Reactive Pollutant Dispersion Research Articles

Effect of buoyancy on dispersion of reactive pollutants in urban canyons

In this paper, we aim to investigate the interplay between chemistry, flow dynamics and temperature using high fidelity computational fluid dynamics (CFD) models in an urban environment. A detailed numerical model based on large eddy simulation (LES) is developed considering the temperature and buoyancy effect and the non-equilibrium chemical processes. The model is used to study flow and reaction inside experimental and real-size street canyons. Street canyons are chosen for this study, as they represent the smallest unit of urban environments where detailed flow simulations combined with chemical reactions can be performed with high numerical accuracy. The effect of thermal driven flow is studied in reacting and non-reacting conditions to understand the role of buoyancy in accurately modeling pollutant reaction and dispersion. It is shown that buoyancy has a significant effect on the dynamics of the flow, by altering the main vortex structure inside the canyon and by increasing turbulent kinetic energy. It is also found that the chemical reactions strongly affect final concentrations of pollutants, which indicates the potential need for implementation of more advanced chemical models in future work. The importance of correct boundary conditions to accurately predict pollutant concentrations are discussed. Finally, by comparing the LES results with experimental field measurements in a real street canyon, the limitation of using periodic boundary conditions, as it is commonly used in the literature, is discussed. Moreover, it is shown that implementation of a variable photolysis rate is likely needed.

Impacts of the Tree Canopy and Chemical Reactions on the Dispersion of Reactive Pollutants in Street Canyons

Traffic-related air pollution in street canyons can cause health problems for pedestrians. In order to clarify the behavior of reactive pollutants, such as NOx and O3, in street canyons, a computational fluid dynamics (CFD) model coupled with a chemistry model and tree canopy model was developed, and then, a set of numerical experiments were performed to investigate the impacts of chemical reactions and aerodynamic effects of trees planted in a canyon. The results were compared with the observation data. Through the results of the numerical experiments designed to simulate a realistic urban street canyon, it was found that chemical reactions have a dominant impact on the NO/NO2 ratio and O3 concentration. While the tree canopy had little impact on the NO/NO2 ratio, it had a moderate impact on the flow field in the canyon and the amount of NOx and O3 in the canyon. In accordance with the aerodynamic effects of tree canopies, the local NOx concentration in the experiments increased and decreased by up to 51% and 11%, respectively. The current findings of this study demonstrate the utility of the proposed model for conducting air quality investigations in urban areas.

The influence of solar natural heating and NOx-O3 photochemistry on flow and reactive pollutant exposure in 2D street canyons

This study incorporates solar radiation model and NOx-O3 photochemistry into computational fluid dynamics (CFD) simulations with the standard k-ε model to quantify the integrated impacts of turbulent mixing, solar heating and chemical processes on vehicular passive (CO) and reactive (NOx, O3) pollutant dispersion within two-dimensional (2D) street canyons. Various street aspect ratios (H/W = 1, 3, 5) and solar-radiative scenarios (LST 0900, 1200, 1500) are considered. The initial source ratio of NO2 to NO is 1:10 and the background O3 concentration is 100 ppb (mole fraction). The reference Reynolds numbers are ~106–107 and Froude number ranges from 0.23 to 1.14. Personal intake fraction (P_IF) and its spatially-averaged values at the leeward-side (⟨P_IF⟩lee), windward-side (⟨P_IF⟩wind) and both street sides (⟨P_IF⟩) are adopted to evaluate pollutant exposure in near-road buildings.As H/W = 1 and 3, the clockwise single vortex is formed under neutral condition. Leeward/ground solar heating at LST 0900/1200 slightly enhance such vortex and reduce ⟨P_IF⟩. However, as H/W = 3, the single dominant vortex is separated into two counter-rotating vortices by windward solar heating at LST 1500, thus this ⟨P_IF⟩wind is significantly larger than the neutral case. As H/W = 5, the lower-level secondary anticlockwise vortex appears under neutral condition inducing much weaker wind and extremely higher pedestrian-level concentration. This two-main-vortex structure is destroyed by leeward/ground heating into single-main-vortex pattern, but dissociates into three counter-rotating vortices by windward heating. These three radiative scenarios raise pedestrian-level velocity in neutral case by about two orders, and reduce overall ⟨P_IF⟩ by two times to one order. For all cases, NO2 exposure is generally about 40%–380% larger than passive CO exposure, which indicates the conversion of NO into NO2 by depleting O3 is dominant in present NOx-O3 titration interactions. Finally, solar heating only raises air temperature by up to 2–3 K and influences chemical rate slightly, thus this impact on reactive pollutant dispersion is less significant than its effect by the enhanced turbulent mixing.

Numerical studies of passive and reactive pollutant dispersion in high-density urban models with various building densities and height variations

Vehicular pollutant exposure in the near-road buildings of high-density urban areas has been rarely studied. This paper investigates the impacts of high-density building morphology on passive (CO) and reactive (NOx-O3) pollutant dispersion in three-dimensional (3D) urban-like models, e.g. medium (street width W = 30 m,λp = 0.25, λf = 0.6) and compact (W = 18 m,λp = 0.39,λf = 0.94) layouts with uniform (H = 72 m) or various heights (H1 = 48 m,H2 = 96 m). Personal intake fraction P_iF and its spatially-averaged value for the entire building wall (<P_iF>W) are utilized for exposure assessment.Some meaningful findings are proposed: 1) When wind blows through neighborhood-scale (~1 km) urban models, it decelerates rapidly and then reaches a flow balance where pedestrian-level velocity and pollutant exposure vary depending on building densities and height variations. 2) Uniform-height cases attain overall <P_iF>W of ~0.3–1.3 ppm in target streets. The compact layout (λf = 0.94) experiences weaker vertical exchange where downward helical flows transport more pollutants to windward wall than medium-type (λf = 0.6), inducing two-order larger windward-side <P_iF>W (~1 ppm) than leeward-side (~0.01 ppm). 3) Building height variations obtain greater velocity and smaller <P_iF>W (~0.2–1.0 ppm) in front of taller buildings than those behind them (<P_iF>W~2.6–4.2 ppm). 4) No matter with or without height variations, densifying building arrays (λf = 0.6 to 0.94) raises windward-side <P_iF>W and mitigates leeward-side <P_iF>W, but overall <P_iF>W for both sides of target streets only change slightly. 5) With present NOx-O3 photochemistry (source emissions NO:NO2 = 10:1, background [O3] = 20 ppbv), NO2 exposure is 40%–230% larger than passive CO exposure, but NO is near to CO. This study implies the near-road pollutant exposure in high-density urban models is sensitive to building morphology which should be carefully designed.

Numerical investigations of reactive pollutant dispersion and personal exposure in 3D urban-like models

With satisfactory validation by experimental data, we perform computational fluid dynamic(CFD) simulations with the standard k-ε model to investigate how NO–NO2–O3 photochemistry and turbulent mixing influence reactive pollutant dispersion and vehicular NOx exposure in 21-row(neighborhood-scale~1 km) three-dimensional(3D) medium-dense urban models with an approaching wind parallel(perpendicular) to the main(secondary) streets. Personal intake fraction P_iF and its spatially-averaged values for the entire building (i.e. building intake fraction <P_iF>B) are adopted for reactive/passive exposure analysis with/without NOx-O3-photochemistry.Some meaningful findings are proposed: 1) There are flow adjustment processes coupling turbulent mixing and chemical reactions through urban areas (i.e. secondary Street 1 to 20). NO–NO2–O3 photochemistry induces O3 depletion and NO conversion into NO2 producing significant increase in NO2 exposure and slight decrease in NO exposure compared with passive dispersion. 2) With span-wise NOx sources, Street 10 in the fully-developed region experiences weaker wind and subsequently greater <P_iF>B(0.207 ppm) than Street 3(0.135 ppm) in the upstream flow-adjustment region. <P_iF>B descends exponentially from the target building toward downstream, and Street 10 experiences quicker decay rates. 3) With stream-wise NOx sources along the main street, <P_iF>B first ascends, then reaches equilibrium values (e.g.0.046–0.049 ppm). 4) If background O3 concentration [O3] rises from 20 ppbv to 40 and 100 ppbv, more NO is oxidized by O3 to generate NO2. As [O3] = 20 ppbv, if NO–NO2 emission ratio decreases from 10 to 5, NO2 exposure is partly offset but NO exposure change little. Present methodologies are confirmed effective to investigate impacts of more complicated meteorological conditions and chemical mechanisms on exposure in urban districts.

Integrated impacts of turbulent mixing and NOX-O3 photochemistry on reactive pollutant dispersion and intake fraction in shallow and deep street canyons

We employ computational fluid dynamics (CFD) simulations with NO-NO2-O3 chemistry to investigate the impacts of aspect ratios (H/W = 1,3,5), elevated-building design, wind catchers and two background ozone concentrations ([O3]b = 100/20 ppb) on reactive pollutant dispersion in two-dimensional (2D) street canyons. Personal intake fraction of NO2 (P_IFNO2) and its spatial mean value in entire street (i.e. street intake fraction <P_IFNO2>) are calculated to quantify pollutant exposure in near-road buildings. Chemical reaction contribution of NO2 exposure (CRC<P_IF>), O3 depletion rate (dozone) and photostationary state defect (δps) are used to analyze the interplay of turbulent and chemical processes.As H/W increases from 1, 3 to 5 with [O3]b = 100 ppb, the flow pattern turns from single-main-vortex structure to two-counter-rotating-vortex structure, and pedestrian-level velocity becomes 1–2 orders smaller. The high-dozone regions and low-|δps| regions get larger with more complete chemical reactions. Consequently, passive <P_IFNO2> rises 1 order (4.09–5.71 ppm to 41.76 ppm), but reactive <P_IFNO2> only increases several times (17.80–21.28 ppm to 58.50 ppm) and the contribution of chemistry (CRC<P_IF>) decreases with higher H/W. Thus, chemistry raises <P_IFNO2 > more effectively in shallow street canyons (H/W = 1–3). In deep street canyons (H/W = 5), elevated-building design and wind catchers destroy two-counter-rotating-vortex structure, improve street ventilation and reduce passive <P_IFNO2> by 2 and 1 orders (41.76 ppm to 0.38–5.16 ppm), however they only reduce reactive <P_IFNO2> by about 97.5% and 75% (58.50 ppm to 1.61–14.48 ppm). Such building techniques induce lower O3 depletion rate but greater chemical contribution. Finally, raising [O3]b from 20 to 100 ppb causes greater O3 depletion rate and chemical contribution and produces larger <P_IFNO2>. For deep street canyons, the impact of higher [O3]b on <P_IFNO2> is weaker than that in shallow street canyons, while it becomes stronger when fixing elevated-building design and wind catchers. This study provides some innovative findings on reactive pollutant exposure in 2D street canyons and offers effective CFD methodologies to evaluate pollutant exposure with more complicated chemistry and urban configurations.

Evaluation of an operational air quality model using large-eddy simulation

Abstract The large-eddy simulation (LES) model uDALES is used to evaluate the predictive skill of the operational air quality model SIRANE. The use of LES in this study presents a novel approach to air quality model evaluation, avoiding sources of uncertainty and providing numerical control that permits systematic analysis of targeted parametrisations and assumptions. A case study is conducted over South Kensington, London with the morphology, emissions, meteorological conditions and boundary conditions carefully matched in both models. The dispersion of both inert (NOx) and reactive (NO, NO2 and O3) pollutants under neutral, steady-state conditions is simulated for a south-westerly and westerly wind direction. A quantitative comparison between the two models is performed using statistical indices (the fractional bias, FB, the normalised mean squared error, NMSE, and the fraction in a factor of 2, FAC2). SIRANE is shown to successfully capture the dominant trends with respect to canyon-averaged concentrations of inert NOx (FB = −0.08, NMSE = 0.08 and FAC2 = 1.0). The prediction of along-canyon velocities is shown to exhibit sources of systematic error dependant on the angle of incidence of the mean wind (FB = −0.18). The assumption of photostationarity within SIRANE (deviations from equilibrium of up to 170% exist close to busy roads) is also identified as a significant source of systematic bias resulting in over- and underpredictions of NO2 (FB = −0.18) and O3 (FB = 0.14) respectively. The validity of the assumed uniform in-canyon concentration is assessed by analysing the pedestrian, leeward and windward concentrations resolved in uDALES. The use of canyon-averaged concentrations to predict pedestrian level exposure is shown to result in significant underestimations. Linear regression is used to effectively capture the relationship between pedestrian- and canyon-averaged concentrations in uDALES. Correction factors are derived ( m ≈ 1.62 and R 2 = 0.92 for inert NOx) that are capable of significantly improving SIRANE's ability to assess pedestrian level exposure for the conditions investigated in this study. The prevalence of intersections and advective nature of the shear layer are highlighted as important differences between modelling real heterogeneous urban morphology and idealised infinite canyons.

Large-Eddy Simulations of Reactive Pollutant Dispersion in the Convective Boundary Layer over Flat and Urban-Like Surfaces

Turbulent flow and reactive pollutant dispersion in the convective boundary layer (CBL) over flat and urban-like surfaces are investigated using a large-eddy simulation model with NO–NO2–O3 chemistry, with the urban-like surface represented by a block array. The CBL over a flat surface with and without ambient flow (FW and FNW cases, respectively) and the CBL over a block array with and without ambient flow (BW and BNW cases, respectively) are simulated. Wind shear in the entrainment zone increases the turbulence intensity and enhances the heat exchange in the entrainment zone. The urban-like surface induces greater wind shear in the entrainment zone, thus the largest turbulence intensity and heat exchange are found in the BW case. High NO concentration appears in updraft regions, whereas high O3 concentration appears in downdraft regions. The segregation of NO and O3 reduces the O3 decomposition in the CBL. The magnitude of the vertical gradients of NO, NO2, and O3 concentrations in the entrainment zone is smallest in the BW case, indicating that the largest reactive pollutant exchange occurs in the BW case. It seems that the greater wind shear in the entrainment zone induced by the urban-like surface also enhances the reactive pollutant exchange in the entrainment zone. The magnitude of the O3 production rate in the entrainment zone is large due to the mixing of mixed-layer air with air in the entrainment zone, especially around updraft regions. Since the segregation of NO and O3 interrupts the O3 decomposition, the turbulent component of the O3 production rate is generally positive in the CBL. The reduction of the O3 decomposition due to the segregation in the entrainment zone is smallest in the BW case. The effects of segregation on the chemical reactions are reduced due to the strengthened turbulent motions in the BW case.

Parameterisation study of chemically reactive pollutant dispersion over idealised urban areas based on the Gaussian plume model

Dispersion of pollutants emitted from vehicles over urban areas largely affects pedestrian-level air quality. In this study, turbulent dispersion of reactive pollutants in the ABL over hypothetical urban areas in the form of idealised street canyons is investigated using large-eddy simulation (LES). Nitric oxide (NO) is emitted from the first street canyon into the urban ABL doped with ozone (O3). First of all, we make use of the advection-diffusion equation with chemical kinetics to derive the theoretical relation between the dispersion coefficients of tracer and reactive pollutants. Next, the source depletion analogy is used to determine the plume shape instead of the conventional Gaussian plume model. Finally, regression to the LES output unveils that the vertical dimensionless NO concentration profiles exhibit self-similarity for a range of background O3 concentrations. A new parameterisation of reactive plume dispersion over urban areas, whose performance is remarkable over the mean plume rise, is thus suggested.

Parameterisation study of chemically reactive pollutant dispersion over idealised urban areas based on the Gaussian plume model

Dispersion of pollutants emitted from vehicles over urban areas largely affects pedestrian-level air quality. In this study, turbulent dispersion of reactive pollutants in the ABL over hypothetical urban areas in the form of idealised street canyons is investigated using large-eddy simulation (LES). Nitric oxide (NO) is emitted from the first street canyon into the urban ABL doped with ozone (O3). First of all, we make use of the advection-diffusion equation with chemical kinetics to derive the theoretical relation between the dispersion coefficients of tracer and reactive pollutants. Next, the source depletion analogy is used to determine the plume shape instead of the conventional Gaussian plume model. Finally, regression to the LES output unveils that the vertical dimensionless NO concentration profiles exhibit self-similarity for a range of background O3 concentrations. A new parameterisation of reactive plume dispersion over urban areas, whose performance is remarkable over the mean plume rise, is thus suggested.

Effects of Heat Intensity and Inflow Wind on the Reactive Pollution Dispersion in Urban Street Canyon

This paper investigates the impacts of heating intensity and inflow wind speed on the characteristics of reactive pollutant dispersion in street canyons using the computational fluid dynamic (CFD) model that includes the transportation of NO, NO2, and O3 coupled with NO-NO2-O3 photochemistry. The results indicated that the heat intensity and inflow wind speed have a significant influence on the flow field, temperature field and the characteristics of reactive pollutant dispersion in and above the street canyon. With the street canyon bottom heating intensity increasing, NO, NO2 and O3 concentrations in street canyon are decreased. The O3 concentration reductions are even more than the NO and NO2 concentrations. Improving the inflow wind speed can significantly reduce the NO and NO2 concentrations within street canyons. But the O3 concentrations have a slight rise with wind speed increasing. The results would be useful for understanding the interrelation among reactive vehicle emissions, and provide references for urban planners.

A CFD modelling study of reactive pollutant dispersion in an urban street canyon

This study numerically explores the diffusion rules of pollutants in an idealized urban street canyon where photochemical reactions are present in the neighbourhood. The airborne pollutants dispersion and reaction in street canyons are further investigated in this study. There is a relatively tense traffic pollutants release in urban neighborhoods, including nitrogen oxides and hydrocarbons. Photochemical reaction occurs at strong solar radiation, forming photochemical smog and other environment issues. We selected typical O3 + NO→O2 + NO2 reactions. Effects of the pollutant source location on pollutant dispersion and reaction are also investigated. Numerical simulations were conducted on chemically activity reactive air nitrogen oxides in urban street canyons with W (street canyon wide)/H (building height) = 1. Provided three different locations from the line source street canyon bottom exhaust nitric oxide diffusion and react with ozone in the free stream. The chemical reaction has a significant impact on the concentration of pollutants in the valley volume.

A numerical study of reactive pollutant dispersion in street canyons with green roofs

Roof greening is a new technique for improvement of outdoor thermal environment which influences air quality through its impacts on thermal and flow field. In order to examine effects of green roofs on reactive pollutant dispersion within urban street canyons, a computational fluid dynamics (CFD) model was employed which contained NO-NO2-O3 photochemistry and energy balance models. Simulations were performed for street canyons with different aspect ratios (H/W) of 0.5, 1.0, and 2.0 such that leaf area density (LAD) of green roofs changed. It was found that roof greening led distribution of pollutants to alter for H/W = 0.5 and 1.0 cases in such a manner that their averaged concentrations had small variations as LAD changed. However, by increasing LAD in H/W = 2.0, ventilation efficiency of nitrogen oxides increased since the flow was enhanced within the canyon. Additionally, averaged concentration of ozone in H/W = 2.0 increased with increasing LAD, owing to downward flux of ozone at roof level. Results show that roof greening is a good strategy which can be used in order to improve air quality and thermal environment, especially within deep street canyons.

Time scale analysis of chemically reactive pollutants over urban roughness in the atmospheric boundary layer

Most practical dispersion models assume inert pollutants but air pollutants are often chemically reactive. There is thus a need for including pollution chemistry in plume dispersion models. In this study, turbulent dispersion of reactive pollutants in the neutral atmospheric boundary layer (ABL) over hypothetical urban area in the form of an array of idealised street canyons is investigated using large-eddy simulation (LES). Nitric oxide (NO) is emitted from the first street canyon into the urban ABL, mixes with ozone (O3) and produces nitrogen dioxide (NO2) to model bimolecular chemical reaction. The time scales of NO oxidation and dispersion are compared in details. It is found that they are coupled with each other and so collectively modify the dispersion coefficient.

Simulations of photochemical smog formation in complex urban areas

In the present study we numerically investigated the dispersion of photochemical reactive pollutants in complex urban areas by applying an integrated Computational Fluid Dynamics (CFD) and Computational Reaction Dynamics (CRD) approach. To model chemical reactions involved in smog generation, the Generic Reaction Set (GRS) approach is used. The GRS model was selected since it does not require detailed modeling of a large set of reactive components. Smog formation is modeled first in the case of an intensive traffic emission, subjected to low to moderate wind conditions in an idealized two-dimensional street canyon with a building aspect ratio (height/width) of one. It is found that Reactive Organic Components (ROC) play an important role in the chemistry of smog formation. In contrast to the NOx/O3 photochemical steady state model that predicts a depletion of the (ground level) ozone, the GRS model predicts generation of ozone. Secondly, the effect of direct sunlight and shadow within the street canyon on the chemical reaction dynamics is investigated for three characteristic solar angles (morning, midday and afternoon). Large differences of up to one order of magnitude are found in the ozone production for different solar angles. As a proof of concept for real urban areas, the integrated CFD/CRD approach is applied for a real scale (1 × 1 km2) complex urban area (a district of the city of Rotterdam, The Netherlands) with high traffic emissions. The predicted pollutant concentration levels give realistic values that correspond to moderate to heavy smog. It is concluded that the integrated CFD/CRD method with the GRS model of chemical reactions is both accurate and numerically robust, and can be used for modeling of smog formation in complex urban areas.

CFD modeling of reactive pollutant dispersion in simplified urban configurations with different chemical mechanisms

Abstract. An accurate understanding of urban air quality requires considering a coupled behavior between the dispersion of reactive pollutants and atmospheric dynamics. Currently, urban air pollution is mostly dominated by traffic emission, where nitrogen oxides (NOx) and volatile organic compounds (VOCs) are the primary emitted pollutants. However, modeling reactive pollutants with a large set of chemical reactions, using a computational fluid dynamic (CFD) model, requires a large amount of computational (CPU) time. In this sense, the selection of the chemical reactions needed in different atmospheric conditions becomes essential in finding the best compromise between CPU time and accuracy. The purpose of this work is to assess the differences in NO and NO2 concentrations by considering three chemical approaches: (a) passive tracers (non-reactive), (b) the NOx–O3 photostationary state and (c) a reduced complex chemical mechanism based on 23 species and 25 reactions. The appraisal of the effects of chemical reactions focuses on studying the NO and NO2 dispersion in comparison with the tracer behavior within the street. In turn, the effect of including VOC reactions is also analyzed taking into account several VOC ∕ NOx ratios of traffic emission. Given that the NO and NO2 dispersion can also be affected by atmospheric conditions, such as wind flow or the background concentration from season-dependent pollutants, in this work the influence of wind speeds and background O3 concentrations are studied. The results show that the presence of ozone in the street plays an important role in NO and NO2 concentrations. Therefore, greater differences linked to the chemical approach used are found with higher O3 concentrations and faster wind speeds. This bears relation to the vertical flux as a function of ambient wind speed since it increases the pollutant exchange between the street and the overlying air. This detailed study allows one to ascertain under which atmospheric conditions the inclusion of chemical reactions are necessary for the study of NO and NO2 dispersion. The conclusions can be applied to future studies in order to establish the chemical reactions needed in terms of an accurate modeling of NO and NO2 dispersion and the CPU time required in a real urban area.

A numerical investigation of reactive air pollutant dispersion in urban street canyons with tree planting

Vegetation acts as a momentum and thermal sink, affecting the mixing of species and temperature-dependent constants of reaction rates. Numerical simulations were performed to investigate the effects of vegetation on the dispersion of reactive pollutants using a computational fluid dynamic (CFD) model coupled with NO-NO2-O3 photochemistry. Moreover, characteristics of temperature and flow fields were analyzed for different aspect ratios and leaf area densities. The results showed that flow is reversed in the presence of trees, and it enhances as leaf area density (LAD) increases; additionally, vegetation creates downward and vortex flows. The results also revealed that the dispersion of nitrogen oxides is influenced by the flow patterns; nevertheless, chemical reactions are significant for the dispersion of ozone. In addition, the vegetation is observed to weaken ventilation efficiency of NO and NO2; however, ventilation efficiency of O3 improves in LAD = 0.5 and 1.0. Aspect ratios and leaf area densities are also found to interact with each other; consequently, the optimum LAD is different for each aspect ratio. The larger regions with maximum concentrations of nitrogen oxides at the height of 2 m for aspect ratios of 0.5, 1.0, and 2.0 correspond to LAD = 2.0, 1.5, and 1.0, respectively. Furthermore, vegetation as compared to tree-free environment, mostly leads to a better chemical equilibrium.

Effects of building–roof cooling on the flow and dispersion of reactive pollutants in an idealized urban street canyon

This study investigated the characteristics of the flow and dispersion of reactive pollutants in three-dimensional idealized street canyons in the presence of building-roof cooling to provide implications in how a green-roof system contributes to mitigating of pedestrian exposure to near-roadway air pollution in street canyons. The pollutant chemistry was simulated using a coupled CFD-chemistry model. In the presence of building-roof cooling, winds and temperature fields inside the building canopy of the street canyon were significantly modified. Building-roof cooling, intensified the street-canyon vortex strength (up to 26.6% in downdraft, 10.4% reverse flow, and 7.7% in updraft). Building-roof cooling also decreased air temperature in the street canyon by supplying cooler air near the building roof. The changes in the in-canopy distributions of primary pollutants (NOX, VOCs and CO) due to building-roof cooling were mainly caused by the modified mean flow rather than the chemical reactions. High concentrations of primary pollutants occurred near the upwind building because the reverse flows were dominant at street level, making this area the downwind region of emission sources. Ozone concentrations were lower than the background concentration near the ground, where NOX concentrations were high. Building-roof cooling decreased primary pollutant concentrations by approximately −2.4% compared to those under non-cooling conditions. By contrast, building-roof cooling increased O3 concentrations by about 1.1% by reducing NO concentrations in the street canyon compared to concentrations under non-cooling conditions.

SOLUÇÃO ANALÍTICA PARA UM MODELO DE DISPERSÃO DE POLUENTES COM REAÇÃO FOTOQUÍMICA NA CAMADA LIMITE ATMOSFÉRICA

This paper presents an analytical solution for three-dimensional advection-diffusion equation applied to the dispersion of reactive pollutants emitted into the Atmospheric Boundary Layer (PBL). Some substances when emitted in PBL suffer photochemical reactions, so a source term is included in the advection-diffusion equation to represent this reaction, turning the model more realistic. The pollutant concentration fields obtained by the proposed solution are compared with the mixing ratio data obtained by monitoring the air quality in the metropolitan region of Porto Alegre. With the analysis of the results can be seen that the inclusion of the term proposed to represent a photochemical reaction of a reactive pollutant, a is a new proposal for the prediction of pollutant concentration in PBL.

Characteristics of flow and reactive pollutant dispersion in urban street canyons

In this study, the effects of aspect ratio defined as the ratio of building height to street width on the dispersion of reactive pollutants in street canyons were investigated using a coupled CFD-chemistry model. Flow characteristics for different aspect ratios were analyzed first. For each aspect ratio, six emission scenarios with different VOC–NOX ratios were considered. One vortex was generated when the aspect ratio was less than 1.6 (shallow street canyon). When the aspect ratio was greater than 1.6 (deep street canyon), two vortices were formed in the street canyons. Comparing to previous studies on two-dimensional street canyons, the vortex center is slanted toward the upwind building and reverse and downward flows are dominant in street canyons. Near the street bottom, there is a marked difference in flow pattern between in shallow and deep street canyons. Near the street bottom, reverse and downward flows are dominant in shallow street canyon and flow convergence exists near the center of the deep street canyons, which induces a large difference in the NOX and O3 dispersion patterns in the street canyons. NOX concentrations are high near the street bottom and decreases with height. The O3 concentrations are low at high NO concentrations near the street bottom because of NO titration. At a low VOC–NOX ratio, the NO concentrations are sufficiently high to destroy large amount of O3 by titration, resulting in an O3 concentration in the street canyon much lower than the background concentration. At high VOC–NOX ratios, a small amount of O3 is destroyed by NO titration in the lower layer of the street canyons. However, in the upper layer, O3 is formed through the photolysis of NO2 by VOC degradation reactions. As the aspect ratio increases, NOX (O3) concentrations averaged over the street canyons decrease (increase) in the shallow street canyons. This is because outward flow becomes strong and NOX flux toward the outsides of the street canyons increases, resulting in less NO titration. In the deep street canyons, outward flow becomes weak and outward NOX flux decreases, resulting in an increase (decrease) in NOX (O3) concentration.