Influence of vehicular Black Carbon concentrations on Urban Heat Islands in Rome (Italy)

Urban areas represent critical environments being sources/sinks of a variety of pollutants. Cities have been in continuous expansion: over 50% of the world’s population live in urban centres. Priority actions are quantifying urban pollution sources, which generate an Urban Pollution Island (UPI) and a consistent amount of anthropogenic heat contributing to the Urban Heat Island (UHI) consisting of significantly higher temperatures in urban areas than in the surroundings. The UHI is one of the most pronounced surface climate changes caused by human activities and urbanization. It arises from the large differences between rural and urban land surface properties. During the day the UHI is due to the different heat and humidity release of rural/urban canopies. At night, the heat stored by the buildings is the major responsible for the urban warming. The link between UHI and UPI is extremely important: many urban activities are both pollutants and heat sources; emissions change the radiative properties of the urban atmosphere hence the surface energy budget (Li, 2018) and temperatures influence atmospheric chemistry and pollutant dispersion. The study focused on the complex mechanism by which traffic-related air pollutions affects the UHI in Rome (Italy) analysing timeseries of data continuously collected in the urban background station of Villa Ada started from July 2020 to May 2022. The station includes measurements of air quality variables such as PM10, PM2.5 and PM1, gases and meteorological parameters. The temperature measurements for the UHI estimation were provided by a Network weather stations, still evolving, developed in 2019 by CNR-ISAC in the framework of the LIFE ASTI European project. The analysed data included also Black Carbon measurements collected continuously by an aethalometer (AE33 Magee Scientific). The instrument uses the optical properties of aerosol to distinguish the BC fraction due to fossil fuel combustions (BCff) and that due to biomass burning (BCbb). The variability of BCff will be used as a proxy for the traffic aerosol impact in the atmosphere. UHI intensity has been evaluated by a degree of imperviousness by the method proposed in Schatz and Kucharick (2015). The study evaluates the potential relationships between BCff and UHI under different meteorological and hours of the day to account for seasonal and daily variations. the study will analyse the dependence between traffic and micrometeorological parameters, like the turbulent kinetic energy (TKE). As previously stated, the BC may play a role in decreasing turbulent mixing, due to its properties of absorbing solar radiation which cause an alteration of the radiative balance at the surface and, therefore, a decrease in the term of buoyancy flux which is directly linked to the PBL height and TKE production. Thereby a reduction in the height of the PBL would be produced (Cecilia, 2023) and a decrease in the vertical temperature gradient (Liu, 2019), generating an aerosolPBL interaction feedback. In a preliminary attempt to estimate the BC influence on UHI, a moderate correlation has been observed at the hours of traffic maxima times under high pressure and low wind. In the time slot 06:00-09:00, BCff concentrations increase due to vehicular traffic, these concentrations seem to influence the UHI trend. High BCff concentrations appear to be associated with high UHI values with a linear correlation shown in Figure 1. A research project has been submitted to study in deep the influence of BC on UHI.