Reactive Pollutant Dispersion Research Articles

Modelling the dispersion and transport of reactive pollutants in a deep urban street canyon: Using large-eddy simulation

This study investigates the dispersion and transport of reactive pollutants in a deep urban street canyon with an aspect ratio of 2 under neutral meteorological conditions using large-eddy simulation. The spatial variation of pollutants is significant due to the existence of two unsteady vortices. The deviation of species abundance from chemical equilibrium for the upper vortex is greater than that for the lower vortex. The interplay of dynamics and chemistry is investigated using two metrics: the photostationary state defect, and the inferred ozone production rate. The latter is found to be negative at all locations within the canyon, pointing to a systematic negative offset to ozone production rates inferred by analogous approaches in environments with incomplete mixing of emissions. This study demonstrates an approach to quantify parameters for a simplified two-box model, which could support traffic management and urban planning strategies and personal exposure assessment.

Dispersion and photochemical evolution of reactive pollutants in street canyons

Dispersion and photochemical evolution of reactive pollutants in street canyons with canyon aspect ratios of 1 and 2 are investigated using a computational fluid dynamics (CFD) model coupled with the carbon bond mechanism IV (CBM–IV). Photochemical ages of NOx and VOC are expressed as a function of the NO2-to-NOx and toluene-to-xylene ratios, respectively. These are found to be useful for analyzing the O3 and OH oxidation processes in the street canyons. The OH oxidation process (O3 oxidation process) is more pronounced in the upper (lower) region of the street canyon with a canyon aspect ratio of 2, which is characterized by more (less) aged air. In the upper region of the street canyon, O3 is chemically produced as well as transported downward across the roof level, whereas O3 is chemically reduced in the lower region of the street canyon. The O3 chemical production is generally favorable when the normalized photochemical ages of NOx and VOC are larger than 0.55 and 0.28, respectively. The sensitivities of O3 chemical characteristics to NOx and VOC emission rates, photolysis rate, and ambient wind speed are examined for the lower and upper regions of the street canyon with a canyon aspect ratio of 2. The O3 concentration and the O3 chemical production rate divided by the O3 concentration increase as the NOx emission rate decreases and the VOC emission rate and photolysis rate increase. The O3 concentration is less sensitive to the ambient wind speed than to other factors considered. The relative importance of the OH oxidation process compared to the O3 oxidation process increases with increasing NOx emission rate and photolysis rate and decreasing VOC emission rate. In this study, both O3 and OH oxidation processes are found to be important in street-canyon scale chemistry. The methodology of estimating the photochemical ages can potentially be adopted to neighborhood scale chemistry.

Effects of building aspect ratio, diurnal heating scenario, and wind speed on reactive pollutant dispersion in urban street canyons

A photochemistry coupled computational fluid dynamics (CFD) based numerical model has been developed to model the reactive pollutant dispersion within urban street canyons, particularly integrating the interrelationship among diurnal heating scenario (solar radiation affections in nighttime, daytime, and sun-rise/set), wind speed, building aspect ratio (building-height-to-street-width), and dispersion of reactive gases, specifically nitric oxide (NO), nitrogen dioxide (NO2) and ozone (O3) such that a higher standard of air quality in metropolitan cities can be achieved. Validation has been done with both experimental and numerical results on flow and temperature fields in a street canyon with bottom heating, which justifies the accuracy of the current model. The model was applied to idealized street canyons of different aspect ratios from 0.5 to 8 with two different ambient wind speeds under different diurnal heating scenarios to estimate the influences of different aforementioned parameters on the chemical evolution of NO, NO2 and O3. Detailed analyses of vertical profiles of pollutant concentrations showed that different diurnal heating scenarios could substantially affect the reactive gases exchange between the street canyon and air aloft, followed by respective dispersion and reaction. Higher building aspect ratio and stronger ambient wind speed were revealed to be, in general, responsible for enhanced entrainment of O3 concentrations into the street canyons along windward walls under all diurnal heating scenarios. Comparatively, particular attention can be paid on the windward wall heating and nighttime uniform surface heating scenarios.

Large-eddy simulation of reactive pollutant dispersion for the spatial instability of photostationary state over idealised 2D urban street canyons

Dispersion of chemically reactive pollutants in the urban canopy layer (UCL) over twelve idealised two-dimensional (2D) street canyons of unity aspect ratio in isothermal conditions is examined. The reversible NO x -O 3 mechanism is integrated into a large-eddy simulation (LES) model. The ground-level NO emission in the first street canyon and the background O 3 in the prevailing wind initiate the NO 2 production. The chemical equilibrium is measured by the photostationary state (PSS) which is a function of the time scales of turbulent mixing and chemical reactions. PSS of the first street canyon increases with increasing NO emission. For small amount of ground-level NO emission and background O 3 in the prevailing flows, (say NO/O 3 = 1/1), PSS increases gradually from the second to last street canyons. It increases faster if the NO-to-O 3 ratio is raised, say 1,000/30. Further increasing the NO-to-O 3 ratio, say 10,000/1, PSS exhibits a non-linear behaviour in which a trough is observed in the fifth street canyon.

A CFD modeling study of the impacts of NOx and VOC emissions on reactive pollutant dispersion in and above a street canyon

Abstract A computational fluid dynamics (CFD) model that includes the carbon bond mechanism IV (CBM-IV) is developed and used to investigate reactive pollutant dispersion in and above a street canyon with an aspect ratio of 1. Fourteen emission scenarios of NOx and volatile organic compounds (VOCs) are considered. Dispersion types are classified into NO-type, NO2-type, and O3-type dispersion that exhibit concentration maxima at the street bottom, near the center of the street canyon, and above the street canyon, respectively. For the base emission scenario, the number of reactive species is 9 in the NO-type dispersion, 10 in the NO2-type dispersion, and 15 in the O3-type dispersion. As the NOx emission level decreases or the VOC emission level increases, some species in the O3-type dispersion are shifted to the NO2-type dispersion. The VOC-to-NOx emission ratio is found to be an important factor in determining the transition of dispersion type. In this transition process, OH plays a key role through a radical chain including HO2, RO, and RO2. Because of their high OH reactivities, XYL (xylene) and OLE (olefin carbon bond) among VOCs are largely responsible for the transition of dispersion type. The O3 sensitivity is examined by reducing NOx or VOC emission level by a half. Because the NO titration of O3 is more pronounced than the NO2 photolysis and the radical chain process in the street canyon, the O3 concentration therein is negatively correlated with the NOx emission level and weakly correlated with the VOC emission level. As a result, the street canyon is a negatively NOx-sensitive regime.

Large eddy simulation of fire-induced buoyancy driven plume dispersion in an urban street canyon under perpendicular wind flow

The dispersion of fire-induced buoyancy driven plume in and above an idealized street canyon of 18 m (width) × 18 m (height) × 40 m (length) with a wind flow perpendicular to its axis was investigated by Fire Dynamics Simulator (FDS), Large Eddy Simulation (LES). Former studies, such as that by Oka [T.R. Oke, Street design and urban canopy layer climate, Energy Build. 11 (1988) 103–113], Gayev and Savory [Y.A. Gayev, E. Savory, Influence of street obstructions on flow processes within street canyons. J. Wind Eng. Ind. Aerodyn. 82 (1999) 89–103], Xie et al. [S. Xie, Y. Zhang, L. Qi, X. Tang, Spatial distribution of traffic-related pollutant concentrations in street canyons. Atmos. Environ. 37 (2003) 3213–3224], Baker et al. [J. Baker, H. L. Walker, X. M. Cai, A study of the dispersion and transport of reactive pollutants in and above street canyons—a large eddy simulation, Atmos. Environ. 38 (2004) 6883–6892] and Baik et al. [J.-J. Baik, Y.-S. Kang, J.-J. Kim, Modeling reactive pollutant dispersion in an urban street canyon, Atmos. Environ. 41 (2007) 934–949], focus on the flow pattern and pollutant dispersion in the street canyon with no buoyancy effect. Results showed that with the increase of the wind flow velocity, the dispersion pattern of a buoyant plume fell into four regimes. When the wind flow velocity increased up to a certain critical level, the buoyancy driven upward rising plume was re-entrained back into the street canyon. This is a dangerous situation as the harmful fire smoke will accumulate to pollute the environment and thus threaten the safety of the people in the street canyon. This critical re-entrainment wind velocity, as an important parameter to be concerned, was further revealed to increase asymptotically with the heat/buoyancy release rate of the fire.

Simulations of the dispersion of reactive pollutants in a street canyon, considering different chemical mechanisms and micromixing

The Stochastic Fields (SF) or Field Monte Carlo method has been used to model the dispersion of reactive scalars in a street canyon, using a simple chemistry and the CBM-IV mechanism. SF is a Probability Density Function (PDF) method which allows both means and variances of the scalars to be calculated as well as considering the effect of segregation on reaction rates. It was found that the variance of reactive scalars such as NO 2 was very high in the mixing region at roof-top level with rms values of the order of the mean values. The effect of segregation on major species such as O 3 was found to be very small using either mechanism, however, some radical species in CBM-IV showed a significant difference. These were found to be the seven species with the fastest chemical timescales. The calculated photostationary state defect was also found to be in error when segregation is neglected.

Further studies of flow and reactive pollutant dispersion in a street canyon with bottom heating

This study numerically investigates how flow and reactive pollutant dispersion in a street canyon with a canyon aspect ratio of one vary with the street-bottom heating intensity. For this, numerical simulations are performed over a wide range of street-bottom heating intensities (Δ T=0–15 °C in 1 °C intervals) using a Reynolds-averaged Navier–Stokes equations (RANS) model with NO–NO 2–O 3 photochemistry. The pollutants NO and NO 2 are emitted from near the street bottom in the presence of background O 3. A primary vortex is formed in the street canyon in all the simulated cases, but the location of the vortex center at the cross-canyon plane becomes quasi-stationary or meanders in the street canyon, depending on the street-bottom heating intensity. The time series of the street canyon-averaged NO concentration at the cross-canyon plane exhibits a quasi-steady, oscillatory, or fluctuating pattern, depending on the street-bottom heating intensity. As the street-bottom heating intensity increases, the averaged NO and NO 2 concentrations tend to decrease and the magnitude of the roof-level area-integrated and time-averaged vertical mean (turbulent) flux between Δ T=2 and 13 °C tends to increase (decrease). Some peculiar features are found in the cases of Δ T=11, 12, and 13 °C in which there are strong downward motion near the downwind building wall, downward motion below the roof level near the upwind building wall, and strong reverse flow in the lower region of the street canyon. Moreover, the vortex center is shifted toward the upwind building wall and does not meander in the street canyon after a certain period of time. Following the downward motions below the roof level, the O 3-containing ambient air is considerably entrained into the street canyon, resulting in a large increase in the O 3 concentration in the street canyon. In the cases of Δ T=11, 12, and 13 °C, the magnitude of the vertical mean flux is larger than that of the vertical turbulent flux. The direct effect of the inhomogeneous temperature distribution on the O 3 concentration via the temperature-dependent photolysis rate and reaction rate constant increases with the street-bottom heating intensity, although the averaged fractional difference in O 3 concentrations is small.

Modeling reactive pollutant dispersion in an urban street canyon

Reactive pollutant dispersion in an urban street canyon with a street aspect ratio of one is numerically investigated using a computational fluid dynamics (CFD) model. The CFD model developed is a Reynolds-averaged Navier–Stokes equations (RANS) model with the renormalization group (RNG) k–ε turbulence model and includes transport equations for NO, NO 2, and O 3 with simple photochemistry. An area emission source of NO and NO 2 is considered in the presence of background O 3 and street bottom heating (Δ T=5 °C) with an ambient wind perpendicular to the along-canyon direction. A primary vortex is formed in the street canyon and the line connecting the centers of cross-sectional vortices meanders over time and in the canyon space. The cross-canyon-averaged temperature and reactive pollutant concentrations oscillate with a period of about 15 min. The averaged temperature is found to be in phase with NO and NO 2 concentrations but out of phase with O 3 concentration. The photostationary state defect is small in the street canyon except for near the roof level and the upper downwind region of the canyon and its local minimum is observed near the center of the primary vortex. The budget analysis of NO (NO 2) concentration shows that the magnitude of the advection or turbulent diffusion term is much larger (larger) than that of the chemical reaction term and that the advection term is largely balanced by the turbulent diffusion term. On the other hand, the budget analysis of O 3 concentration shows that the magnitude of the chemical reaction term is comparable to that of the advection or turbulent diffusion term. The inhomogeneous temperature distribution itself affects O 3 concentration to some extent due to the temperature-dependent photolysis rate and reaction rate constant.

Recent progress in CFD modelling of wind field and pollutant transport in street canyons

With rapid development in computer hardware and numerical algorithms, computational fluid dynamics (CFD) techniques are widely utilized to study the wind field and pollutant transport in urban street canyons. The recent advancements and achievements in street-canyon pollution research using mathematical modelling approaches are reviewed in this paper. The standard, renormalized-group (RNG), and realizable k – ε turbulence closure schemes are the most commonly used Reynolds-averaged Navier–Stokes (RANS) models in street-canyon flow research, including the studies on the effects of street-canyon aspect ratio, building configuration, ambient wind direction, inflow turbulence intensities, vehicle-induced turbulence, and thermal stratifications. Another approach to turbulence simulation inside street canyons is large-eddy simulation (LES) which can handle a broad range of turbulent motions in a transient manner. These two approaches have their merits and the choice between them will be a compromise between accuracy and cost. Several guidelines on this choice as well as some comments on the 2D and 3D CFD simulations are given. The outputs from wind field models can be used with pollutant transport models to calculate the pollutant distribution inside street canyons. The most commonly employed pollutant transport models include Lagrangian, Eulerian, and Lagrangian/Eulerian hybrid models. The advantages and shortcomings of these models are summarized. Several other modelling concerns, such as chemically reactive pollutant dispersion and boundary conditions treatment, are also discussed.

The conceptual development of a simple scale-adaptive reactive pollutant dispersion model

The paper develops the basis for a self-consistent, operationally useful, reactive pollutant dispersion model, for application in urban environments. The model addresses the multi-scale nature of the physical and chemical processes and the interaction between the different scales. The methodology builds on existing techniques of source apportionment in pollutant dispersion and on reduction techniques of detailed chemical mechanisms.

A study of the dispersion and transport of reactive pollutants in and above street canyons—a large eddy simulation

Abstract This study extends a large eddy simulation model used to simulate turbulent flow in an urban street canyon to the dispersion and transport of reactive pollutants (NO, NO2 and O3). The original model, based on the regional atmospheric modelling system, has previously been validated against wind tunnel experiments for wind flow and turbulence. A real scale street canyon with an aspect ratio of one was studied with a neutrally stratified atmosphere and initial wind perpendicular to the street axis. After initialising and running the model to generate a stationary turbulent flow a constant discharge source of NOx was specified to imitate steady traffic flow through the street canyon. Line and area emissions sources for low and medium emissions of NOx were considered. A background concentration of ozone was specified and the gases were allowed to mix and react. Spatial variation of pollutants within the canyon was calculated to be significant. The photostationary state defect, ( δ ps = k [ NO ] [ O 3 ] / J NO 2 [ NO 2 ] - 1 ) , was found to be a sensitive indicator of reactive mixing within the canyon, the greater the defect the greater the local state of chemical instability. Low values for δ ps were found at the centre of the street canyon, within the primary vortex, and at the ground level corner of the windward wall, where a secondary eddy was predicted. High values were found above ground level along the windward wall, where air was entrained into the canyon along the outer edge of the primary vortex, and downstream of, and close to the NOx emission. Above the canyon a region of high δ ps corresponds to the edge of the escaping canyon plume.

Assessing the impact of the New Athens airport to urban air quality with contemporary air pollution models

The new airport of Athens will be constructed in the Spata area to the east of the Athens basin. In an attempt to study how the airport operation influences air quality in Athens, the wind flow and pollutant transport in the Athens basin and the Spata area are studied by applying a set of contemporary models, all constituents of the EUMAC Zooming Model (EZM): (1) the nonhydrostatic prognostic mesoscale model MEMO for simulating air flow and the dispersion of inert pollutants, and (2) two photochemical dispersion models, the three-dimensional model MARS and the three-layer model MUSE for describing the dispersion of reactive pollutants. Simulations were performed for meteorological conditions favouring the occurrence of air pollution episodes. Emphasis is put on the influence of the airport emissions on air quality assuming that the airport is operating either at its old location (Hellenikon) or at Spata. Comparison of simulation results for one selected scenario achieved with all three models reveals similar diurnal variations of nitrogen oxides in the Athens basin and the Spata area. The model results show that under conditions favoring air mass penetration from Athens to the Spata area the resulting pollutant transport causes an increase in air pollution levels without, however, leading to the exceedance of air quality standards. In the opposite case, the pollutant transport cannot have a noticeable adverse influence on the Athens air quality because of both the relatively high urban air pollution levels and the fact that the penetration depth is small.

Einfluβ chemischer reaktionen auf die turbulente dispersion von schmutzstoffen in der atmosphäre

The farfield dispersion of reactive pollutants in the atmosphere is treated from the viewpoint of chemical engineering. The ratio (“influence factor”) formed of the average densities of a pollutant for the cases with and without chemical reactions is determined as a thermodynamic function of average temperature and average density. Two chemical engineering applications (combustion of exhaust gas in the atmosphere, afterburning device for smelling organic pollutants) illustrate the approach to the problem of pollution abatement presented in this paper.