that the solar surface reflectivity of the RP reduces from 33-38% when installed to 19-30% across the eight neighborhoods. Similar reflectivity degradation was found by Ko et al in Los Angeles 41 . These reflectivity reductions may result in a lower cooling effect for Tsfc and thus Tair, yet an improvement for Tmrt.…”

“…Our observational study agrees with prior and current urban climate modeling 45 and the Los Angeles observational studies on a single 27 and multiple RP types 41 about the impact of RP in a neighborhood. This is promising when considering the challenges of accurately measuring or modeling large-scale urban effects across entire neighborhoods.…”

“…The overall magnitude of the RP Tsfc reduction agrees with observations in Los Angeles that showed a moderate Tsfc cooling of 4-6°C for a similar change in albedo 27,41 .…”

“…Fading reflectivity via "wear and tear" is an important consideration given the demand for more cleaning and maintenance, i.e., to restore higher solar reflectivity 30,41 which helps maintain lower Tsfc in the residential neighborhood (this demand may be less in areas that receive more rain and less dust). Notably, the reflectivity results should be interpreted with caution since they only refer to a portion of the roadway (northern side, except location D1S) near the curb and potentially least driven on.…”

“…Research on reflective urban materials, particularly roofs, under hot daytime summer conditions has been growing in recent years using models and simulations [24][25][26]38,39 , microclimate observations 27,40,41 , and laboratory studies 30 . Further real-world field studies are warranted to understand RP thermal performance, specifically concerning the interaction between different heat exposure metrics, i.e., surface (Tsfc), air (Tair), and mean radiant temperatures (Tmrt).…”