The Urban Heat Island (UHI) effect, where urban areas experience higher air temperatures than surrounding rural areas, is a well-documented phenomenon. This effect stems from the replacement of natural vegetation with impervious surfaces that absorb and retain more solar radiation, and a reduction in turbulent heat transport due to wind dissipation caused by urban structures. The UHI effect leads to several negative consequences including increased energy consumption for cooling, higher water usage, reduced thermal comfort, and detrimental effects on public health. These impacts vary within cities and are correlated with the extent of impervious surfaces. Given the constraints of limited municipal budgets, efficient mitigation strategies are crucial. Several interventions have been proposed, including green infrastructure (vegetation), blue infrastructure (water bodies), and gray infrastructure (modified surfaces). This study focuses on gray infrastructure, specifically the use of reflective surfaces, to mitigate heat pollution. While the potential of reflective surfaces has been demonstrated in large-scale studies, their application is often limited in practice due to cost. Furthermore, large-scale studies may average out important local-scale effects. This research addresses the need for strategies that maximize the benefits of partial application of reflective surfaces at the neighborhood level.

A considerable amount of research over the past two centuries has explored the UHI effect and its causes. Studies highlight the impact of replacing vegetation with low-albedo impervious surfaces and the reduction in wind-driven heat dissipation. The UHI effect's detrimental impact on energy consumption, water usage, thermal comfort, and public health has been consistently demonstrated. Various mitigation strategies have been explored in the literature. Green infrastructure, such as green roofs and walls, offers benefits through evapotranspiration and shading. Blue infrastructure, utilizing water bodies and irrigation, aids in evaporative cooling. Gray infrastructure solutions include permeable surfaces, energy-harvesting surfaces, and reflective surfaces, each with its own mechanisms for reducing heat. Reflective surfaces, with their higher albedo, absorb less solar energy, leading to reduced heat pollution. While large-scale studies indicate significant potential for city-wide application of reflective surfaces, limitations exist. The effectiveness of reflective surfaces is influenced by factors such as wind direction, spatial distribution within a neighborhood, and potential reflection onto other surfaces or pedestrians. Moreover, the albedo of reflective surfaces degrades over time, impacting their long-term effectiveness. Existing studies often focus on full-scale implementation, overlooking the practical challenges and need for optimization in partial application scenarios, which are more common in reality.

This study employs a Computational Fluid Dynamics (CFD) model to investigate the effectiveness of partially applying reflective surfaces (50% coverage) within a prototypical neighborhood. The model considers various spatial distributions of these surfaces: upstream, parallel, and downstream relative to the prevailing wind direction. Three building configurations are analyzed: low-density (aspect ratio H/W = 1.0) with westerly and northwesterly winds, and high-density (H/W = 2.0) with a westerly wind. The meteorological conditions used represent the warmest hour of the year in Chicago (July 19th, 3:00 pm). A 1D model, previously developed, is utilized to determine surface temperatures for both conventional and reflective surfaces based on albedo, thermal diffusivity, and emissivity. The CFD model simulates 3D wind speed and air temperature fields, with the 2m air temperature extracted for analysis. The 'departure' from the conventional case (no reflective surfaces) is calculated for each simulation. The 'benefit-to-cost' (B/C) ratio is introduced as a quantitative metric. The benefit is calculated as the area integral of the temperature departure, normalized to the full reflective case. The cost is represented by the proportion of the neighborhood area covered by reflective surfaces. A B/C ratio > 1 signifies a more cost-effective strategy than full coverage.

The simulation results demonstrate that full coverage of reflective surfaces leads to a significant reduction in air temperature (approximately 1.9°C in the studied cases). However, the analysis of partial application scenarios reveals spatial dependence. The upstream placement of reflective surfaces consistently resulted in a B/C ratio greater than 1, indicating greater cost-effectiveness compared to full coverage. This was observed across all wind directions and building densities. In contrast, the downstream placement yielded a B/C ratio consistently less than 1, suggesting reduced effectiveness relative to cost. The parallel placement showed a B/C ratio around 1, signifying comparable effectiveness to full coverage relative to its reduced cost. The highest B/C ratio (1.26) was observed for the high-density (H/W = 2.0) westerly wind configuration with upstream reflective surfaces. The spatial distribution of the cooling effect varied significantly among the different placement scenarios. Upstream application resulted in cooling not only in the upstream area but also downstream, while downstream application only benefited the downstream area. These findings suggest that even with limited resources, significant heat mitigation can be achieved by strategically placing reflective surfaces in the upstream part of neighborhoods, counterintuitively to where the highest pollution is observed.

The study's findings underscore the importance of strategic placement of reflective surfaces for maximizing heat mitigation effectiveness. Focusing on the upstream side in relation to the prevailing wind direction significantly enhances the benefit-to-cost ratio. This contrasts with the intuitive approach of targeting areas with the highest heat pollution. The results indicate that the benefits relative to cost diminish as the reflective surfaces are positioned further downstream, an effect amplified in higher-density neighborhoods. This implies a need for site-specific analysis considering prevailing wind patterns to optimize the allocation of limited resources. The study provides valuable insights for urban planners and policymakers, offering a data-driven approach to mitigating urban heat islands with limited budgetary constraints. The model demonstrates the practical implementation of optimizing partial application of reflective surfaces, guiding informed decisions about urban heat mitigation.

This study demonstrates that a limited application of reflective surfaces can significantly mitigate urban heat pollution, particularly when strategically placed upstream relative to prevailing winds. The benefit-to-cost ratio analysis highlights the cost-effectiveness of this approach compared to full coverage. Future research should investigate the combined effects of integrating reflective surfaces with green and blue infrastructure solutions, analyze long-term impacts considering albedo degradation, and incorporate additional factors such as pedestrian thermal comfort into the evaluation.

This study focuses solely on the reduction in 2m air temperature as a measure of heat mitigation. Other factors, such as pedestrian thermal comfort, long-term albedo changes due to surface degradation, and the environmental impact of manufacturing reflective surfaces, were not considered. The model assumes uniform surface temperatures for roofs, walls, and roads, which might not reflect reality. Additionally, statistical uncertainties were not incorporated into the analysis. Future work should address these limitations for a more comprehensive understanding.