Thermal Characteristics and Material Optimization of the Underlying Surface in the Central Plaza of Beijing Olympic Park

Objective The large, continuous and hard urban underlying surface is an important cause of the deterioration of the near-surface thermal environment, and an increase in albedo can usually coolit. With global warming and rapid urbanization, Urban heat island (UHI) has significantly impacted urban living comfort, air quality, and energy consumption. Mitigating urban high temperature and optimizing the urban thermal environment have become core issues in building low-carbon, green, and sustainable cities. Existing research indicates that the differences in thermal properties between hard underlying surfaces may affect the urban near-surface energy balance, significantly impacting surface temperature (ST). In recent years, scholars at home and abroad have conducted multi-indicator research on the thermal environment of different hard underlying surfaces, with a view to exploring their thermal environment effects, temperature, humidity and microclimate change characteristics, as well as thermal comfort in specific activity spaces. However, such research seldom combines actual measurements with numerical simulations for thermal environment optimization. This research aims to investigate the thermal environment characteristics and temperature interval ranges of six typical underlying surfaces in the central plaza of Beijing Olympic Park through actual measurements and numerical simulations using the ENVI-met model. The research also explores the specific impact of overall albedo changes on the thermal environment, and provides suggestions for the material selection and configuration of hard underlying surfaces in public spaces in Beijing.

Methods In the central plaza of Beijing Olympic Park, the surface temperature of six typical underlying surfaces (asphalt, concrete, brick, granite, gravel, and grass) is measured to analyze the thermal environment characteristics and temperature interval range. The research also adopts the ENVI-met numerical simulation method to explore the specific impact of numerical changes in the overall albedo on the thermal environment. ENVI-met is a three-dimensional microclimate numerical simulation software based on computational fluid dynamics (CFD) principles. The model is built using actual measurement data for an 860 m × 580 m research area, with the core research area covering an area of 260 m × 500 m. Different albedo scenarios are simulated by applying high-reflectance coatings to the hard underlying surfaces, with the overall albedo being increased to 0.40, 0.60, and 0.80 in scenarios S₁, S₂, and S₃, respectively. The simulation results are compared to the base scenario (S₀) to analyze the cooling effect of increased albedo on surface and air temperatures.

Results The measurement results show that different underlying surfaces produce different thermal effects due to variations in specific heat capacity and thermal conductivity. Seasonal variation patterns indicate that in spring, summer, and autumn, the surface temperatures of hard underlying surfaces (asphalt, concrete, brick, and granite) are significantly higher than the air temperature, indicating their role in heating the air. However, in winter, most hard surfaces’ temperatures drop below the air temperature after 16:00. One-way ANOVA results indicate that the average surface temperature of grass in spring, summer, and autumn is the lowest and significantly different from other underlying surfaces (p<0.01). In winter, the average surface temperature of grass is higher than all hard underlying surfaces except asphalt. Among hard surfaces, granite has the lowest annual average surface temperature, significantly lower than asphalt, concrete, and brick, while asphalt has the highest annual average surface temperature, significantly higher than concrete, granite, and brick. Overall, hotter hard surfaces like asphalt, concrete, and brick have a wider temperature range compared to cooler surfaces like granite. For similar types of surfaces, darker ones have a higher temperature range than lighter ones. In summer, dark surfaces with a lower daily average temperature may be hotter than light surfaces with a higher daily average temperature. The research also finds that increasing the overall albedo can effectively reduce surface and air temperatures. The maximum cooling intensity is observed at an albedo of approximately 0.56, with cooling effects increasing initially and then decreasing as albedo increases.

Conclusion This research reveals the thermal environment characteristics of common hard underlying surfaces in urban squares in Beijing, showing that asphalt surfaces are the hottest while granite surfaces are the coolest. Additionally, the surface temperature of grass in spring, summer, and autumn is significantly lower than that of hard surfaces. The research finds that among similar hard surfaces, lighter types have lower daily average temperatures and smaller temperature variations compared to darker types. Furthermore, light-colored surfaces with higher annual average temperatures may be cooler in summer than dark-colored surfaces with lower annual average temperatures, indicating that selecting light-colored materials can more effectively mitigate the urban heat island effect. ENVI-met simulation results show that increasing the overall albedo of urban surfaces can significantly reduce surface and air temperatures. The research area achieves the maximum cooling intensity at an albedo of around 0.56, with the optimal albedo range for cooling benefits being between 0.50 and 0.60. These insights may provide valuable guidance for the design and material selection of hard surfaces in public spaces, thus helping enhance urban thermal comfort and sustainability.