Investigating the Climate and Air Quality Impacts of Adopting Solar Reflective Cool Walls and Roofs in Los Angeles

Climate change and urban air pollution are two of society’s greatest environmental challenges. In this research we investigate air quality co-benefits and penalties of urban heat mitigation strategies. This policy-relevant research also elucidates understanding of how intentionally changing the urban surface energy balance can lead to changes in meteorology with subsequent impacts on pollutant concentrations.

This study for the first time assesses the influence of employing solar reflective “cool” walls on the urban surface energy budget, summertime climate, and air quality of the Los Angeles basin. We systematically compare the effects of cool walls to cool roofs, a heat mitigation strategy that has been widely studied and employed, using a consistent modeling framework (Weather Research and Forecasting model). Adoption of cool walls leads to increases in urban grid cell albedo that peak in the early morning and late afternoon, when the ratio of solar radiation onto vertical walls versus horizontal surfaces is at a maximum. In Los Angeles County, daily cumulative increase in grid cell reflected solar radiation from increasing wall albedo by 0.80 is 783 kJ m^-2, 43% of that for increasing roof albedo. Cool walls reduce canyon air temperatures in Los Angeles by 0.43 K (daily average), with the peak reduction (0.64 K) occurring at 09:00 LST and a secondary peak (0.53 K) at 18:00 LST. Per 0.10 wall (roof) albedo increase, cool walls (roofs) can reduce summertime daily average canyon air temperature by 0.05 K (0.06 K).

While many past studies have investigated the climate impacts of adopting cool surfaces, few studies have investigated their effects on air pollution, especially on particulate matter (PM). This research for the first time investigates the influence of widespread deployment of cool walls on urban air pollutant concentrations. Simulations using a coupled meteorology-chemistry model (WRF-Chem) show that cool walls and roofs can reduce urban air temperatures, wind speeds, and planetary boundary heights in the Los Angeles basin. Consequently, increasing wall (roof) albedo by 0.80, an upper bound scenario, leads to maximum daily 8-hour average ozone concentration reductions of 0.35 ppbv (0.83 ppbv) in Los Angeles County. However, cool walls (roofs) increase daily average PM_2.5 concentrations by 0.62 (0.85) μg m^-3. In addition, we investigate the competing processes driving changes in concentrations of speciated PM_2.5. Increases in primary PM (elemental carbon and primary organic aerosols) concentrations can be attributed to reductions in ventilation of the Los Angeles Basin. Increases in concentrations of semi-volatile species (e.g., nitrate) are mainly driven by increases in gas-to-particle conversion due to reduced atmospheric temperatures. Results reported here can be used to inform policies on urban heat island mitigation or climate change adaptation.