cienceDirect

Atmospheric Environment 44 (2010) 2940–2941

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Atmospheric Environment

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New Directions: Passive control of personal air pollution exposurefrom traffic emissions in urban street canyons

The adverse health risks of personal exposure to air pollutionhave been widely reported and in the urban environment traffichas been shown to be responsible for the majority of air pollutionemissions. As is commonplace in many cities, where tall buildingssurround a busy roadway an air mass is partially enclosed withinits boundaries, restricting the dispersion of pollutants. Thisscenario defines an urban street canyon, personal exposure amongroad users in this micro-environment has been shown to beelevated and it has been estimated that 30% of the daily intake ofbenzene is acquired by spending 1 h travelling in a car in urbanareas (WHO, 2000).

As a result of this environmental health risk, numerous investi-gations have been carried out intomethods of reducing air pollutionfrom traffic comprising numerous different approaches to theproblem. These have included the investigation of policy tools toreduce emissions such as congestion charging and carbon taxes;transportmodelling to reduce congestion, vehicle technologydevel-opment to reduce emissions intensity such as catalytic convertersetc. However an alternative and complementary approach to theproblem of personal exposure among urban road users has recentlyemerged and is gaining research focus as an air quality remediationtool.

The passive control of air pollution emissions through themanipulation of natural pollutant dispersion patterns in urbanstreet canyons is being put forward as a viable option in the protec-tion of human health. In contrast to methods which aim to reduceair pollution emissions this technique aims to increase local disper-sion to reduce air pollution concentrations. Using low boundarywalls, trees, on-street parking, hedgerows, noise pollution barriersand other common urban features, investigators have highlightedthe ability of such obstacles to influence the dispersion of air pollu-tion from traffic in a typical street canyon. Investigations haverevealed that these obstacles may be positioned within an urbanstreet canyon in such a way as to increase the amount of dispersionwhich takes place between the tailpipe emission of air pollutantsand their inhalation by pedestrians/cyclists on the edge ofa roadway. Therefore these obstacles form passive controls on airpollution levels which could be incorporated into urban environ-mental policy as a further mechanism of human health protection.

Field experiments in Ireland demonstrated the impact of anexisting low boundary wall (LBW) in a busy street canyon, on thedispersion of vehicular emissions. The wall, located between theroad and footpath, acted as a baffle plate directing the flow ofpollutant laden air away from the footpath. The investigation foundconcentrations of carcinogenic VOCs (benzene and 1, 3 butadiene)to be 1.7–2.0 times lower on the footpath behind the LBW

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(McNabola et al., 2007). Concentrations of particulate matter(PM2.5) were 2.8 times lower. Further numerical investigations ofLBWs under various wind speed and direction conditions showedthat this form of passive control located between the road and foot-path or located in the centre of the roadway could reduce personalexposure to benzene on the footpath by 40–75% depending mainlyon the ambient wind direction (McNabola et al., 2009).

Various wind tunnel experiments and numerical investigationshave been carried out into the influence of tree planting on thedispersion of traffic emissions in street canyons. Investigationshave shown larger pollutant concentrations near the leeward walland lower concentrations near the windward wall as a result oftree planting in street canyons (Gross, 1997; Ries and Eichhorn,2001; Gromke et al., 2008; Buccolieri et al., 2009). In addition totheir influence on street canyon dispersion, numerous investiga-tions have shown that trees and other forms of vegetation act toinduce the deposition of particulate matter and heavy metals,such as lead, inhibiting the spread of air pollution (Heath et al.,1999; Bowker et al., 2007; Ottele et al., 2010).

On-street car parking systems have also been shown to act aspassive controls on air pollution, presenting a temporary barrierto the dispersion of air pollutants, operating in much the samemanner as a LBW. Parked cars however present a much widerand taller boundary than LBWs and as a result when on-street park-ing in a particular street canyon is full to capacity, greater reduc-tions in personal exposure on the footpath have been found thanthose achievable using LBWs. However, as the number of carsparked in spaces reduces so too does the effectiveness of on-streetparking as a passive control on air pollution.

Similarly research has been carried out into the effect of noisepollution barriers on air pollution dispersion. These have in effectbeen shown to be dual purpose in the urban environmentproviding reduction in noise and air pollution through differingmechanisms for those positioned behind the barrier (King et al.,2009; Finn et al., 2010). On open highways, noise barriers havebeen shown to reduce pollutants such as CO and PM number by15–50% (Baldauf et al., 2008). In addition, when combined withmature trees noise pollution barriers have been found to furtherimprove these reductions.

Furthermore, investigations have been carried out into the effectof roadway design on pollutant dispersion. Cut-section roads orroads on embankments have been shown to induce greater turbu-lence and pollutant mixing than roads which are at existing groundlevel (Baldauf et al., 2009). More complex pedestrian ventilationssystems have also beenproposed to ventilate footpathswith cleanerair via ducting from roof level (Mirzaei and Haghighat, 2010).

A. McNabola / Atmospheric Environment 44 (2010) 2940–2941 2941

In general terms passive controls can present a barrier betweentailpipes and pedestrians in a street canyon. Pollutants emitted froma tailpipe, where the air flow is being directed towards the footpath,must pass over/around the passive control increasing the amount ofdispersion that takes place before it reaches the footpath. Passivecontrols can also act to significantly alter air flow patterns withina street canyon whereby air pollution emissions are re-directedaway from the edge of the roadway resulting in very significantreductions in personal exposure. The main factors behind howa passive control operates are the wind direction with respect tothe street orientation and the geometry of the street.

It should be noted however, that under certain conditions somepassive controls have been shown to have either negative or negli-gible effects on dispersion. For example, under certain wind direc-tions air pollution levels behind noise barriers have been found toincrease (Baldauf et al., 2008). Increasing the height of a noisebarrier between a footpath and roadway was found to reduceconcentrations on the footpath but concentrations on the roadwayincreased significantly (King et al., 2009). Further research is there-fore required into the impacts of passive controls on air pollutionlevels on the roadway. In addition, care must be taken to positionpassive controls appropriately, taking into account the prevailingwind conditions and possible negative impacts.

In summary, research in this area is gathering momentum witha view to producing results which investigate the effectiveness ofthe various known options for the passive control of air pollutionin urban street canyons. Undoubtedly there are further passivecontrol options, viable in urban street canyons, which have yet tobe investigated in this manner. In addition, numerous aspects ofair pollution dispersion in street canyons have yet to be addressedby this research area such as: passive control at junctions; in asym-metric or realistic street canyons; real world demonstration ofpassive control in practice, to name a few. A unified approach isneeded in this area to focus efforts among the scientific communityto produce comprehensive guidelines on the implementation ofpassive controls for the myriad of varying geometric and meteoro-logical conditions in urban street canyons. The dissemination ofsuch guidelines could help to alter urban planning policy to incor-porate passive control of air pollution in future urban development,thus helping to improve environmental health worldwide.

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Aonghus McNabola*Department of Civil, Structural and Environmental Engineering,

Trinity College Dublin, Ireland* Tel.: þ353 896 3837; fax: þ353 1 677 3072.

E-mail address: amcnabol@tcd.ie