Vertical variations of particle number concentration and size distribution in a street canyon in Shanghai, China

Measurements of particle number size distribution in the range of 10-487 nm were made at four heights on one side of an asymmetric street canyon on Beijing East Road in Shanghai, China. The result showed that the number size distributions were bimodal or trimodal and lognormal in form. Within a certain height from 1.5 to 20 m, the particle size distributions significantly changed with increasing height. The particle number concentrations in the nucleation mode and in the Aitken mode significantly dropped, and the peaking diameter in the Aitken mode shifted to larger sizes. The variations of the particle number size distributions in the accumulation mode were less significant than those in the nucleation and Aitken modes. The particle number size distributions slightly changed with increasing height ranging from 20 to 38 m. The particle number concentrations in the street canyon showed a stronger association with the pre-existing particle concentrations and the intensity of the solar radiation when the traffic flow was stable. The particle number concentrations were observed higher in Test I than in Test II, probably because the small pre-existing particle concentrations and the intense solar radiation promoted the formation of new particles. The pollutant concentrations in the street canyon showed a stronger association with wind speed and direction. For example, the concentrations of total particle surface area, total particle volume, PM2.5 and CO were lower in Test I (high wind speed and step-up canyon) than in Test II (low wind speed and wind blowing parallel to the canyon). The equations for the normalized concentration curves of the total particle number, CO and PM2.5 in Test I and Test II were derived. A power functions was found to be a good estimator for predicting the concentrations of total particle number, CO and PM2.5 at different heights. The decay rates of PM2.5 and CO concentrations were lower in Test I than in Test II. However, the decay rate of the total particle number concentration in Test I was similar to that in Test II. No matter how the wind direction changed, for example, in the step-up case or wind blowing parallel to the canyon, the decay rates of the total particle number concentration were larger than those of PM2.5 and CO concentrations. For example, CO concentrations decreased by 0.33 and 0.69 at the heights ranging from 1.5 to 38 m in Test I and Test II, while the total particle number concentrations decreased by 0.72 and 0.85 within the same height ranges in Test I and Test II. It is concluded that the coagulation process, besides the dilution process, affected the total particle number concentration.     

Dispersion in a street canyon for a wind direction parallel to the street axis

We investigate pollutant dispersion in a street canyon for an external wind direction parallel to the street axis, a case which has been poorly documented in the literature. The study is performed numerically and analytically by means of a model based on a series of simplifying assumptions. The range of validity of these assumptions is discussed by comparing analytical and numerical results for two different street aspect ratios. Our results show that, for a critical length of the street, ground level concentration can be higher than those observed in a street canyon whose axis is perpendicular to the external wind direction. We show that this critical length depends on the street aspect ratio.     

Numerical analysis of the street canyon thermal conductance to improve urban design and climate

Built environment is increasingly dependent on the scientific knowledge which integrates urban design and climate. In the work presented here, the canyon thermal conductance which quantifies the heat transported outside of a canyon street, is analyzed to improve on understanding of how to accomplish this integration. A two-dimensional, steady, k$k$–ε$\epsilon$ turbulence model is used to study the influence of a windward heated wall on the air flow circulation in a street canyon with building height-to-street width ratio (aspect ratio) from 0.7 to 1.5. The numerical results presented here suggest that the air flow regime is strongly affected by buoyancy and three configurations are predicted: (I) and (II) with high Froude numbers (≈10¹$\approx {10}^1$) result in one or two stable counter-rotating vortices, with an intenser upper vortex; air flow regime (III), with low Froude numbers (≈10^-1$\approx {10}^{-1}$), is dominated by the lower vortex whose intensity is enhanced by a strong upward current close to the heated surface confining the upper vortex to a strict leeward zone of the canyon. Transitional Froude numbers are found as a function of canyon aspect ratio for transitions between regimes. The relevance of the results for urban design are quantified and analyzed in terms of canyon thermal conductance. The main conclusion is that, for one vortex skimming air flow regime, the canyon thermal conductance linearly increases with wind intensity, being larger streets more exposed to thermal losses. Multiple vortices in the air flow regime significantly decrease the canyon thermal conductance and, therefore, narrow streets provide protection from heat losses on windy and cloudy days and nights.     

CFD modeling of pollution dispersion in a street canyon: Comparison between LES and RANS

CFD modeling using RANS and LES of pollutant dispersion in a three-dimensional street canyon is investigated by comparison with measurements. The purpose of this study is to confirm the accuracy of LES in modeling plume dispersion in a simple street canyon model and to clarify the mechanism of the discrepancy in relation to RANS computation. Simple LES modeling is shown by comparison with wind tunnel experiments to give better results than conventional RANS computation (RNG) modeling of the distribution of mean concentration. The horizontal diffusion of concentration is well reproduced by LES, mainly due to the reproduction of unsteady concentration fluctuations in the street canyon.     

Experimental study of temperature and airflow distribution inside an urban street canyon during hot summer weather conditions. Part II: Airflow analysis

This paper presents the results of an urban measurement campaign performed in a street canyon in Athens, Greece. A number of field experimental procedures were organized during hot weather conditions, on a 24-h basis for five consecutive days during July 2002. Wind velocity measurements were conducted inside and outside the street canyon together with air and surface temperature measurements. Based on the results of air and surface temperature measurements, a further analysis is performed for the investigation of airflow inside the canyon when the ambient flow is parallel, perpendicular and oblique relative to the long canyon axis. The observed airflow characteristics are associated with the impact of thermal effects mainly induced from ground heating due to the incident solar radiation. However, the role of the finite length canyon effects related to wind circulation near street intersections, on the observed airflow patterns, is also identified.     

Effects of building aspect ratio and wind speed on air temperatures in urban-like street canyons

The objective of this study is to simulate the characteristic role of building aspect ratio (AR) and wind speed on air temperatures during different street canyon heating situations. A two-dimensional Renormalization Group (RNG) k–? turbulence model is employed to solve the Reynolds-averaged Navier–Stokes (RANS) and energy transport equations. A comparison of the results from the adopted model with those reported by similar experimental and numerical works demonstrated that the model is quite reliable when simulating temperature and wind profiles. The model is employed to predict air temperatures in idealized street canyons of aspect ratios (building-height-to-street-width ratio) of 0.5–8 with ambient wind speeds of 0.5–4 m/s. Three situations were identified for simulating diurnal heating of street canyon. It is noted that air temperatures are positively correlated with the bulk Richardson number (R_b) in most of the cases. The results show that the air temperature difference between high and low AR street canyon (Δθ_AR) was the highest during the nighttime (i.e., around 7.5 K between AR8 and AR0.5), but low or even negative during the daytime. It is also found that air temperatures rose as high as 1.3 K when ambient wind speed decreased from 4 m/s to 0.5 m/s. It is also revealed that the Δθ_AR during different diurnal situations and the nighttime and daytime air temperature difference between urban and rural areas (Urban Heat Island, UHI) closely resemble one another. Conclusively, the results of this study have highlighted the importance of street canyon AR and wind speed on urban heating.     

Analysis of convective heat and mass transfer at the vertical walls of a street canyon

The combined effects of urbanization and global climate warming give rise to increased temperatures in urbanized areas, a phenomenon called the urban heat island effect. The higher air temperatures have a negative impact on the energy demand for cooling and on the comfort and health of the people residing in urban areas. One way to mitigate the excess heat in urban areas is to make use of evaporative cooling, for example from ponds, from surfaces wetted by wind-driven rain, or from vegetated surfaces. Therefore understanding the interaction between the urban microclimate and evaporation processes is of interest. In the current paper, a two-step methodology for the simulation of drying processes is proposed and subsequently applied to study the drying behavior of both vertical walls of a square-shaped street canyon. Simulation results reveal the importance of using location-dependent convective heat and mass transfer coefficients (CHTC and CMTC) to capture the spatially non-uniform drying behavior of the wall. Whether the CMTC is obtained via Computational Fluid Dynamics simulations or via a simplified method based on the Chilton-Colburn analogy only slightly affects the drying behavior. The potential for evaporative cooling of wet walls in a hot climate is demonstrated in a final example.     

Effect of wind direction and speed on the dispersion of nucleation and accumulation mode particles in an urban street canyon

There have been many studies concerning dispersion of gaseous pollutants from vehicles within street canyons; fewer address the dispersion of particulate matter, particularly particle number concentrations separated into the nucleation (10-30 nm or N10-30) or accumulation (30-300 nm or N30-300) modes either separately or together (N10-300). This study aimed to determine the effect of wind direction and speed on particle dispersion in the above size ranges. Particle number distributions (PNDs) and concentrations (PNCs) were measured in the 5-2738 nm range continuously (and in real-time) for 17 days between 7th and 23rd March 2007 in a regular (aspect ratio approximately unity) street canyon in Cambridge (UK), using a newly developed fast-response differential mobility spectrometer (sampling frequency 0.5 Hz), at 1.60 m above the road level. The PNCs in each size range, during all wind directions, were better described by a proposed two regime model (traffic-dependent and wind-dependent mixing) than by simply assuming that the PNC was inversely proportional to the wind speed or by fitting the data with a best-fit single power law. The critical cut-off wind speed (Ur,crit) for each size range of particles, distinguishing the boundary between these mixing regimes was also investigated. In the traffic-dependent PNC region (UrUrUr,critUr,crit), concentrations were inversely proportional to Ur irrespective of any particle size range and wind directions. The wind speed demarcating the two regimes (Ur,critUr,crit) was 1.23+/-0.55 m s(-1) for N10-300, (1.47+/-0.72 m s(-1)) for N10-30 but smaller (0.78+/-0.29 m s(-1)) for N30-300.     

Analysis and evaluation of soundscapes in public parks through interviews and measurement of noise

The purpose of this work was to investigate the sound environment of public parks using a soundscape study model that analyzes not only noise but also all the types of sound of a given area, as well as other environmental factors. To this end, acoustic measurements were made in the parks under study and interviews were held with their frequent visitors. Noise measurements were conducted in 55 points, and a total of 335 people were interviewed in the 4 parks studied. The parks selected for this study are located in areas very close to streets with intense vehicle flow, raising the hypothesis that this proximity impairs the acoustic comfort of their visitors. The findings confirm the strong influence of traffic noise on the soundscapes of the parks. Noise measurements showed that in all parks, between 50 and 100% of the points evaluated displayed sound levels above 55 dB(A), the level established by Curitiba's Municipal Law 10625 as the limit permitted for green areas during daytime. Other conditions in the parks' environments were also identified, which interfere jointly in the soundscape and in its perception, such as spatial factors of each park, the urban setting of its surroundings, and the sounds originating inside the parks.     

Numerical study on the effects of aspect ratio and orientation of an urban street canyon on outdoor thermal comfort in hot and dry climate

This paper discusses the contribution of street design, i.e. aspect ratio (or height-to-width ratio, H/W) and solar orientation, towards the development of a comfortable microclimate at street level for pedestrians. The investigation is carried out by using the three-dimensional numerical model ENVI-met, which simulates the microclimatic changes within urban environments in a high spatial and temporal resolution. Model calculations are run for a typical summer day in Ghardaia, Algeria (32.40°N, 3.80°E, 469 m a.s.l.), a region characterized by a hot and dry climate. Symmetrical urban canyons, with various height-to-width ratios (i.e. H/W=0.5, 1, 2 and 4) and different solar orientations (i.e. E–W, N–S, NE–SW and NW–SE), have been studied. Special emphasis is placed on a human bio-meteorological assessment of these microclimates by using the physiologically equivalent temperature (PET).     

Small-scale spatial variability of particle concentrations and traffic levels in Montreal: a pilot study

Little is known about the particulate exposure of populations living along major urban roads. The objective of this pilot study was to explore the small-scale spatial and temporal variability of the absorption coefficient of PM2.5 filters, as a surrogate for elemental carbon, in relation to levels of PM2.5, at residential sites with varying traffic densities in a large Canadian city. Concurrent 24-h measurements were performed at four residential sites during 7 weeks. A gradient existed across all four sites for the absorption coefficient of the filters (and NO2 levels). In contrast, the levels of PM2.5 were quite similar at all sites. The difference in the filter absorption coefficient of PM2.5 filters, between an urban background and a residential traffic site (with about 30000 vehicles/day), expressed as a percentage of the background site, was 40%. These results indicate that spatial variability in PM2.5 absorption coefficient can be observed with traffic intensity on a small scale within a North American city and suggests that regression modelling approaches similar to those used in European studies could be used to estimate exposure of the general population to traffic-related particles on a local scale in North America.     

The physicochemical characterisation of microscopic airborne particles in south Wales: a review of the locations and methodologies

As part of the NERC-URGENT thematic programme, research was undertaken into the physicochemistry and bioreactivity of microscopic airborne particulate matter in south Wales. This paper reviews the collecting and characterisation methods used in the research; some of the results obtained are shown as examples. Four main collecting locations were chosen: Cardiff (urban); Port Talbot (urban/industrial); Park Slip West coal opencast pit (industrial/rural); the Black Mountains (rural/background). Collections initially used a 30-l/min Negretti PM10 filter collection system, however in the later stages of the project increased use was made of a 1100-l/min impaction system (nicknamed the super-sucker). This latter device was developed at Harvard University USA, however was adapted and optimised at Cardiff University. Methods for the extraction of PM10 off polycarbonate filters and polyurethane substrates were developed, with particular attention being paid to minimise physical or chemical changes during the extraction, and the extracts being in an appropriate state for bioreactivity assessment. Physicochemical characterisation of the PM10 included the empirical measurement of shape and size using electron microscopy and semi-automated image analysis. The determinations of the water-soluble and -insoluble chemical components were undertaken by ion chromatography and inductive coupled plasma-mass spectrometry. The bioreactivity of south Wales airborne particles is not covered by this review.