Urban Sustainable Solutions for Energy and Air Quality Impacts Using Green Infrastructure

As populations rise, cities grow and the land surface heats up causing urban heat island (UHI) phenomena. These occur because city walls, streets and roofs hold in the sun’s warmth, making cities warmer than the land surrounding them. UHI effects threaten urban sustainability and public health in cities by raising death rates, increases energy demand for cooling, and makes life uncomfortable even for healthy residents. With time, these effects are expected to increase unless well planned mitigation strategies are implemented.

In this study, we examined whether green roofs, which are covered with plants, and cool roofs, which are surfaced with reflective materials, could reduce UHI effects in Chicago and address impacts on energy and air quality. We used Weather Research and Forecasting (WRF) model coupled with a modified dynamic building energy (BEP+BEM) urban scheme to adapt to green and cool roof algorithms and quantified the reductions in electricity loads in terms of W/m² and thus assessed heating and cooling demand for buildings in urban areas. Preliminary results show substantial reductions (>20W/m²) in air conditioning energy consumption with green and cool roofs over urban areas. To address the impact on air quality using green and cool roofs, we employed a chemistry version of WRF (WRF-CHEM) model. Results showed that green and cool roofs not only affected the surface and near-surface conditions but also modified the structure of the boundary layer (< 2500 m). Lower atmosphere temperature, winds, and relative humidity changed due to reduction in UHI, as vertical mixing and boundary-layer depth decreased with green and cool roofs during daytime convective period. The reduced vertical mixing, weaker convective rolls and lower horizontal velocity in lower atmosphere (< 0.5 km) lead to increased retention time of pollutant- laden air close to the surface during daytime, thus causing air quality issues. This effect would then lead to increased exposure rates of urban dwellers, a detrimental air quality impact, because of reduced transport of pollutants to upper levels and eventually downstream areas, a disadvantage from the air quality standpoint.