Trade-off between urban heat island mitigation and air quality in urban valleys

Abstract

The number of studies on urban cooling strategies has been continuously growing. A common premise is that urban heat island (UHI) mitigation has a net beneficial effect on urban environmental quality and sustainability. Here we evaluate how UHI mitigation affects air quality in an idealized urban valley by means of large-eddy simulations. A passive tracer represents air pollution transport in three UHI mitigation scenarios and under two initial stability conditions. Contrary to the common premise, our results demonstrate that UHI mitigation can worsen air quality in urban valleys, via the alteration of the mechanisms of air pollution transport. Our results also show the theoretical possibility of finding moderate UHI mitigation strategies in which UHI is reduced while limiting the impacts on air quality. A fundamental implication is that this strategies should be explored through case-specific realistic simulations for guiding decision-making in real systems. Large and often expensive urban transformations should not be accomplished under the generalized assumption that UHI mitigation improves environmental quality. Building a more general understanding of the potential impacts of UHI mitigation, as well as of the mechanisms behind, is a continuing challenge with important implications for urban management and planning.