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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 Climate, 21, ISSN:2212-0955

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The urban population has reached the highest number ever recorded. Based on recent studies, this number is going to increase even further, with strong consequences for the urban microclimate. In this perspective, the analysis of microclimatic conditions in the urban environment is getting more significant. This study analyzes microclimatic conditions in a real urban area. The real compact inhomogeneous urban area under study is located in Nicosia old town, in Cyprus. High-resolution Computational Fluid Dynamics (CFD) simulations are performed based on the 3D unsteady Reynolds-Averaged Navier-Stokes equations to assess the air temperature, surface temperatures and wind speed distributions. The CFD results are compared with on-site measurements for the same area. The results show that CFD can predict air temperatures in compact real urban areas, with an average deviation of approximately 7%. Based on the results, conclusions are going to be made regarding microclimatic conditions in the case study area.

Because of the rapid trend toward urbanization around 54% of the world’s population currently live in urban areas (UNFPA, 2007). The urbanized population is projected to increase and reach up to 66% by 2050 (United Nations, 2014). In addition, due to climatic changes phenomena like the Urban Heat Island (UHI) effect and heat waves are becoming more severe, with dramatic consequences for the urban population (Toparlar et al., 2015; Montazeri et al., 2017). Considering this trend, problems related to urban overpopulation are expected to get more intense in the next years, hence in this perspective, the analysis of microclimatic conditions in urban areas is getting more significant. Through the detailed literature review, it appears that most of the studies focusing on the analysis of microclimatic conditions use observational approaches such as on-site measurements, since this method provides information about the real complexity of the microclimatic conditions in urban areas, without any similarity issues. While computational power is radically increasing, the use of numerical simulation approaches is getting more popular. Most of the numerical studies use the Energy Balance Model which is based on the law of energy conservation. However, the use of Computational Fluid Dynamics (CFD) is considered advantageous since it can provide information of both temperature and velocity fields, and can be used in higher resolution scales. In this perspective, this study analyzes microclimatic conditions in a real compact urban area, with the use of CFD simulations, and results are compared with on-site measurements of the same area. The real compact inhomogeneous urban area under study is located in Nicosia old town, in Cyprus. High-resolution Computational Fluid Dynamics (CFD) simulations are performed based on the 3D unsteady Reynolds-Averaged Navier-Stokes equations to assess the air temperature and wind speed distributions. The on-site measurements took place as a part of the European funded project with code name TOPEUM (Neophytou et al., 2011), with collected data of air and surface temperature. The results show that CFD can predict air temperatures in compact real urban areas, with an average deviation of 6.8%, and surface temperatures with average deviation of approximately 6 to 7%. Based on the results, conclusions are going to be made regarding thermal comfort in the case study area.

Urban morphology characterization is crucial for the parametrization of boundary-layer development over urban areas. One complexity in such a characterization is the three-dimensional variation of the urban canopies and textures, which are customarily reduced to and represented by one-dimensional varying parametrization such as the aerodynamic roughness length \(z_{0}\) and zero-plane displacement \(d\). The scope of the paper is to provide novel means for a scale-adaptive spatially-varying parametrization of the boundary layer by addressing this 3-D variation. Specifically, the 3-D variation of urban geometries often poses questions in the multi-scale modelling of air pollution dispersion and other climate or weather-related modelling applications that have not been addressed yet, such as: (a) how we represent urban attributes (parameters) appropriately for the multi-scale nature and multi-resolution basis of weather numerical models, (b) how we quantify the uniqueness of an urban database in the context of modelling urban effects in large-scale weather numerical models, and (c) how we derive the impact and influence of a particular building in pre-specified sub-domain areas of the urban database. We illustrate how multi-resolution analysis (MRA) addresses and answers the afore-mentioned questions by taking as an example the Central Business District of Oklahoma City. The selection of MRA is motivated by its capacity for multi-scale sampling; in the MRA the “urban” signal depicting a city is decomposed into an approximation, a representation at a higher scale, and a detail, the part removed at lower scales to yield the approximation. Different levels of approximations were deduced for the building height \(\bar{{H}}\) and planar packing density \(\lambda _\mathrm{p}\). A spatially-varying characterization with a scale-adaptive capacity is obtained for the boundary-layer parameters (aerodynamic roughness length \(z_{0}\) and zero-plane displacement \(d\)) using the MRA-deduced results for the building height and the planar packing density with a morphometric model; an attribute that is shown to be of great advantage to multi-scale and multi-resolution numerical weather prediction models.

The study of urban-scale energy consumption due to buildings follows the archetype approach where samples of representative buildings are analysed and then results are extrapolated across the city. Lack of reliable data has so far been the main reason hindering different approaches; nevertheless, significant improvements have been achieved recently. In this work we use the Westminster Borough of London City, as a case study to illustrate how a dynamic high-resolution dataset for the energy demands of the whole area can be analyzed using the Multi-Resolution Analysis (MRA) to provide insights into the urban- and sub-urban (e.g. district) scale energy demand and therefore suggest how such an analysis can be exploited to guide smart urban energy-demand management. Specifically, we analyze full-day (24-hr) evolutions of the urban-scale energy heating/cooling demands together with high-resolution information on the building density, height and the population. By introducing the scale-adaptive approach, our analysis provides suggestions for e.g. peak hour identification and localization, as well as the identification for example of best-neighboring candidate zone area for extra energy supply in case of power failures.

The rate of removal of pollutants within simplified urban street canyons is investigated using air flow fields obtained from Particle Image Velocimetry (PIV) experiments and Computational Fluid Dynamics (CFD) simulations of the resulting pollutant dispersion. In particular, the link between the pollutant removal capacity and the air-flow exchange velocity, a characteristic velocity widely used to characterize this pollutant removal capacity, is examined. First, velocity fields through a series of homogeneous urban street canyons with flat roofs are obtained using ensemble-averaged PIV measurements as obtained within water channel experiments. The experimental results for the fully-developed street canyon air velocity field are used to drive numerical simulations of turbulent pollutant dispersion from a pollutant release source located within the street canyon. The rate of removal of pollutant from the canyon is deduced from the numerical simulations and analyzed in terms of mean convective and turbulent exchange mechanisms. It is found that rate of pollutant removal as expressed through a pollutant-exchange velocity is dominated by the turbulent pollutant diffusion in the street canyon case. It is found that the air-exchange velocity cannot be used as a representative measure for pollutant removal, since it does not take into account all physics involved in the removal process. It is only a measure for the convective pollutant flux when the pollutants are uniformly distributed in the control volume.

In this work we investigate the influence of real world conditions, including heterogeneity and natural variability of background wind, on the air flow and pollutant concentrations in a heterogeneous urban street canyon using both a series of field measurements and controlled laboratory experiments. Field measurements of wind velocities and Carbon Monoxide (CO) concentrations were taken under field conditions in a heterogeneous street in a city centre at several cross-sections along the length of the street (each cross-section being of different aspect ratio). The real field background wind was in fact observed to be highly variable and thus different Intensive Observation Periods (IOPs) represented by a different mean wind velocity and different wind variability were defined. Observed pollution concentrations reveal high sensitivity to local parameters: there is a bias towards the side closer to the traffic lane; higher concentrations are found in the centre of the street as compared to cross-sections closer to the junctions; higher concentrations are found at 1.5 height from the ground than at 2.5 m height, all of which are of concern regarding pedestrian exposure to traffic-related pollution. A physical model of the same street was produced for the purpose of laboratory experiments, making some geometrical simplifications of complex volumes and extrusions. The physical model was tested in an Atmospheric Boundary Layer water channel, using simultaneously Particle Image Velocimetry (PIV) and Planar Laser Induced Fluorescence (PLIF), for flow visualisation as well as for quantitative measurement of concentrations and flow velocities. The wind field conditions were represented by a steady mean approach velocity in the laboratory simulation (essentially representing periods of near-zero wind variability). The laboratory investigations showed a clear sensitivity of the resulting flow field to the local geometry and substantial three-dimensional flow patterns were observed throughout the modelled street. The real-field observations and the laboratory measurements were compared. Overall, we found that lower variability in the background wind does not necessarily ensure a better agreement between the airflow velocity measured in the field and in the lab. In fact, it was observed that in certain cross sections, the airflow was more affected by the particular complex architectural features such as building extrusions and balconies, which were not represented in the simplified physical model tested in the laboratory, than by the real wind field variability. For wind speed comparisons the most favourable agreement (36.6% of the compared values were within a factor of 2) was found in the case of lowest wind variability and in the section with the most simple geometry where the physical lab model was most similar to the real street. For wind direction comparisons the most favourable agreement (45.5% of the compared values was within ±45°) was found in the case with higher wind variability but in the cross-sections with more homogeneous geometrical features. Street canyons are often simplified in research and are often modelled as homogenous symmetrical canyons under steady flow, for practical purposes; our study as a whole demonstrates that natural variability and heterogeneity play a large role in how pollution disperses throughout the street, and therefore further detail in models is vital to understand real world conditions.

The significance of the residential building sector in terms of energy consumption is well acknowledged. In this view the knowledge of the way the residential building stock of Cyprus behaves in terms of energy consumption is quite valuable since it will assist policy makers to formulate targeted measures aiming the improvement of energy efficiency and setting current legal standards and benchmarks in the energy performance certificate, a requirement of the 2002/91/EC Directive. Unfortunately, the existing knowledge on this subject is quite poor. As a remedy a research project supported by a national research grant is in operation since December 2008. This paper presents the outline, goals and methodology of this research project and the findings regarding the energy behaviour and other characteristics of the residential building stock of Cyprus. From the analysis of the results for 500 residential buildings, it seems that the energy demand and primary energy required is lower than that of other European countries. Moreover, it seems that the age of residential buildings has low correlation with the energy demand while in contrast with other Northern and Central European countries, it is clear that the contribution of cooling energy requirements to the overall energy demand is quite significant.