The purpose of this study is to assess the impact of the lockdown measures in 2020 on the urban air quality in Nicosia capital city, in Cyprus—an island-country in the East Mediterranean—which is often affected by transboundary dust pollution. The study focuses on three criteria pollutants, nitrogen dioxide (NO₂), carbon monoxide (CO) and Particulate Matter (PM₁₀), taken from three Air Quality Monitoring Stations; two urban stations and one reference-background. The results of this study show that the decrease in traffic, which is the main source of high concentrations of pollutants in the urban area, reached up to 66.5% during the lockdown. At the beginning of the lockdown period, it exhibited a downward trend of 29% for CO concentration, and downward trend 43% for NO₂ and PM₁₀ concentrations. The NO₂ concentration exhibited an upward trend towards the end of the lockdown; with the indication that this was due to meteorological conditions relevant to the monitoring stations and the transport of NO₂ concentrations from sources that cannot be tracked. PM₁₀ concentrations exhibited a varying behaviour as observed in the trends, where the decreasing trend was followed by an increasing trend due to transboundary air pollution episodes occurring in the same period.

Urban heat intensification and its implications for environmental health due to solar radiation absorption is a key driver of the Urban Heat Island (UHI) effect, particularly in Mediterranean climates. In dense urban fabrics, shortwave radiation is repeatedly reflected within street canyons, affecting pedestrian-level radiative exposure. This study introduces the Urban Canyon Shortwave Trapping Index (STI_c), a dimensionless parameter quantifying geometry-induced shortwave accumulation near the ground. Shortwave fluxes were simulated using the SOLWEIG model over the centre of Nicosia, Cyprus, under typical summer conditions. Wall albedo was systematically varied (0.2–0.8) to represent a range of common building materials and cooling strategies, including Nature-Based Solution (NBS). Results show that STI_c increases with both wall reflectivity and canyon depth, reaching values above 2 in high-albedo, high aspect ratio configurations. Maximum thermal trapping occurs between 11:00 and 14:00 local time, coinciding with peak in solar altitude. Spatial variability of STI_c also increases with wall albedo, indicating greater sensitivity to morphological differences under more reflective conditions. These findings support the integration of both morphological and material-based strategies in UHI mitigation and thermal comfort improvement and sustainable urban development.

Urban heat stress is an escalating concern in Mediterranean cities, which are climate-change hotspots, and is associated with increased mortality and adverse health outcomes. These risks highlight the need for evidence-based strategies to reduce thermal exposure in dense urban environments, with urban trees representing a key nature-based mitigation measure. Computational Fluid Dynamics (CFD) is widely used to study urban microclimates; however, many studies rely on idealized canyon geometries, and only a limited number incorporate thermophysiological indices to assess how urban microclimates influence human thermal perception1. This study uses a coupled CFD–SOLWEIG framework with UTCI-based assessment to enable integrated evaluation of urban microclimate and pedestrian thermal stress. High-resolution simulations were conducted and validated in a dense Mediterranean urban fabric in the historical centre of Nicosia (Cyprus) using ANSYS Fluent. Unsteady Reynolds-Averaged Navier–Stokes simulations were performed over a 24-hour summer period using detailed urban geometry to resolve airflow and air temperature, while mean radiant temperature was calculated using the SOLWEIG2 radiative model. These variables were combined in the UTCI-Fiala model to estimate UTCI at pedestrian level3. Urban trees were represented as porous canopy volumes to account for aerodynamic drag, shading, and evapotranspirational cooling effects and to quantify their mitigation role. Simulation results were validated using field observations, yielding mean absolute deviations of 1.48°C for air temperature (R²=0.89) and 0.68m s⁻¹ for wind speed (R²=0.67). UTCI exhibits a pronounced diurnal cycle driven by solar radiation, increasing from below 30°C in the early morning to above 45°C around midday. Even isolated urban trees can locally mitigate thermal stress by reducing radiative loading and air temperature through shading and evapotranspiration. UTCI reductions reach up to 5.1°C during peak solar hours at pedestrian level beneath the tree canopy, while evapotranspiration alone contributes reductions of up to 2.4°C under shaded conditions. Vegetation also generates localized cooling zones extending downstream up to three times the tree crown width, depending on local wind conditions. These results highlight the capacity of urban trees to significantly reduce pedestrian thermal stress and demonstrate the potential of the CFD–SOLWEIG–UTCI framework to support climate-responsive urban planning in dense built environments, providing quantitative evidence to inform heat mitigation strategies and urban design.1. Antoniou, N., Montazeri, H., Blocken, B. & Neophytou, M. K. A. On the impact of climate change on urban microclimate, thermal comfort, and human health: multiscale numerical simulations. Build. Environ. (2024).2. Lindberg, F., Holmer, B. & Thorsson, S. SOLWEIG 1.0 – Modelling spatial variations of 3D radiant fluxes and mean radiant temperature in complex urban settings. Int J Biometeorol 52, 697–713 (2008).3. Fiala, D., Havenith, G., Bröde, P., Kampmann, B. & Jendritzky, G. UTCI-Fiala multi-node model of human heat transfer and temperature regulation. Int J Biometeorol 56, 429–441 (2012)

Urban heat island (UHI) is a well-known phenomenon1 that affects thermal comfort of city dwellers, building energy consumption, and city sustainability in general. The planting of urban trees has been identified as one of the most effective methods for mitigating UHI effect and improving urban microclimate. The flow characteristics of an idealized symmetric urban street canyon containing trees of varying sizes is being investigated in this study. Particle Image Velocimetry (PIV) measurements in a water channel at the Environmental Fluid Mechanics Laboratory of the University of Cyprus, were used, to investigate the combination of buoyant and inertial forces in the canyon. A non-dimensional analysis of the measurements2 was conducted to quantify the competition between thermally driven and inertially driven flow in the presence of varying sizes of tree obstacles. This study aims to provide insights into the urban microclimate by examining the flow pattern, turbulence characteristics, and air exchanges between the canyon and the boundaries above. Results show that trees have a significant impact on the formation and strength of vortical flow within the canyon. Smaller trees stimulate the formation of stronger vortices, while larger trees lead to the elimination of the vortex and the existence of recirculation cells. The location of the heated surface also affects the vortex structure and elevation within the canyon. In addition, trees can reduce normalized vertical velocities, which can be restrictive to the “breathability”3,4 of the canyon. The volumetric ventilation rate 5 through the roof-level opening that refers to the time flow travels one circuit inside the canyon, is of high interest in the context of pollution escape/removal from the canyon, as well as the removal of other scalars (e.g., relating to heat, thermal comfort, and ¨breathability¨ of the urban canyon). This study found that in the case of a leeward heated surface, air removal from the canyon can change to air entrainment with the increase in tree size, whereas in the case of a windward heated surface, air removal dominates regardless of tree size. The study encompasses an examination of the vertical profiles of Reynolds Stresses (RS) and the extent of Roughness SubLayer (RSL). Our findings demonstrate that the range of layers extension, spans from 1.10 H (minimum) to 1.60 H (maximum), where H represents the height of the building. Moreover, a substantial increase in the RSL extent was observed in windward heated surfaces scenarios in comparison to leeward heated surfaces. In conclusion, our study sheds light on the impact of trees on flow characteristics and air exchange in urban street canyons. The findings of this study can inform urban areas' design and improve their overall sustainability.