Benzo[a]pyrene in Moscow road dust: pollution levels and health risks

Benzo[a]pyrene (BaP) is one of the priority pollutants in the urban environment. For the first time, the accumulation of BaP in road dust on different types of Moscow roads has been determined. The average BaP content in road dust is 0.26 mg/kg, which is 53 times higher than the BaP content in the background topsoils (Umbric Albeluvisols) of the Moscow Meshchera lowland, 50 km east of the city. The most polluted territories are large roads (0.29 mg/kg, excess of the maximum permissible concentration (MPC) in soils by 14 times) and parking lots in the courtyards (0.37 mg/kg, MPC excess by 19 times). In the city center, the BaP content in the dust of courtyards reaches 1.02 mg/kg (MPC excess by 51 times). The accumulation of BaP depends on the parameters of street canyons formed by buildings along the roads: in short canyons (< 500 m), the content of BaP reaches maximum. Relatively wide canyons accumulate BaP 1.6 times more actively than narrow canyons. The BaP accumulation in road dust significantly increases on the Third Ring Road (TRR), highways, medium and small roads with an average height of the canyon > 20 m. Public health risks from exposure to BaP-contaminated road dust particles were assessed using the US EPA methodology. The main BaP exposure pathway is oral via ingestion (> 90% of the total BaP intake). The carcinogenic risk for adults is the highest in courtyard areas in the south, southwest, northwest, and center of Moscow. The minimum carcinogenic risk is characteristic of the highways and TRR with predominance of nonstop traffic.

The impact of heat and inflow wind variations on vertical transport around a supertall building - The One Vanderbilt field experiment

The impact skyscrapers have on wind flow remains poorly characterized, thus affecting atmospheric dispersion predictions in dense urban centers. A new mobile observatory equipped with remote sensors controlled by a smart sampling protocol was developed to collect high-resolution (18 m, 15 s) observations throughout the atmospheric layer below 1.5 km. A series of four deployments was performed around the One Vanderbilt skyscraper (H₁ = 427 m) located in Manhattan, NY to document wind flow and temperature in canyons with relatively high width-to-depth ratios (H₂/W ~ 1.2-7.5; H₂ being the height of the adjacent building) and steepness (H₁/H₂= 2.1-11.2) and that under a range of inflow wind and solar heating conditions. A series of flow features were common to all case studies with head-on winds. A stagnation point was observed 2/3 of the way up the impeded portion of the One Vanderbilt, pointing to the importance of the upwind building height in controlling vertical air flow. In the canyons parallel to the flow, three sets of mirroring counterrotating vortices were detected pointing to the fact that H₂ is not as important a parameter in controlling flow in canyons parallel to the inflow wind. Plumes of rapidly rising air were detected near building heat vents under both 10 m s^-1 and 3 m s^-1 inflow wind conditions, at night and in the morning respectively. This suggests that anthropogenic heat may be an important energy source especially in the absence of solar heating. In the presence of solar heating, a systematic tendency for upward flow was observed above H₁. We associate this pattern to the presence of rising thermals, a common mechanism for planetary boundary layer growth. Below H₂, complete flow reversal (relative to mechanically driven circulations) was detected ~20 % of the time, showing evidence of dominant thermal effects even under 7 m s^-1 inflow wind conditions.

Understanding the effects of roadside hedges on the horizontal and vertical distributions of air pollutants in street canyons

Built-up environments limit air pollution dispersion in street canyons and lead to complex trade-offs between green infrastructure (GI) usage and its potential to reduce near-road exposure. This study evaluated the effects of an evergreen hedge on the distribution of particulate matter (PM₁, PM_2.5, PM₁₀), black carbon (BC) and particle number concentrations (PNCs) in a street canyon in West London. Instrumentation was deployed around the hedge at 13 fixed locations to assess the impact of the hedge on vertical and horizontal concentration distributions. Changes in concentrations behind the hedge were measured with reference to the corresponding sampling point in front of the hedge for all sets of measurements. Results showed a significant reduction in vertical concentrations between 1 and 1.7 m height, with maximum reductions of -16% (PM₁ and PM₁₀) and -17% (PM_2.5) at ∼1 m height. Horizontal concentrations revealed two zones between the building façade and the hedge, with opposite trends: (i) close to hedge (within 0.2 m), where a reduction of PM₁ and PM_2.5 was observed, possibly due to dilution, deposition and the barrier effect; and (ii) 0.2-3 m from the hedge, showing an increase of 13-37% (PM₁) and 7-21% (PM_2.5), possibly due to the blockage effect of the building, restricting dispersion. BC showed a significant reduction at breathing height (1.5 m) of between -7 and -50%, followed by -15% for PNCs in the 0.02-1 µm size range. The ELPI + analyser showed a peak of ∼30 nm. The presence of the hedge led to a ∼39 ± 32% decrease in total PNCs (0.006-10 µm), suggesting a greater removal in different modes, such as a 83 ± 12% reduction in nucleation mode (0.006-0.030 µm), 74 ± 15% in ultrafine (≤0.1 µm), and 34 ± 30% in accumulation mode (0.03-0.3 µm). These findings indicate graded filtering of particles by GI in a near-road street canyon environment. This insight will guide the improved design of GI barriers and the validation of microscale dispersion models.

Buoyancy effects on the flows around flat and steep street canyons in simplified urban settings subject to a neutral approaching boundary layer: Wind tunnel PIV measurements

The present wind tunnel particle image velocimetry (PIV) measurements document flows around flat and steep street canyons subject to thermal conditions at different levels, ranging from the Richardson number of 0.31 to 2.07. A steepness ratio, that is, the ratio of windward and leeward building heights, is proposed to characterise the geometrical influence of street canyons surrounded by buildings of non-uniform height. To study the thermal effects of building façades and ground on surrounding flow, surfaces of building models and the ground between them are heated up and maintained at three different temperatures to induce buoyant flows of different strength. The transition of the canyon flow from the typical rooftop shear-layer driven vortex to the buoyant plume type of flow is clearly revealed from the measurement results, which enhances the air removal that takes place at the roof-level of the two canyons. However, due to the different steepness of the canyons, the air removal rate from the steep canyon of a steepness ratio 2.52 is approximately 50% of that from the flat canyon with a steepness ratio of 1.53 in the buoyant plume-driven case because the downward flush flow along the windward façade suppresses the ascending plumes in the steep canyon. At the pedestrian level, the wind field is jointly dominated by the interplay between canyon-wide vortical flow and the buoyant plume rising ascending from the ground. The dynamics of non-isothermal flow in flat and steep canyons are revealed in detail, the implication of which is that the steepness of street canyons has to be considered in urban morphology planning, as well as in simplified geometrical representations of street canyons and in simplified urban canopy models.

Effect of triangular roof angle on dispersion of gaseous pollutants and particulate matter

The Euler-Lagrangian method is adopted to simulate the dispersion of gaseous pollutants and particulate matter (PM) in isolated street canyons, and the influences of the roof angle on the flow structures and distributions of gaseous pollutants and PM are analyzed in detail. Numerical simulation results indicate that gaseous pollutants and PM in the canyons present three typical single main clockwise vortex, transition vortex, and double vortex structures, which are identified at increasing roof slopes. Gaseous pollutants and PM demonstrate the lowest concentration of pollutants when a single vortex structure exists. The concentration of gaseous pollutants and PM reaches the highest value in pedestrian-level areas when the flow field is in a transitional vortex structure. Unlike gaseous pollutants, the concentration of PM does not always decrease with increasing altitude, and higher PM concentrations sometimes occur in the mid-level areas of the canyon. A small roof incline angle is generally recommended for discharging gaseous pollutants and PM.

A decision support tool for evaluating the air quality and wind comfort induced by different opening configurations for buildings in canyons

This study formulated a new index for evaluating both the air quality and wind comfort induced by building openings at the pedestrian level of street canyons. The air pollutant concentrations and wind velocities induced by building openings were predicted by a series of CFD simulations using ANSYS Fluent software based on standard k-ɛ model. The types of opening configurations investigated inside isolated and non-isolated canyons included separations, voids and permeable elements. It was found that openings with permeability values of 10% were adequate for improving the air quality and wind comfort conditions for pedestrians after considering the reduction in development floor areas. Openings were effective in improving the air quality in isolated canyons and different types of opening configurations were suggested for different street aspect ratios. On the contrary, openings were not always found effective for non-isolated canyons if there were pollutant sources in adjacent street canyons. As such, it would also be recommended introducing openings to adjacent canyons along with openings to the target canyons. The formulated index can help city planners and building designers to strike an optimal balance between air quality and wind comfort for pedestrians when designing and planning buildings inside urban streets and thus promoting urban environmental sustainability.

Variability of airborne particle metrics in an urban area

In the present study a mobile monitoring approach (i.e. bike with onboard instruments) was proposed and applied to investigate the spatial variability of all the key airborne particle metrics in an Italian urban area from a statistical point of view. Particle number, alveolar-deposited surface area, and PM₁₀ concentrations were measured through hand-held monitors and compared to simultaneous background concentrations by means of non-parametric tests and further post-hoc tests (Kruskal-Wallis test). Streets characterized by exposure levels statistically higher than the background levels for all the particle metrics were identified for different seasons in a pilot urban area (Cassino, Italy). A higher number of hot spots was detected for metrics affected by ultrafine particles (i.e. number and alveolar-deposited surface area concentrations) with respect to PM₁₀. The effect of metrological requirements of the instrumentation on the proposed method was also discussed.

From street canyon microclimate to indoor environmental quality in naturally ventilated urban buildings: Issues and possibilities for improvement

Many buildings in urban areas are more or less naturally ventilated. A good understanding of the current status and issues of indoor environmental quality (IEQ) in naturally ventilated urban buildings and the association with urban microclimate is fundamental for improving their IEQ. This paper reviews past studies on (a) the microclimate in urban street canyons, (b) the potential influence of such microclimate on IEQ of nearby naturally ventilated buildings, and (c) the real-life IEQ status in these buildings. The review focuses mainly on studies conducted by on-site measurements. The microclimate in urban street canyons is characterized by low wind speed, high surface temperature difference, high pollutant concentration, and high noise level. Insufficient ventilation rates and excessive penetration of outdoor pollutants are two key risks involved in naturally ventilated urban buildings. Existing knowledge suggests that reasonable urban planning and careful building envelope design are the primary methods to ensure acceptable IEQ and maximize the utilization of natural ventilation. However, quantitative studies of both microclimate in street canyons and IEQ in buildings are still highly insufficient in many aspects, which make cross comparison and influencing factors analysis currently impossible. Based on the limitations of previous studies and the current issues of naturally ventilated urban buildings, suggestions are made for future studies to better understand and improve IEQ in naturally ventilated urban buildings.

Impact of trees on pollutant dispersion in street canyons: A numerical study of the annual average effects in Antwerp, Belgium

Effects of vegetation on pollutant dispersion receive increased attention in attempts to reduce air pollutant concentration levels in the urban environment. In this study, we examine the influence of vegetation on the concentrations of traffic pollutants in urban street canyons using numerical simulations with the CFD code OpenFOAM. This CFD approach is validated against literature wind tunnel data of traffic pollutant dispersion in street canyons. The impact of trees is simulated for a variety of vegetation types and the full range of approaching wind directions at 15° interval. All these results are combined using meteo statistics, including effects of seasonal leaf loss, to determine the annual average effect of trees in street canyons. This analysis is performed for two pollutants, elemental carbon (EC) and PM10, using background concentrations and emission strengths for the city of Antwerp, Belgium. The results show that due to the presence of trees the annual average pollutant concentrations increase with about 8% (range of 1% to 13%) for EC and with about 1.4% (range of 0.2 to 2.6%) for PM10. The study indicates that this annual effect is considerably smaller than earlier estimates which are generally based on a specific set of governing conditions (1 wind direction, full leafed trees and peak hour traffic emissions).

Spatially-varying surface roughness and ground-level air quality in an operational dispersion model

Urban form controls the overall aerodynamic roughness of a city, and hence plays a significant role in how air flow interacts with the urban landscape. This paper reports improved model performance resulting from the introduction of variable surface roughness in the operational air-quality model ADMS-Urban (v3.1). We then assess to what extent pollutant concentrations can be reduced solely through local reductions in roughness. The model results suggest that reducing surface roughness in a city centre can increase ground-level pollutant concentrations, both locally in the area of reduced roughness and downwind of that area. The unexpected simulation of increased ground-level pollutant concentrations implies that this type of modelling should be used with caution for urban planning and design studies looking at ventilation of pollution. We expect the results from this study to be relevant for all atmospheric dispersion models with urban-surface parameterisations based on roughness.