Relating microclimate, human thermal comfort and health during heat waves: an analysis of heat island mitigation strategies through a case study in an urban outdoor environment

Ever growing populations in cities are associated with a major increase in road vehicles and air pollution. The overall high levels of urban air pollution have been shown to be of a significant risk to city dwellers. However, the impacts of very high but temporally and spatially restricted pollution, and thus exposure, are still poorly understood. Conventional approaches to air quality monitoring are based on networks of static and sparse measurement stations. However, these are prohibitively expensive to capture tempo-spatial heterogeneity and identify pollution hotspots, which is required for the development of robust real-time strategies for exposure control. Current progress in developing low-cost micro-scale sensing technology is radically changing the conventional approach to allow real-time information in a capillary form. But the question remains whether there is value in the less accurate data they generate. This article illustrates the drivers behind current rises in the use of low-cost sensors for air pollution management in cities, whilst addressing the major challenges for their effective implementation.

The rise of low-cost sensing for managing air pollution in cities

High-resolution spectral mapping of urban thermal properties with Unmanned Aerial Vehicles

How outdoor microclimate mitigation affects building thermal-energy performance: a new design-stage method for energy saving in residential near-zero energy settlements in Italy

Ultrafine particles (UFP; diameter less than 100 nm) are ubiquitous in urban air, and an acknowledged risk to human health. Globally, the major source for urban outdoor UFP concentrations is motor traffic. Ongoing trends towards urbanisation and expansion of road traffic are anticipated to further increase population exposure to UFPs. Numerous experimental studies have characterised UFPs in individual cities, but an integrated evaluation of emissions and population exposure is still lacking. Our analysis suggest that average exposure to outdoor UFPs in Asian cities is about four-times larger than those in European cities but impacts on human health are largely unknown. This article reviews some fundamental drivers of UFP emissions and dispersion, and highlights unresolved challenges, as well as recommendations to ensure sustainable urban development whilst minimising any possible adverse health impacts.

Ultrafine particles in cities

This work investigates the role of materials selected for different urban surfaces (e.g. on building walls, roofs and pavements) in the intensity of the urban heat island (UHI) phenomenon. Three archetypal street-canyon geometries are considered, reflecting two-dimensional canyon arrays with frontal packing densities (λf) of 0.5, 0.25 and 0.125 under direct solar radiation and ground heating. The impact of radiative heat transfer in the urban environment is examined for each of the different built packing densities. A number of extreme heat scenarios were modelled in order to mimic conditions often found at low- to mid-latitudes dry climates. The investigation involved a suite of different computational fluid dynamics (CFD) simulations using the Reynolds-Averaged Navier–Stokes equations for mass and momentum coupled with the energy equation as well as using the standard k-e turbulence model. Results indicate that a higher rate of ventilation within the street canyon is observed in areas with sparser built packing density. However, such higher ventilation rates were not necessarily found to be linked with lower temperatures within the canyon; this is because such sparser geometries are associated with higher heat transfer from the wider surfaces of road material under the condition of direct solar radiation and ground heating. Sparser canyon arrays corresponding to wider asphalt street roads in particular, have been found to yield substantially higher air temperatures. Additional simulations indicated that replacing asphalt road surfaces in streets with concrete roads (of different albedo or emissivity characteristics) can lead up to a ~5 °C reduction in the canyon air temperature in dry climates. It is finally concluded that an optimized selection of materials in the urban infrastructure design can lead to a more effective mitigation of the UHI phenomenon than the optimisation of the built packing density.

Konstantinos Marakkos

Papers

The role of materials selection in the urban heat island effect in dry mid-latitude climates