In 2019, Los Angeles County coated streets across a 0.64-square-kilometer neighborhood; surfaces were up to 9°F cooler, but ten months later the cooling effect had fallen by about 1.8°F

Los Angeles County's Department of Public Works, in 2019, applied a solar-reflective coating on roads in an area covering 0.64 km² of Covina, California, in order to determine whether a cooling effect could be achieved using a simple paving technology. As Ko and colleagues later reported in Environmental Research Letters, the surface temperature fell by as much as 9°F (5°C), but after ten months of weathering and vehicle traffic, that cooling effect had weakened by 1.8°F. A more recent, larger experiment in the Pacoima neighborhood has also provided useful data on the method’s practicality.A 2022 paper analyzed the 2019 Covina deployment's albedo shiftAnalysis of the impact of the Covina project is provided in the study, "Measuring the impacts of a real-world neighborhood-scale cool pavement deployment on albedo and temperatures in Los Angeles," published in the peer-reviewed journal Environmental Research Letters in 2022. According to the authors, average pavement albedo increased by 0.18, from 0.08 to 0.26, resulting in a maximum neighborhood‑averaged temperature reduction of 5 °C at the surface and 0.2 °C in the near‑surface air. The authors also report that cooling efficiency declined by about 1 °C as albedo fell by roughly 30%. According to the authors, this is among the first field studies to measure air-temperature reductions from cool pavements, and they note that further work is needed to assess trade-offs for pedestrian comfort. To quantify the impact of cool pavements on surface and air temperatures, the researchers took both mobile and stationary measurements, comparing coated streets with nearby uncoated streets to distinguish the coating’s cooling effect from broader seasonal temperature changes.Ten months later, albedo had fallen back to between 0.16 and 0.24However, the effect faded quickly. About ten months after installation, researchers found that the coating’s albedo had fallen to between 0.16 and 0.24, depending on the location, from 0.26 immediately after application. That change in albedo reduced the road-surface cooling effect by about 1°C, or 1.8°F, compared with the initial result. For city officials, the result matters because it shifts cool pavement from a one-time installation to an ongoing maintenance task. If the effect decreases by one-fifth during only ten months, then the pavement needs re-coating quite regularly if the city wishes to retain the initial cooling effect.A 2022 Pacoima project treated over 700,000 square feet of asphaltA year-long observational study, Micrometeorological effects and thermal-environmental benefits of cool pavements: findings from a detailed observational field study in Pacoima, California, published in Environmental Research Communications, tracked results across an entire year using a mix of ground sensors, drone imagery, and satellite data. The study found that the reflective surface reduced surface and air temperatures in the urban canopy layer by less than 2°C, though it at times raised the mean radiant temperature by up to 1.2°C, with no evidence of glare or photochemical damage. Using a broader suite of instruments, the researchers were able to examine not only temperature impacts but also secondary effects of reflective surfaces, such as glare, air‑quality changes associated with UV reflectance, and pedestrian thermal comfort.September 2022 heatwave measurements showed up to 3.4°F of air coolingAs the Pacoima study reported, coated surfaces had air temperatures 1.9°C, or 3.4°F, cooler than those on control surfaces during the afternoons of the September 2022 heatwave. On other summer days that were not within the heatwave period, the highest variation in surface temperature between the coated surfaces and the control surfaces was 12.2°C, which was higher than the variations observed in the previous cool pavement research. Just like the case in Covina stated above, the degree of the cooling effect was very variable depending on location and weather.Together, the two studies point to a shared maintenance problemBoth of these research projects, however, end up with one and the same idea through different perspectives. The studies suggest that reflective coatings can reduce surface and air temperatures, but effectiveness varies with coating age, type and weather conditions. In this respect, the Covina study is particularly valuable for tracking how quickly the reflective effect degrades over time, while the Pacoima study, which also spans roughly a year, is especially useful for showing how the cooling effect varies with weather conditions, including during a summer heatwave.Neither project, however, settles how effective cool pavements are at reducing pedestrian-level air temperatures. According to the scientists involved in the Pacoima project, reflective pavements can help with heat mitigation but are not a substitute for the shade trees provide. The larger takeaway from both studies is that the cooling effect of cool pavements is not permanent, a factor cities need to weigh when planning maintenance schedules, reapplication frequency, and how long a given installation stays effective.Catch the latest World News and Live updates. Download the TOI app.