Street Canyon Dispersion Models Research Articles

The impact of measures to reduce ambient air PM<sub>10</sub> concentrations originating from road dust, evaluated for a street canyon in Helsinki

Abstract. We have numerically evaluated how effective selected potential measures would be for reducing the impact of road dust on ambient air particulate matter (PM10). The selected measures included a reduction of the use of studded tyres on light-duty vehicles and a reduction of the use of salt or sand for traction control. We have evaluated these measures for a street canyon located in central Helsinki for four years (2007–2009 and 2014). Air quality measurements were conducted in the street canyon for two years, 2009 and 2014. Two road dust emission models, NORTRIP (NOn-exhaust Road TRaffic Induced Particle emissions) and FORE (Forecasting Of Road dust Emissions), were applied in combination with the Operational Street Pollution Model (OSPM), a street canyon dispersion model, to compute the street increments of PM10 (i.e. the fraction of PM10 concentration originating from traffic emissions at the street level) within the street canyon. The predicted concentrations were compared with the air quality measurements. Both road dust emission models reproduced the seasonal variability of the PM10 concentrations fairly well but under-predicted the annual mean values. It was found that the largest reductions of concentrations could potentially be achieved by reducing the fraction of vehicles that use studded tyres. For instance, a 30 % decrease in the number of vehicles using studded tyres would result in an average decrease in the non-exhaust street increment of PM10 from 10 % to 22 %, depending on the model used and the year considered. Modelled contributions of traction sand and salt to the annual mean non-exhaust street increment of PM10 ranged from 4 % to 20 % for the traction sand and from 0.1 % to 4 % for the traction salt. The results presented here can be used to support the development of optimal strategies for reducing high springtime particulate matter concentrations originating from road dust.

Intercomparison of Atmospheric Dispersion Models Applied to an Urban Street Canyon of Irregular Geometry

The NOx concentrations measured at the sampling site of Cordoba Avenue, Buenos Aires City, characterised by an irregular geometry on both sides of the street, are used to intercompare results of five urban street canyon dispersion models, STREET, OSPM, AEOLIUSF, STREET-BOX and SEUS. Three different wind directions with respect to the street axis are considered, i.e., leeward, windward and parallel. Additionally, two wind speed classes are considered, above and below 2 m s–1. In order to evaluate the models performance, observed and calculated concentrations are compared using different statistical measures, i.e., mean values, bias, mean square error and fractional error. In general, all models estimate better leeward conditions and wind speeds above 2 m s–1, with SEUS providing the overall best result.

Scalar Fluxes Near a Tall Building in an Aligned Array of Rectangular Buildings

Scalar dispersion from ground-level sources in arrays of buildings is investigated using wind-tunnel measurements and large-eddy simulation (LES). An array of uniform-height buildings of equal dimensions and an array with an additional single tall building (wind tunnel) or a periodically repeated tall building (LES) are considered. The buildings in the array are aligned and form long streets. The sensitivity of the dispersion pattern to small changes in wind direction is demonstrated. Vertical scalar fluxes are decomposed into the advective and turbulent parts and the influences of wind direction and of the presence of the tall building on the scalar flux components are evaluated. In the uniform-height array turbulent scalar fluxes are dominant, whereas the tall building produces an increase of the magnitude of advective scalar fluxes that yields the largest component. The presence of the tall building causes either an increase or a decrease to the total vertical scalar flux depending on the position of the source with respect to the tall building. The results of the simulations can be used to develop parametrizations for street-canyon dispersion models and enhance their capabilities in areas with tall buildings.

Improving the modeling of road dust levels for Barcelona at urban scale and street level

Road dust emission is an emerging issue in air quality due to the lack of remediation measures in contrast to vehicle exhaust emissions. The evidence of receptor modeling studies allows for quantifying impact on a few receptors, but the high cost of PM chemical speciation data and the questionable representativeness of single monitoring sites, limit considerably the development of population exposure estimates and epidemiologic studies based on georeferenced data. This study attempts to initiate and promote urban-scale dispersion modeling for road dust emissions, which will allow for a more robust estimate of population exposure and health outcomes. The TNO URBIS (URBan Information System) model was applied in the city of Barcelona, implementing a Gaussian line source and a street canyon dispersion model, together with new experimental estimates of road dust emission factors and algorithm to describe the time variability. Annual, daily and hourly road dust contributions were simulated and validated against observation of PM10, mineral dust and hourly PM2.5-10 concentrations. Results show that road dust contributed 9-15% to PM10 levels at background sites, and 23-44% at traffic sites. Highest contributions were modeled in the commercial/residential district where most of population live and work (Eixample) structured by 120 m wide square blocks, separated by roads with >10,000 vehicles per day. Street level contributions rise up to 20 μg/m3 (96% of roads) and an additional 3% of roads within 20-40 μg/m3. Hourly simulations of road dust contributions revealed to benefit from the implementation of the new emission module (Amato et al., 2012), able to describe the exponential recovery of road dust emission potential after rain events, when compared to common approach such as the use of constant emission factor or an ON/OFF approach. Correlation coefficients with observed data varied from 0.61, 0.58 and 0.43 for annual, daily and hourly means, respectively, revealing a clear improvement in terms of both spatial and temporal variability. However, more efforts need to be done in validating the model in different climatic scenarios and evaluating the seasonal variation of road dust emissions, due to droughts or Saharan dust events. © 2015 Elsevier Ltd.

Monitoring and modelling carbon monoxide concentrations in a deep street canyon: application of a two-box model

Carbon monoxide concentrations were monitored at 3, 10 and 30 meters in a deep street canyon with an aspect ratio of (H/W)≈5.8 and were modelled using a two–box model developed in a previous CFD study. The monitoring campaign lasted 5 days, from 11th to 15th July, 2011. The turbulent kinetic energy at the rooftop level and traffic flow was also measured in the same period. Experimental data were used to evaluate parameters (mass transfer velocities and overall mass transfer velocity) of the box model. The daily pattern shows a significant increase of the overall mass transfer velocity from 9:00 to 11:00 and a decrease until 14:00. Turbulent kinetic energy measured at the rooftop level seems to play a major role with respect to wind velocity in determining the mass transfer between the canyon and the atmosphere above. The evaluation of the overall mass transfer velocity contributes to the use of operational street canyon dispersion models in the case of deep street canyons.

Evaluation of a road dust suspension model for predicting the concentrations of PM10 in a street canyon

We have slightly refined, evaluated and tested a mathematical model for predicting the vehicular suspension emissions of PM10. The model describes particulate matter generated by the wear of road pavement, traction sand, and the processes that control the suspension of road dust particles into the air. However, the model does not address the emissions from the wear of vehicle components. The performance of this suspension emission model has been evaluated in combination with the street canyon dispersion model OSPM. We used data from a measurement campaign that was conducted in the street canyon Runeberg Street in Helsinki from 8 January to 2 May, 2004. The model reproduced fairly well the seasonal variation of the PM10 concentrations, also during the time periods, when studded tyres and anti-skid treatments were commonly in use. For instance, the index of agreement (IA) was 0.83 for the time series of the hourly predicted and observed concentrations of PM10. The predictions of the model were found to be sensitive to precipitation and street traction sanding. The main uncertainties in the predictions are probably caused by (i) the cleaning processes of the streets, which are currently not included in the model, (ii) the uncertainties in the estimation of the sanding days, and (iii) the uncertainties in the evaluation of precipitation. This study provides more confidence that this model could potentially be a valuable tool of assessment to evaluate and forecast the suspension PM10 emissions worldwide. However, a further evaluation of the model is needed against other datasets in various vehicle fleet, speed and climatic conditions.

Using the OSPM model on pollutant dispersion in an urban street canyon

An observational campaign was conducted in the street canyon of Zhujiang Road in Nanjing city in 2007. Hourly mean concentrations of PM10 were measured at street and roof levels. The Operational Street Pollution Model (OSPM) street canyon dispersion model was used to calculate the street concentrations and the results were compared with the measurements. The results show that there is good agreement between measured and predicted concentrations. The correlation coefficient R 2 values (R 2 is a measure of the correlation of the predicted and measured time series of concentrations) are 0.5319, 0.8044, and 0.6630 for the scatter plots of PM10 corresponding to light wind speed conditions, higher wind speed conditions, and all wind speed conditions, respectively. PM10 concentrations tend to be smaller for the higher wind speed cases and decrease rapidly with increasing wind speed. The presentations of measured and modelled concentration dependence on wind direction show fairly good agreement. PM10 concentrations measured on the windward side are relatively smaller, compared with the corresponding results for the leeward side. This study demonstrates that it is possible to use the OSPM to model PM10 dispersion rules for an urban street canyon.

Intake fraction distributions for benzene from vehicles in the Helsinki metropolitan area

The intake fraction (iF) gives a measure of the portion of a source's emissions that is inhaled by an exposed population over a defined period of time. This study examines spatial and population-based iF distributions of a known human carcinogen, benzene, from a ubiquitous urban source, local vehicular traffic, in the Helsinki Metropolitan Area using three computational methods. The first method uses the EXPAND model (EXPosure to Air pollution, especially to Nitrogen Dioxide and particulate matter), which incorporates spatial and temporal information on population activity patterns as well as urban-scale and street canyon dispersion models to predict spatial population exposure distributions. The second method uses data from the personal monitoring study EXPOLIS (Air Pollution Exposure Distributions of Adult Urban Populations in Europe) to estimate the intake fractions for individuals in the study. The third method, a one-compartment box model provides estimates within an order-of-magnitude or better for non-reactive agents in an urban area. Population intake fractions are higher using the personal monitoring data method (median iF 30 per million, mean iF 39 per million) compared with the spatial model (annual mean iF 10 per million) and the box model (median iF 4 per million, mean iF 7 per million). In particular, this study presents detailed intake fraction distributions on several different levels (spatial, individual, and generic) for the same urban area.

Evaluation of the OSPM model combined with an urban background model against the data measured in 1997 in Runeberg Street, Helsinki

In 1997, a measuring campaign was conducted in a street canyon (Runeberg Street) in Helsinki. Hourly street level measurements and on-site electronic traffic counts were conducted throughout the whole of 1997; roof level measurements were conducted for approximately two months during the so-called intensive measuring campaign, from 3 March to 30 April 1997. Hourly mean concentrations of NO x , NO 2, O 3 and CO were measured at street and roof levels; the relevant hourly meteorological parameters were measured at roof level. We present here an evaluation of the Operational Street Pollution Model (OSPM) street canyon dispersion model against the data measured during the whole of 1997. As the roof level concentrations and meteorological measurements were not available for the whole year, we utilised computed or meteorologically pre-processed values. The use of modelled urban background concentrations and meteorological values (instead of on-site roof level measurements) did not lessen the agreement between modelled and measured average concentration values at street level. The agreement between the temporal variations of predictions and measured data was also fairly good; for instance, the corresponding index of agreement values for NO x , NO 2 and CO were 0.89, 0.81 and 0.87, respectively. However, as expected, the agreement in the temporal variations was somewhat better using actual measured on-site data during the intensive measuring campaign, than when using modelled urban background concentrations and meteorological values. This study demonstrates that it is possible to utilise the street canyon dispersion model OSPM with reasonable accuracy using modelled urban background and pre-processed meteorological values as model input.

Traffic performance-based air quality management

Traffic is a major strain on air quality in cities, as most of the hazardous emissions in urban areas are caused by it. Road traffic causes such emissions as COz, NO,, NOz, HC and CO. Road traffic induces also particles, of which especially small particles are extremely hazardous to health. Accurate modelling of traffic and traffic-generated pollution is important in estimating the exposure caused to people. Being able to correctly model traffic and its emissions helps in finding different traffic control strategies that could be useful in improving the situation in extensive air quality management systems. Granberg et a1 [l] presented a combined application of traffic microsimulation and street canyon dispersion models, which was tested on a case study in Helsinki. The purpose of the study presented here is to continue on the same field and to introduce the traffic-based pollution and concentration model used in Helsinki area as well to investigate the connection between urban transport and air quality. The study will also handle the exposure of people to pollution, and shed light on its consequences. The current air quality measurement systems used in Helsinki area will be introduced. Finally, some traffic control measures and their effect on emissions will be tested and presented.

High-resolution wind-tunnel datasets from the University of Hamburg: validation of street canyon dispersion models

Air quality is one of the major environmental concerns in cities as it has a direct impact on human health. One of the biggest sources of air pollution is urban traffic. To predict air quality in urban areas, significant effort and considerable funding has been, and is being, spent in developing numerical models for urban dispersion and in obtaining experimental datasets. So far, less effort has been expended on providing formalised procedures and reference data suitable for model validation. At the Meteorological Institute of the University of Hamburg, a project has been started to compile a database of wind-tunnel measurements, which can be used for validation of microscale dispersion models. Laser Doppler Anemometer measurements were made on several configurations of the building arrangements. Mean velocity, as well as the intensity of turbulence and the Reynolds stress, were obtained.

A measurement campaign in a street canyon in Helsinki and comparison of results with predictions of the OSPM model

In 1997, a measuring campaign was conducted in a street canyon (Runeberg St.) in Helsinki. Hourly mean concentrations of CO, NO x , NO 2 and O 3 were measured at street and roof levels, the latter in order to determine the urban background concentrations. The relevant hourly meteorological parameters were measured at roof level; these included wind speed and direction, temperature and solar radiation. Hourly street level measurements and on-site electronic traffic counts were conducted throughout the whole of 1997; roof level measurements were conducted for approximately two months, from 3 March to 30 April in 1997. CO and NO x emissions from traffic were computed using measured hourly traffic volumes and evaluated emission factors. The Operational Street Pollution Model (OSPM) was used to calculate the street concentrations and the results were compared with the measurements. The overall agreement between measured and predicted concentrations was good for CO and NO x (fractional bias were −4.2 and +4.5%, respectively), but the model overpredicted the measured NO 2 concentrations (fractional bias was +22%). The agreement between the measured and predicted values was also analysed in terms of its dependence on wind speed and direction; the latter analysis was performed separately for two categories of wind velocity. The model qualitatively reproduces the observed behaviour very well. The database, which contains all measured and predicted data, is available for further testing of other street canyon dispersion models. The dataset contains a larger proportion of low wind speed cases, compared with other available street canyon measurement datasets.

COMBINED APPLICATION OF TRAFFIC MICROSIMULATION AND STREET CANYON DISPERSION MODELS, AND EVALUATION OF THE MODELLING SYSTEM AGAINST MEASURED DATA

In this study, the traffic microsimulation model HUTSIM, used by Helsinki University of Technology, and a street canyon dispersion model OSPM were integrated. HUTSIM is designed specially for simulating signal controlled intersections and small areas consisting of several intersections. In 1997 a measuring campaign was conducted at a street canyon in Helsinki. The above mentioned location and time are considered in the study discussed in this paper, in order to be able to compare the simulation results with measured traffic flow and concentration values, as well as the corresponding previous computational results. The results of flow and emission computations obtained with the two modelling systems, involving traffic micro- and macrosimulation models, were first compared with each other. The predicted atmospheric pollutant concentrations were then compared with the measured data. The results were promising.