Evaluation of heat and air pollution mitigation effects of urban green infrastructures and palisades in a Mediterranean urban high-density area - Scenario analyses based on microclimatological modelling

Urban high-density areas are particularly affected by climate change effects. Reduced ventilation, evapotranspiration, albedo as well as increased surface sealing and traffic emissions significantly influence urban microclimates. The ongoing densification of cities results in pollutant trapping and higher heat loads. Magnitudes of the Urban Heat Island (UHI) and the Urban Pollution Island (UPI) effects are expected to intensify in many cities worldwide according to future climate projections. Thermal discomfort, health consequences and higher mortality rates caused by heat stress and air pollution will become an increasing risk for inhabitants. Urban Green Infrastructures (UGI) are widely recognized to mitigate UHI and UPI effects. However, their effectiveness is highly dependent on local conditions, while non-strategical implementations can even cause adverse effects at local level. Therefore, sound-scientific analyses are required to achieve best possible cooling and air quality improvement. Microclimatological modelling is an efficient tool to evaluate the performance of climate change adaptation strategies in urban environments. This study aims to investigate the impact of palisades and UGI on air temperature as well as Carbon Monoxide (CO) concentrations at pedestrian level. The physically-based 3D-gridded ENVI-met model was used to simulate the microclimate of a street canyon (Via Guglielmo Marconi) in a densely-developed residential study area  (Bologna, Italy). To enhance model outputs, ENVI-met database parametrization was adjusted using cadastral datasets and field measurements. The simulations are driven and validated based on local meteorological measurements within the study area. Different scenarios were designed implementing technical- and nature-based solutions (NBSs) in the street canyon including trees, hedges, green walls and palisades. Model outputs were compared to the reference run to statistically evaluate the heat and air pollution mitigation potential. The results show that palisades have a non-significant impact on CO concentrations and even cause warming for pedestrians. Street trees and hedges moderately reduce air temperature by up to -0.45 K, but significantly increase CO concentrations by up to +9.37 %. In contrast, for green walls, a maximum cooling effect of 0.76 K but a non-significant increase in CO concentrations was found. The study concludes that while UGI can effectively reduce local temperatures, careful design and placement are crucial to minimize negative effects on air quality. These findings can have important implications for decision-making in urban planning. Further research could analyze the effects on bio-meteorological thermal comfort indices in the context of climate change adaptation, or compare the results with study areas with different ventilation or pollution conditions.