Dynamics of the urban heat island and its effect on local air quality: the Bologna iSCAPE case study

In this work we address the linkage between urban heat island (UHI) and air quality in the metropolitan area of Bologna (Italy), which is one of the pilot cities in H2020-iSCAPE (www.iscapeproject.eu). Occurrence of heat waves in the region is projected to increase in the future and superimposition with the UHI phenomenon is expected to have strong impacts on local circulation and pollutant dispersion patterns. Dynamic features of the UHI formation and evolution is examined by a combination of data analyses from two intensive field campaigns carried out in August 2017 and February 2018 integrated with high-resolution numerical modelling using an urbanized version of the model WRF. The experimental campaigns included a suite of instruments deployed in two neighbourhoods of Bologna city to evaluate profiles of turbulent fluxes of momentum and sensible heat, air temperature via distributed low-cost sensors, surface temperature using infra-red camera, particulate matter, ozone and traffic-related compounds concentrations. Data are also integrated with boundary-layer height measurements obtained by a Vaisala ceilometer (CL31) and radiosonde soundings available from a nearby station. Stand-alone preliminary model runs using WRF-3.8 and validated with several weather stations show that in presence of a heat-wave (the worst condition for the urban environment in south European cities, such as the one that occurred in July 2015 and in 2017), the UHI contribution exceeds rural temperatures of 4-6 °C during the night. It is shown that air quality in street canyons worsen under UHI phenomenon for which a simple physics-based relationship is proposed.