Perpendicular Wind Direction Research Articles

Statistical assessment of the concentration fluctuations in street canyons via time-resolved wind tunnel experiments

Time-resolved wind tunnel experiments are performed for three periodic street canyon configurations: one of uniform (H/W = 1) and two of variable building height (consisting of 0.5H and 1.5H tall towers in aligned and staggered arrangements) for perpendicular wind direction. The velocity field is mapped using Laser Doppler Anemometry (LDA), and the tracer gas concentration is sampled using Fast Flame Ionization Detection (FFID). The concentration field is characterized by the mean concentration and the characteristic percentiles of the concentration-time series, such as the median and the peak, represented by the 99th percentile, in a total of 1138 sampling points. The probability distribution of the concentration in each point is modeled using the gamma distribution, which is fitted based on the mean and the variance of the measured time series. The results show that the mean pedestrian exposure to air pollutants is significantly lower in the case of heterogeneous roof height, although the mean concentration at the leeward corner of the source canyon can locally exceed those of the uniform street canyons for both tower arrangements of variable building height. Moreover, it is shown that the concentration peaks for both tower configurations can significantly exceed those of the uniform canyons case, especially near the sources.

The impact of dynamic traffic and wind conditions on green infrastructure performance to improve local air quality

Road traffic represents the dominant source of air pollution in urban street canyons. Local wind conditions greatly impacts the dispersion of these pollutants, yet street trees complicate ventilation in such settings. This case study adopts a novel modelling framework to account for dynamic traffic and wind conditions to identify the optimal street tree configuration that prevents a deterioration in air quality. Measurement data from a shallow to moderately deep street canyon (average 0.5 H/W aspect ratio and four lanes of 1-way traffic) in Dublin, Ireland was used for model calibration. The computational fluid dynamics (CFD) models were used to examine scenarios of dynamic traffic flows within each traffic lane with respect to its impact on local PM2.5 concentrations on adjacent footpaths, segmenting air quality monitoring results based on different wind conditions for model calibration. The monitoring campaign identified higher PM2.5 concentrations on the leeward (north) footpath, with average differences of 14.1 % (2.15 μg/m3) for early evening peaks. The modelling results demonstrated how street trees negatively impacted air quality on the windward footpath in parallel wind conditions regardless of leaf area density (LAD) or tree spacing, with mixed results observed on the leeward footpath in varying traffic flows and wind speeds. Perpendicular wind direction models and high wind speed exacerbated poor air quality on the windward footpath for all tree spacing models, while improving the air quality on the leeward footpath. The findings advise against planting high-LAD trees in this type of street, with a minimum of 20 m spacing for low-LAD trees to balance reducing local air pollution and ventilation capacity in the street. This study highlights the complexities of those in key decision-marking roles and demonstrates the need to adopt a transparent framework to ensure adequate modelling evidence can inform tree planting in city streets.

CFD simulation of gas dispersion at hydrogen bunkering station

ABSTRACT This research is to simulate hydrogen leakage at bunker station with various wind directions and velocities using Computational Fluid Dynamics (CFD) model to understand hydrogen dispersion behaviour and provide general guidelines to establish risk prevent measures and mitigations at early design phase. This case study examines hydrogen plume behavior in various wind conditions, focusing on horizontal and vertical dispersion as well as mean travel distance over time. Regardless of wind direction, hydrogen disperses in alignment with the wind. As time progresses post-leakage, the plume elongates in the wind's longitudinal direction and contracts vertically, maintaining a consistent shape. Wind direction and the direction of hydrogen release notably influence dispersion patterns. When the wind aligns vertically with the release point, hydrogen plume distance increases with higher wind speeds. Conversely, when wind opposes the release direction, plume length tends to decrease at high speeds. Intriguingly, the maximum distance of 27.85 m occurs when wind and leak directions are orthogonal at 180°. For wind speeds up to 5 m/s, all wind directions show a similar increase in plume distance. However, at 7 m/s, scenarios with horizontal, perpendicular wind directions exhibit a distinct change. Analyzing hydrogen dispersion aids in establishing safety criteria and risk mitigation distances for hydrogen leakages in bunker stations during the early design phase.

Large-eddy simulations on pollutant reduction effects of road-center hedge and solid barriers in an idealized street canyon

This study conducted large-eddy simulations (LES) on the pollutant reduction effects of hedge and solid barriers in a three-dimensional idealized street canyon with an aspect ratio of 0.5. The wind direction was perpendicular and oblique (45°) to the street. The results were validated with data from wind tunnel experiments. LES accurately predicted the concentration distribution in the barrier-free case and reproduced well the barrier-induced concentration reduction. In the barrier-free case, a large recirculation vortex was observed. However, the central barriers forced the recirculated airflow in the middle of the canyon and newly formed vortices near the leeward walls. The two counter-direction vortices in the hedge and solid barrier cases transported pollutants toward the center of the canyon and enhanced the vertical pollutant removal at the top of the street canyon. The hedge barrier (solid barrier) reduced spatially-averaged concentration by about 59% (45%) near the leeward wall, 64% (20%) near the windward wall, and 45% (17%) in the whole street canyon compared to the barrier-free case. The effects of leaf area density (LAD) and barrier width were further investigated under the perpendicular wind direction. Increasing the LAD or the width of the hedge barrier decreased concentration near the leeward walls but increased canyon-averaged concentration. Increasing the width of the solid barrier decreased the concentration near the leeward walls and the canyon-averaged concentration. In an oblique wind direction, the hedge and solid barriers reduced by about 30% and 60% the spatially-averaged concentration near the building walls compared to the barrier-free case.

Influence of wind direction on the ventilation and pollutant dispersion in different 3D street canyon configurations: numerical simulation and wind-tunnel experiment.

Configuration of street canyon and the wind environment have a great influence on the self-ventilation capacity of the canyon, but the couple-effect of these two factors could not be considered in the previous study. The purpose of this study is to clarify the couple effect of street canyon configuration and wind environment on the ventilation and pollutant dispersion inside the street canyon. For this purpose, five wind directions of α = 90°, 60°, 45°, 30°, and 0° (α is the angle between the approaching wind and street canyon) and three canyon configurations (flat, step-up, and step-down canyons) were considered with numerical simulation and wind-tunnel experiment. Meanwhile, ACH (air exchange rate) and NEV (net escape velocity) were used to evaluate the ventilation capacity of the canyon. The results reveal that the wind direction has a vital influence on the ventilation in the different canyon configurations. Under the parallel wind direction (α = 0°), the airflow and ventilation capacity inside the three canyons are similar. Relative difference of ACH, named as RDA ((ACHasymmetric-ACHsymmetric)/ACHsymmetric [Formula: see text] 100%), is 1.82%. However, under the oblique (α = 30°, 45°, and 60°) and perpendicular wind direction (α = 90°), the airflow of the step-down canyon is very different from the step-up and flat canyons. In step-down canyons, reverse flow occurs under the oblique and perpendicular wind direction, and the strength of the reverse flow increases as α increases. Due to this reverse flow, the ventilation capacity of the step-down canyon is lower than that of the step-up and flat canyons. As for the ventilation capacity in the pedestrian respiration domain, the ventilation capacity of the leeward pedestrian domain (leeward NEV) is higher in the step-down canyon than in the step-up canyon and the flat canyon (when α = 90°, leeward NEV of step-down canyon is 2.47 times the flat canyon). Conversely, the ventilation capacity of the windward pedestrian domain is lower in step-down canyons than in step-up or flat canyon (when α = 90°, windward NEV of step-down canyon is 0.1 times that of step-up canyon). The aforementioned findings are helpful to understand the effects of canyon configurations together with wind directions on the airflow as well as pollutant concentration inside the canyon. Although further researches are still required to provide practical guidelines, this study present effective methodologies to quantify the influences of street configurations and wind directions on street canyon ventilation for urban design purpose.

Effects of roadside morphologies and moving vehicles on street canyon ventilation

The influence of roadside morphology and the movement of vehicles on street canyon ventilation is important, but their coupling effect is still unclear. This study aimed to determine the best structural design for roadside buildings while considering moving vehicles’ effect on ventilation. The morphologies examined were podium, cantilever, rain shed, balcony, outdoor air-conditioning unit, and reference structures. The ventilation capacity of partial areas and the entire lane in the sidewalk and cycle lane was analyzed based on the age of air and net escape velocity (NEV). When a vehicle moves through a street canyon, the ventilation capacity of the zone part at the rear of the vehicle is improved, especially for the cycle lane on the windward side. However, for the entire lane, moving vehicles reduce the ventilation capacity by about 0.2–1.4%, with an average reduction of 0.8%. Therefore, when a vehicle passed through the street canyon, it reduced the ventilation capacity of an entire lane under a perpendicular wind direction. The cantilever morphology exhibited the best ventilation based on the vortex distribution and ventilation indices analysis. Its NEV values were 5% higher than that of the reference.

Characteristic Scales for Turbulent Exchange Processes in a Real Urban Canopy

An experimental field campaign is designed to unveil mechanisms responsible for turbulent exchange processes when mechanical and thermal effects are entwined. The focus is an urban street canyon with a mean aspect ratio H/W of 1.65 in the business centre of a mid-size Italian city (H is the mean building height and W is the mean canyon width). The exchange processes can be characterized by time scales and time-scale ratios specific to either mechanical or thermal process. Time scales describe the mixing caused by momentum and heat exchange within different canyon layers, while their rates are surrogates of their efficacy. Given that homogeneous mixing does not always occur within the canyon, several time scales are estimated at different levels, showing that mechanical and thermal processes may both contribute to enhance mixing. By computing mechanical time scales, it is found that the fastest mixing occurs at the canyon rooftop level for perpendicular or oblique wind directions, while slow mixing occurs for parallel directions. Thermal processes are faster than the mechanical ones and are particularly efficient for perpendicular wind directions. By calculating the time-scale ratios, exchange processes are found to facilitate mixing for most wind directions and to regulate the pollutant-concentration variability in the canyon. This variability can be associated with the local-circulation regime, demarcated as thermally driven or inertially driven using a buoyancy parameter, i.e., the ratio between thermal and inertial forcings. Using this approach, a generalization of the results is proposed, enabling the extension of the current investigation to different street-canyon aspect ratios.

Implicit Definition of Flow Patterns in Street Canyons—Recirculation Zone—Using Exploratory Quantitative and Qualitative Methods

Air pollution is a major health hazard for the population that increasingly lives in cities. Street-scale Air Quality Models (AQMs) are a cheap and efficient way to study air pollution and possibly provide solutions. Having to include all the complex phenomena of wind flow between buildings, AQMs employ several parameterisations, one of which is the recirculation zone. Goal of this study is to derive an implicit or explicit definition for the recirculation zone from the flow in street canyons using computational fluid dynamics (CFD). Therefore, a CFD-Large Eddy Simulation model was employed to investigate street canyons with height to width ratio from 1 to 0.20 under perpendicular wind direction. The developed dataset was analyzed with traditional methods (vortex visualization criteria and pollutant dispersion fields), as well as clustering methods (machine learning). Combining the above analyses, it was possible to extract qualitative features that agree well with literature but most importantly to develop quantitative expressions that describe their topology. The extracted features’ topology depends strongly on the street canyon dimensions and not surprisingly is independent of the wind velocity. The developed expressions describe areas with common flow characteristics inside the canyon and thus they can be characterised as an implicit definition for the recirculation zone. Furthermore, the presented methodology can be further applied to cover more parameters such us oblique wind direction and heated-facades and more methods for data analysis.

Study on localised wind pressure development in gable roof buildings having different roof pitches with experiments, RANS and LES simulation models

In this study, the influence of different roof pitches on the mean pressure distributions around isolated buildings subject to different wind directions were investigated with wind tunnel tests, 3D steady Reynolds-averaged Navier-Stokes (RANS) simulation and large eddy simulation (LES). Wind tunnel experiments were conducted to obtain mean pressure distributions around buildings having three commonly used roof pitches, namely, 1:5, 2:5 and 3:5. The critical high suction areas created by conical vortices were identified, and the influence of roof pitch on these critical areas with high localised pressures was investigated under various wind directions. In addition, computational fluid dynamics (CFD) analysis was performed, and the performance of RANS and LES were evaluated and compared with the results measured from the wind tunnel tests by considering both the accuracy of results and computational cost. The assessment on RANS and LES was used to determine the best guidelines for the flow problems involved in this study.Results from the RANS show good agreement with the experimental results on pressure distribution when the building is subject to perpendicular wind directions. However, a significant improvement was found using LES over RANS in the prediction of near-building flow field and localised pressure under oblique wind directions but with an increased computational cost by a factor of more than 80. Furthermore, the high suction pressures are observed to be more critical on buildings with a lower roof pitch under both perpendicular and oblique wind directions. This indicates that a low roof pitch should be applied with caution especially in windstorm-prone areas.

Effect of Wind Angle Direction on Carbon Monoxide (CO) Concentration Dispersion on Traffic Flow in Padang City

This study aims to analyze the relationship between CO concentration and wind direction. Wind direction in this context is the wind angle to the road on the traffic flow in Padang City. Sampling of CO concentration was conducted for 9 days at 3 monitoring points (each 3-day point) representing the wind angle to the road (a) i.e. at Jend. A. Yani road (0 degrees), Andalas road (30 degrees) and Prof. Dr. Hamka road (60 degrees), using impinger and analyzed by spectrophotometer. The results of the research in the three monitoring sites showed that the concentration of CO ranged between 137.217 and 600.525 μg/Nm3. The highest and lowest concentrations respectively on Prof. Dr. Hamka road and Jend. A. Yani road. The sampling showed that CO concentrations will be decreased if wind direction is changed from perpendicular wind direction (a 90°) to a 60°, 30°, and 0° respectively by 64.62%, 37.77% and 27.09%. It can be concluded that the wind angle direction to the road affects the CO concentrations in the roadside.

Wind-induced single-sided natural ventilation in buildings near a long street canyon: CFD evaluation of street configuration and envelope design

Wind-induced single-sided natural ventilation in buildings was widely investigated based on isolated buildings. However, owing to the presence of surrounding buildings, the wind flow pattern around a building in an urban area becomes very different from that around an isolated building. Considering an urban context, this study investigates the wind-induced single-sided natural ventilation in buildings near a long street canyon under a perpendicular wind direction using CFD method. Four aspect ratios (AR) of the street canyon, from 1.0, 2.0, 4.0 to 6.0, are investigated to examine the influence of street configuration, while eight envelope features are compared to explore the possibility of envelope design in improving natural ventilation performance of urban buildings. Ventilation rate of rooms in buildings is particularly analyzed. AR influences ventilation rate and its distribution among rooms along height of buildings. The percentage decrease of ventilation rate of buildings reaches 67% when AR of a street canyon is increased from 1.0 to 6.0. Envelope design provides a possibility to enhance the adaptability of buildings to dense urban environments. A good envelope design, such as a horizontal feature at the middle of an opening, can break effectively the along-facade flow and thus create a large pressure difference to drive ventilation. The findings of this study are intended to increase the understanding of natural ventilation performance in urban buildings and thus provide information for urban planning and building design.

Modelling urban airflow and natural ventilation using a GPU-based lattice-Boltzmann method

Simulation of urban airflow and ventilation potential is desirable for building design, however the complex and transient nature of flows in urban environments makes this a challenging task. This study aims to evaluate the capability of a lattice-Boltzmann method (LBM) code deployed on a graphical processing unit (GPU) using a large eddy sub-grid turbulence model for cross-flow ventilation of an idealised cubical building at wind-tunnel scale. ANSYS Fluent is used as a numerical comparison. Façade pressure and ventilation of the cube are investigated for parallel and perpendicular wind directions with the building in isolation and regular array format. Pressures, velocities and ventilation rates are compared to experimental data from wind tunnel and full-scale experiments of the Silsoe cube. Simulations compare favourably with experimental values and between each other. When the cube was surrounded by other cubes, simulations suggest that vortex shedding from up-wind buildings provides pulsating ventilation, improving airflow ingress in the parallel wind cases. A parametric study showed that doubling surrounding building height had a small negative effect on ventilation but was mitigated by high levels of downdraft and flow fluctuations in the vertical plane. Comparatively, doubling the central building height had a net positive effect but caused high internal airspeeds for both angles. The LBM code running on one GPU was several orders of magnitude faster than Fluent with similar accuracy. Simulation time using the LBM approach was several orders of magnitude lower than Fluent.

Numerical Study on the Urban Ventilation in Regulating Microclimate and Pollutant Dispersion in Urban Street Canyon: A Case Study of Nanjing New Region, China

Urban ventilation plays an important role in regulating city climate and air quality. A numerical study was conducted to explore the ventilation effectiveness on the microclimate and pollutant removal in the urban street canyon based on the rebuilt Southern New Town region in Nanjing, China. The RNG k − ε turbulence model in the computational fluid dynamics (CFD) was employed to study the street canyon under parallel and perpendicular wind directions, respectively. Velocity inside of the street canyon and temperature on the building envelopes were obtained. A novel pressure coefficient was defined, and three methods were applied to evaluate the urban ventilation effectiveness. Results revealed that there was little comfort difference for the human body under two ventilation patterns in the street canyon. Air stagnation occurred easily in dense building clusters, especially under the perpendicular wind direction. In addition, large pressure coefficients ( C P > 1 ) appeared at the windward region, contributing to promising ventilation. The air age was introduced to evaluate the “freshness” of the air in the street canyon and illustrated the ventilation effectiveness on the pollutant removal. It was found that the young air distributed where the corresponding ventilation was favorable and the wind speed was large. The results from this study can be useful in further city renovation for the street canyon construction and municipal planning.

CFD simulation of flow in a long street canyon under a perpendicular wind direction: Evaluation of three computational settings

A street canyon is an important platform for the understanding of local atmospheric flow and other related processes in the built environment. Many previous studies focused on long street canyons under a perpendicular wind direction, as they represent the worst street canyon microclimate, such as stagnation of wind and accumulation of pollutants. While CFD simulations were widely applied to investigate atmospheric processes in street canyons, appropriate computational settings are important factors influencing the predictive reliability. A non-exhaustive literature review of CFD studies on atmospheric processes in long street canyons indicates an arbitrary selection of three important computational settings, namely computational domain configuration, domain dimensions and inflow boundary conditions. Based on previous water tunnel experimental data for street canyons with aspect ratio equal to 0.5, 1.0 and 2.0, this study evaluates the influence of the three computational settings on CFD prediction of isothermal flow field inside the street canyons. Flow field inside an urban street canyon cannot be reasonably predicted using an isolated street canyon included in a conventional computational domain, which, however, can be well predicted using a T-shape computational domain where a street canyon is connected to a free flow layer above the canyon. A T-shape domain with the upstream length, downstream length and height above a street canyon all equal to the height of the street canyon is appropriate when considering both computational cost and predictive accuracy. It is reasonable to use uniform inflow boundary conditions to represent the free layer above street canyons.

Transport characteristics of PM2.5 inside urban street canyons: The effects of trees and vehicles

Particles emitted from vehicles can threaten human health. Therefore the investigations about the particle dispersion characteristics inside the street canyon are of significance. For the current work, the airflow fields and the particle movement inside an isolated street canyon with three aspect ratios under perpendicular wind direction and two typical wind speeds are studied numerically, for which the particle distributions are obtained by solving the drift-flux model. The conclusions demonstrate that the trees will result in the particles gathering. However, the particle deposition fluxes on the trees are so small which cannot effectively remove the particles from the ambient air. Besides, the vehicles tend to cluster particles around the street axis and slow down the dispersion to the curbside buildings. Therefore, the vehicles should not be ignored in the numerical models, especially for the cases in the wind direction of 90°. Moreover the cases with the aspect ratio of 0.5 are more sensitive to the effects of trees and vehicles. The shop vendors expose to the highest particle concentrations in most of the scenarios.

Outdoor ventilation performance of various configurations of a layout of two adjacent buildings under isothermal conditions

This paper explores the pedestrian-level wind environment around two buildings of unequal heights for the effects of ratio of height of upwind building to that of downwind building, referred to as buildings height ratio, spacing distance between the buildings, and direction of ambient wind, on the outdoor air ventilation. The objective is to propose appropriate spacing distances for various configurations of this type of buildings layout in a suburban area of Kuala Lumpur, which would enable harnessing the full ventilation potential of the local wind conditions. A three-dimensional numerical technique employing computational fluid dynamics simulation of continuity and Reynolds-Averaged Navier-Stokes equations was used to study the turbulent flow field around the full-scale sizes of the buildings. Results indicate the influence of spacing distance between buildings, relative heights of the buildings and direction of ambient wind on outdoor ventilation. Results also show that 3-D turbulent flow processes such as lateral and secondary flows play significant roles in defining the wind flow pattern and ventilation availability. The perpendicular wind direction presents remarkable adverse effects compared to the inclined direction, while a higher upwind building height which would ordinarily be expected to proportionally obstruct wind flow could be of advantage, at low values of building height ratios, in enhancing outdoor ventilation. The findings established the need to give consideration to climatic factors such as wind flow and direction in planning urban cities, as it affects buildings to be situated in close proximity, especially when there is height difference.

Rapid organic aerosol formation downwind of a highway: Measured and model results from the FEVER study

AbstractThe Fast Evolution of Vehicle Emissions from Roadway (FEVER) study was undertaken to strategically measure pollutant gradients perpendicular to a major highway north of Toronto, Canada. A case study period was analyzed when there was an average perpendicular wind direction. Two independent, fast response measurements were used to infer rapid organic aerosol (OA) growth on a spatial scale from 34 m to 285 m at the same time as a decrease was observed in the mixing ratio of primary emitted species, such as CO2 and NOx. An integrated organic gas and particle sampler also showed that near the highway, the aerosol had a larger semivolatile fraction than lower volatile fraction, but over a relatively short distance downwind of the highway, the aerosol transformed to being more low volatile with the change being driven by both evaporation of semivolatile and production of lower volatile organic aerosol. A new 1‐D column Lagrangian atmospheric chemistry model was developed to help interpret the measured increase in the ∆OA/∆CO2 curve from 34 m to 285 m downwind of highway, where the ∆ refers to background‐corrected concentrations. The model was sensitive to the assumptions for semivolatile organic compounds (SVOCs). Different combinations of SVOC emissions and background mixing ratios were able to yield similar model curves and reproduce the observations. Future measurements of total gas‐phase SVOC in equilibrium with aerosol both upwind and downwind of the highway would be helpful to constrain the model.

Pollutant Concentrations in Street Canyons of Different Aspect Ratio with Avenues of Trees for Various Wind Directions

This study summarizes the effects of avenues of trees in urban street canyons on traffic pollutant dispersion. We describe various wind-tunnel experiments with different tree-avenue models in combination with variations in street-canyon aspect ratio W/H (with W the street-canyon width and H the building height) and approaching wind direction. Compared to tree-free street canyons, in general, higher pollutant concentrations are found. Avenues of trees do not suppress canyon vortices, although the air ventilation in canyons is hindered significantly. For a perpendicular wind direction, increases in wall-average and wall-maximum concentrations at the leeward canyon wall and decreases in wall-average concentrations at the windward wall are found. For oblique and perpendicular wind directions, increases at both canyon walls are obtained. The strongest effects of avenues of trees on traffic pollutant dispersion are observed for oblique wind directions for which also the largest concentrations at the canyon walls are found. Thus, the prevailing assumption that attributes the most harmful dispersion conditions to a perpendicular wind direction does not hold for street canyons with avenues of trees. Furthermore, following dimensional analysis, an estimate of the normalized wall-maximum traffic pollutant concentration in street canyons with avenues of trees is derived.

On the segregation of chemical species in a clear boundary layer over heterogeneous land surfaces

Abstract. Using a Large-Eddy Simulation model, we have systematically studied the inability of boundary layer turbulence to efficiently mix reactive species. This creates regions where the species are accumulated in a correlated or anti-correlated way, thereby modifying the mean reactivity. We quantify this modification by the intensity of segregation, IS, and analyse the driving mechanisms: heterogeneity of the surface moisture and heat fluxes, various background wind patterns and non-uniform isoprene emissions. The heterogeneous surface conditions are characterized by cool and wet forested patches with high isoprene emissions, alternated with warm and dry patches that represents pasture with relatively low isoprene emissions. For typical conditions in the Amazon rain forest, applying homogeneous surface forcings and in the absence of free tropospheric NOx, the isoprene-OH reaction rate is altered by less than 10%. This is substantially smaller than the previously assumed IS of 50% in recent large-scale model analyses of tropical rain forest chemistry. Spatial heterogeneous surface emissions enhance the segregation of species, leading to alterations of the chemical reaction rates up to 20%. The intensities of segregation are enhanced when the background wind direction is parallel to the borders between the patches and reduced in the case of a perpendicular wind direction. The effects of segregation on trace gas concentrations vary per species. For the highly reactive OH, the differences in concentration averaged over the boundary layer are less than 2% compared to homogeneous surface conditions, while the isoprene concentration is increased by as much as 12% due to the reduced chemical reaction rates. These processes take place at the sub-grid scale of chemistry transport models and therefore need to be parameterized.

On the Tropospheric Ozone Variations in a Coastal Area of Southwestern Europe under a Mesoscale Circulation

Abstract It is well established that surface ozone levels are greatly affected by orography, solar radiation intensity, meteorological conditions, and the levels of their precursors. In this work, the authors study the sea–land breeze circulation in its relation with the behavior of ozone in a coastal area, located in southwestern Europe, with high levels of solar radiation and an important industrial complex emitting air pollutants. Hourly mean data over a 7-yr period (1999–2005) have been used in the study. Two patterns of sea–land breeze have been identified after analyzing 2298 days of measurements: a “pure” breeze (179 cases) and another one, called a “nonpure” breeze (284 cases), which is the resulting flow of the former and northwesterlies synoptic forcing. Among other results, the highest levels of surface ozone were observed under pure sea–land breeze, with hourly values up to 100 μg m−3 in the mean daily evolution. In contrast, for a nonpure breeze, the 24-h average daily value was lower than the corresponding one under a pure breeze by a factor of 1.16 and could reach 1.60 in representative real cases. These findings give evidence that the formation and accumulation of ozone are favored by the conditions under a pure sea–land breeze: that is, perpendicular wind directions toward the coastline, effective recirculation of air masses, and formation of ozone residual layers above the sea. Because these atmospheric conditions occur in other coastal regions in the world, the conclusions of this study could be extended to them.