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Impact of roadside noise barriers on particle size distributions and pollutants concentrations near freeways Zhi Ning a , Neelakshi Hudda a , Nancy Daher a , Winnie Kam a , Jorn Herner b , Kathleen Kozawa b , Steven Mara b , Constantinos Sioutas a, * a Department of Civil and Environmental Engineering, University of Southern California, 3620 South Vermont Avenue, Los Angeles, CA 90089, USA b California Air Resources Board, Research Division, 1001 ‘‘I’’ Street, P.O. Box 2815, Sacramento, CA 95812, USA article info Article history: Received 19 March 2010 Received in revised form 15 May 2010 Accepted 17 May 2010 Keywords: Particle size distribution Pollutants concentrations Noise barrier Freeway emissions Roadway congurations abstract Increasing epidemiological evidence has established an association between a host of adverse health effects and exposure to ambient particulate matter (PM) and co-pollutants, especially those emitted from motor vehicles. Although PM and their co-pollutants dispersion proles near the open freeway have been extensively characterized by means of both experimental measurements and numerical simulations in recent years, such investigations near freeways with roadside barriers have not been well documented in the literature. A few previous studies suggested that the presence of roadside structures, such as noise barriers and vegetation, may impact the decay of pollutant concentrations downwind of the freeway by limiting the initial dispersion of trafc emissions and increasing their vertical mixing due to the upward deection of airow. Since the noise barriers are now common roadside features of the freeways, particularly those running through populated urban areas, it is pertinent to investigate the impact of their presence on the particles and co-pollutants concentrations in areas adjacent to busy roadways. This study investigated two highly trafcked freeways (I-710 and I-5) in Southern California, with two sampling sites for each freeway, one with and the other without the roadside noise barriers. Particle size distributions and co-pollutants concentrations were measured in the immediate proximity of freeways and at different distances downwind of the freeways. The results showed the formation of a concen- tration decitzone in the immediate vicinity of the freeway with the presence of roadside noise barrier, followed by a surge of pollutant concentrations further downwind at 80e100 m away from freeway. The particle and co-pollutants concentrations reach background levels at farther distances of 250e400 m compared to 150e200 m at the sites without roadside noise barriers. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction In recent years, several toxicological studies have reported the ability of particulate matter (PM) to generate reactive oxygen species in biological systems (Sagai et al., 2000; Donaldson et al., 2002; Xia et al., 2004) and these pro-oxidant species are intimately linked to the genesis of pulmonary (Li et al., 2009) and cardiovascular (Delno et al., 2005) injury, and even neurodegenerative disorders (Campbell, 2004; Peters et al., 2006). In urban areas, the primary source of ambient PM and their co-pollutants come from motor vehicles and trafc-induced emissions (Ning and Sioutas, 2010), which raises serious health concerns for the part of the population who live and/or work in the communities nearby busy roadways or spend several hours daily commuting. Several epidemiological studies have reported a strong and positive association between a communitys proximity to highly trafcked roadways and the risk of adverse health effects among the population in the community, including asthma and other respiratory diseases (Delno, 2002; McConnell et al., 2006), birth and developmental defects (Ritz and Yu, 1999; Wilhelm and Ritz, 2003), cardiovascular diseases (Peters et al., 2004; Delno et al., 2005) and childhood cancer such as leukemia (Harrison et al., 1999; Pearson et al., 2000). Particle and gaseous pollutants dispersion proles near open freeways have been well documented by means of both experi- mental measurements and numerical simulations in recent years. One of the rst such attempts was undertaken by Zhu et al. (2002a, b) in the vicinity of Interstate 405 and 710 freeways in Southern California. It was observed that the particle number and CO concentrations decreased exponentially, and particle size distri- butions shifted towards larger particle diameters as the aerosols were transported away from freeway, which the authors attributed * Corresponding author. Tel.: þ1 213 740 6134; fax: þ1 213 744 1426. E-mail address: [email protected] (C. Sioutas). Contents lists available at ScienceDirect Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv 1352-2310/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2010.05.033 Atmospheric Environment 44 (2010) 3118e3127

Impact of roadside noise barriers on particle size distributions and pollutants concentrations near freeways

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Atmospheric Environment

journal homepage: www.elsevier .com/locate/atmosenv

Impact of roadside noise barriers on particle size distributionsand pollutants concentrations near freeways

Zhi Ning a, Neelakshi Hudda a, Nancy Daher a, Winnie Kama, Jorn Herner b,Kathleen Kozawa b, Steven Mara b, Constantinos Sioutas a,*

aDepartment of Civil and Environmental Engineering, University of Southern California, 3620 South Vermont Avenue, Los Angeles, CA 90089, USAbCalifornia Air Resources Board, Research Division, 1001 ‘‘I’’ Street, P.O. Box 2815, Sacramento, CA 95812, USA

a r t i c l e i n f o

Article history:Received 19 March 2010Received in revised form15 May 2010Accepted 17 May 2010

Keywords:Particle size distributionPollutants concentrationsNoise barrierFreeway emissionsRoadway configurations

* Corresponding author. Tel.: þ1 213 740 6134; faxE-mail address: [email protected] (C. Sioutas).

1352-2310/$ e see front matter � 2010 Elsevier Ltd.doi:10.1016/j.atmosenv.2010.05.033

a b s t r a c t

Increasing epidemiological evidence has established an association between a host of adverse healtheffects and exposure to ambient particulate matter (PM) and co-pollutants, especially those emitted frommotor vehicles. Although PM and their co-pollutants dispersion profiles near the open freeway have beenextensively characterized by means of both experimental measurements and numerical simulations inrecent years, such investigations near freeways with roadside barriers have not been well documented inthe literature. A few previous studies suggested that the presence of roadside structures, such as noisebarriers and vegetation, may impact the decay of pollutant concentrations downwind of the freeway bylimiting the initial dispersion of traffic emissions and increasing their vertical mixing due to the upwarddeflection of airflow. Since the noise barriers are now common roadside features of the freeways,particularly those running through populated urban areas, it is pertinent to investigate the impact oftheir presence on the particles and co-pollutants concentrations in areas adjacent to busy roadways.This study investigated two highly trafficked freeways (I-710 and I-5) in Southern California, with twosampling sites for each freeway, one with and the other without the roadside noise barriers. Particle sizedistributions and co-pollutants concentrations were measured in the immediate proximity of freewaysand at different distances downwind of the freeways. The results showed the formation of a “concen-tration deficit” zone in the immediate vicinity of the freeway with the presence of roadside noise barrier,followed by a surge of pollutant concentrations further downwind at 80e100 m away from freeway.The particle and co-pollutants concentrations reach background levels at farther distances of 250e400 mcompared to 150e200 m at the sites without roadside noise barriers.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

In recent years, several toxicological studies have reported theability of particulatematter (PM) to generate reactive oxygen speciesin biological systems (Sagai et al., 2000; Donaldson et al., 2002; Xiaet al., 2004) and these pro-oxidant species are intimately linked tothe genesis of pulmonary (Li et al., 2009) and cardiovascular (Delfinoet al., 2005) injury, and even neurodegenerative disorders(Campbell, 2004; Peters et al., 2006). In urban areas, the primarysource of ambient PM and their co-pollutants come from motorvehicles and traffic-induced emissions (Ning and Sioutas, 2010),which raises serious health concerns for the part of the populationwho live and/or work in the communities nearby busy roadways orspend several hours daily commuting. Several epidemiological

: þ1 213 744 1426.

All rights reserved.

studies have reported a strong and positive association betweena community’s proximity to highly trafficked roadways and the riskof adverse health effects among the population in the community,including asthma and other respiratory diseases (Delfino, 2002;McConnell et al., 2006), birth and developmental defects (Ritz andYu, 1999; Wilhelm and Ritz, 2003), cardiovascular diseases (Peterset al., 2004; Delfino et al., 2005) and childhood cancer such asleukemia (Harrison et al., 1999; Pearson et al., 2000).

Particle and gaseous pollutants dispersion profiles near openfreeways have been well documented by means of both experi-mental measurements and numerical simulations in recent years.One of the first such attempts was undertaken by Zhu et al. (2002a,b) in the vicinity of Interstate 405 and 710 freeways in SouthernCalifornia. It was observed that the particle number and COconcentrations decreased exponentially, and particle size distri-butions shifted towards larger particle diameters as the aerosolswere transported away from freeway, which the authors attributed

Z. Ning et al. / Atmospheric Environment 44 (2010) 3118e3127 3119

to particle coagulation and turbulent dispersion. Similar experi-mental investigations were also carried out in other cities aroundthe world (Gramotnev and Ristovski, 2004; Zhu et al., 2009). On theother hand, several modeling studies have attempted to explain theparticles evolution near roadways (Jacobson and Seinfeld, 2004;Zhang et al., 2004; Jacobson et al., 2005). Zhang et al. (2004) useda multi-component aerosol dynamic model to fit the measurementdata from Zhu et al. (2002a) studies and demonstrated thatcondensation/evaporation and dilution were the major mecha-nisms in altering aerosol size distribution downwind the freeway. Arecent study by Jacobson et al. (2005) suggested that evaporationcauses smaller semi-volatile particles to shrink in size and thusenhanced their coagulation rate, which played an important role inthe near-source evolution of particle size distribution.

Although these experimental and modeling studies have charac-terized the roadside particle and co-pollutants dispersion profilesquite extensively, the roadways under investigation were often open-field without considering any roadside obstacles. These roadsidestructures, such as noise barrier, vegetation and buildings, mayaffect the characterization of pollutants decay profiles downwind ofroadway by limiting or blocking the initial dispersion of traffic-induced emissions from roadway and increasing their verticalmixingdue to the upward deflection of airflow caused by the obstacles(Holscher et al., 1993; Heist et al., 2009). Given that the noise barriers,built to reduce noise levels in areas nearby roadways, are commonroadside features of highly trafficked freeways, particularly thoserunning throughdensely populated areas, it is pertinent to investigatethe effect of their roadside presence on particles and co-pollutantsconcentration levels in areas adjacent to the busy roadways.

Few recent studies demonstrated that a recirculation cavity,consisting of well-mixed and substantially lower concentrations ofpollutants, exists in the lee of the roadside barrier (Holscher et al.,1993; Bowker et al., 2007; Baldauf et al., 2008b). This “concentra-tion deficit zone” can extend from3 tomore than 20 times the barrierheights depending on the meteorological conditions and barrierconfigurations (Baldauf et al., 2008b; Heist et al., 2009). Modelingstudies also suggested the existence of a “hot” zone at furtherdistances downwind of the recirculation wake region, where thepollutant concentrations are higher than at an equivalent distance inan open areawithout barrier, due to the vertical elevation of roadwayemissions source by the barrier and the subsequent reattachmentof airflow further downwind (Bowker et al., 2007). Based on theprevious investigations, the present study employed an intensivesampling campaign and examined the impact of the roadside noisebarrier on the dispersion profiles of particles and co-pollutants fromfreeway emissions. In this study, we selected two major freewaysin Southern California (I-710 and I-5) with different traffic fleetcompositions. For each freeway, we have selected two sampling sitesfeatured with- and without roadside noise barrier, respectively.The particle size distribution and co-pollutants concentrations weremeasured at different distances downwind of the freeways toinvestigate the effect of noise barrier on the decay of their concen-trations and the evolution of particle size distributions from traffic-induced emissions. The results of the current study, based on real-world experimental measurement, provide direct evidence that theexistence of roadside noise barrier dramatically alters the dynamicsof particle and co-pollutants dispersion and the spatial distribution ofpollutants concentrations in the areas nearby the freeways.

2. Experimental methodology

2.1. Site locations

An intensive summer sampling campaign has been carried outin the present study during JuneeJuly, 2009 to investigate the

impact of roadside noise barrier on the dispersion profile of parti-cles and co-pollutants emitted from freeways. Two highly traffickedfreeways in greater Los Angeles area (I-710 and I-5) were selected,with two sampling sites (one with roadside noise barrier and theother without) located along the span of each freeway. At each ofthe four sites, a stationary sampling station was set up locatedin the immediate proximity of the freeways to characterize thefreeway emissions, while a mobile platform was deployed alongthe trajectories downwind of the traffic emissions from freeway tocollect ambient data at varying distances. Fig. 1 (a, b, c and d) showsthe location of the four sampling sites and the route of the mobileplatform at the downwind area of the selected freeways.

The I-710 non-noise barrier site (Fig. 1a) is located in the stretchof the freeway in Downey, CA. Any major roadway in the upwinddirection of the stationary sampling station was at distance greaterthan 1 km. The inlet of the sampling instruments was extended toas close as 2 m from the freeway edge. The location of the stationand the route of the mobile platform are also highlighted in Fig. 1a.I-710 noise barrier site (Fig. 1c) was located 2 km north of the non-noise barrier site in a residential neighborhood in Bell Gardens, CA,with no major roadways, other than the freeway, upwind to about1 km from the stationary sampling station. The station was set upon the freeway curbside and the inlet reached over the noise barrierinto the freeway with a short distance of 2 m from the freewayedge. The barrier was 3.7 m in height as measured above theadjacent surface road next to the noise barrier and extended morethan few hundred meters from the site in both north and southbound sections along the freeway. The mobile platform routedownwind of the freeway is identified in the Fig. 1c.

The I-5 non-noise barrier site (Fig.1b)was located in the stretch ofthe freeway near LaMirada, CA. Therewere no othermajor roadwaysupwind of the sampling site other than the freeway and the vicinityof the sampling site is mainly comprised of office buildings withoutimmediate industrial sources nearby. The stationary sampling loca-tion was set next to the freeway curb with the inlet reaching within0.5 m to the edge. The noise barrier site in I-5 (Fig. 1d) was located ina residential neighborhood, with the roadside noise barrier extend-ing for hundreds meters along the freeway north of the samplinglocation. The height of the noise barrier at the site was 5.2 m asmeasured above the adjacent surface road next to the noise barrier.The inlet of the stationary sampling station was extended over thebarrier and reached into the freeway zonewith offset about 4m fromthe edge of the freeway.

2.2. Instrumentation

Two sampling instrument sets were deployed simultaneously,one at the stationary sampling station and the other in the mobileplatform. A list of the instruments at both sampling stations withtheir data resolution is shown in Table 1. The stationary station setup included a Scanning Mobility Particle Sizer (Model 3080, TSI)configured to measure the particle size distribution in the size rangeof 10e225nm. Particle-bound black carbon (BC) and co-pollutants ofcarbonmonoxide (CO) and nitrogen dioxide (NO2) concentrations inthe immediate proximity of freeway were also measured at thestation. A camera was set up on top of the sampling station tocapture the traffic on the freewayand anultrasonic anemometerwasset up at a height of 4 m above the freeway road surface at all of thefour sampling sites to collect meteorological data. For the noisebarrier sites, the probe was placed upwind of the barrier to avoidpossible interferences of the barrier on the measurements.

Themobile platform is a 1998 electric Toyota RAV4 SUV equippedwith various on boardmonitoring instruments. The same vehicle hasbeen used in many previous studies (Westerdahl et al., 2005;Fruin et al., 2008; Kozawa et al., 2008) in Southern California.

Fig. 1. Location of the sampling sites: (a) I-710 without roadside barrier; (b) I-5 without roadside barrier; (c) I-710 with roadside barrier; (d) I-5 with roadside barrier. Note: Red dotrepresents the stationary sampling station; the yellow lines represent the route of the mobile platform downwind of the freeway.

Z. Ning et al. / Atmospheric Environment 44 (2010) 3118e31273120

The sampling inlet consists of a 6-inch diameter galvanized steelduct, located 1.5m above the roadway in the rear passenger space ofthe vehicle. The particle size distributions data (6e523 nm) werecollected using aFast Mobility Particle Sizer(Model 3091, TSI).Particle-bound black carbon and co-pollutants (CO, NO2) concen-trationsweremeasured simultaneously to calculate their downwindconcentration ratios. A GPS unit was used to record the geocodeddata of the mobile platform to derive its downwind distance fromthe freeway. The mobile platform was also equipped with an onboard camera to distinguish a time when the measurements wereimpacted by a passing vehicle near the mobile platform.

Quality assurance measures, including flow and zero checksof all instruments, and regular calibration of gaseous pollutantmonitoring instruments, were carried out before and after thesampling campaign. The side-by-side test results showed thattwo sets of continuous monitoring instruments for CO, NO2 and BCdeployed in the stationary site and in the mobile platform reported

Table 1Monitoring instruments deployed in stationary sampling station and the mobile platform

Measurement Stationary sampling station

Geodata GPS (Garmin GPSmap 76CSx)Particle size distribution SMPS: TSI model 3080 (long DMA) w/TSI model 302

(CPC) @ 5 min intervals (10e225 nm range)Particle-bound Black Carbon Aethalometer: Anderson model 14 (dual channel) @CO QTrak e TSI model 7565 @ 1 min intervalsNO2 Teledyne-API model 200A @ 1 min intervalsMeteorological data 3-D ultrasonic anemometer (RS Young model 81 00

values within 5% of each other. All the instruments ran onsynchronized time at both stationary station and the mobileplatform. The two instrument sets were run side-by-side overnightat the beginning and end of each sampling period for the foursampling sites to assess the systematic uncertainty due to thedifference of monitoring instruments and to inspect the time lag(a consequence of different flow rates, inlet lengths, and instrumentresponse time) between the actual sampling and the reported time.

2.3. Data processing

All data from the stationary sampling station were downloadedand exported using proprietary software and data from the mobileplatform were exported to a custom database for all instruments.Data used for data analysis (specified in Table 2) were chosen fromthe sampling time periods with the measured wind direction �45�

from that perpendicular to the freeway, placing the sampling

.

Mobile platform

GPS(Garmin GPSmap 76CSx)2A FMPS: TSI model 3091 @ 20 s intervals

(6e523 nm range)1 min intervals Aethalometer: Magee Scientific @ 1 min intervals

Teledyne-API model 300E for CO @ 20 s intervalsTeledyne-API Model 200E @ 20 s intervals

0) @ 1 min intervals 2-D Ultrasonic anemometer (RS Young) @ 1 s intervals

Table 2Summary of the meteorological parameters and average pollutant concentrations measured in the immediate proximity of the freeways from stationary sampling station.

Sampling sites I-710 I-5

Non-noise barrier Noise barrier Non-noise barrier Noise barrier

Samping dates and time 06/08/09 (2PMe5PM);06/09/09 (3PMe7PM)

06/02/09 (1PMe4PM);06/05/09 (1PMe5PM)

07/06/09 (1PMe4PM);07/02/09 (12PMe4PM)

06/25/09 (1PMe5PM);06/29/09 (1PMe5PM)

Total Vehicle Flowa (vehicles hour�1) Average 12 170 12 212 8460 8677Stdev 563 599 405 300

Truck Flowa (vehicles hour�1) Average 490 516 673 610Stdev 124 101 136 138

T (�C) Average 21.7 23.7 28.4 25.0Stdev 0.9 2.3 1.0 4.5

RH (%) Average 57.0 43.3 48.6 51.0Stdev 4.4 2.5 8.8 3.5

Wind speed (m s�1) Average 2.0 3.1 1.9 0.9Stdev 0.3 0.5 0.4 0.2

Wind Directionb (deg true N) Average 233 229 225 225Stdev 16 10 42 25

CO (ppm) Average 1.5 1.4Stdev 0.4 0.1

BC (mg m�3) Average 11.0 11.6 10.6 9.5Stdev 6.3 1.4 4.2 1.5

NO2 (ppb) Average 152.2 87.3 93.9 79.3Stdev 38.0 8.2 31.2 4.9

SMPS Number Concontration (# cm�3) Average 1.2e5 1.1e5 8.0e4 7.5e4Stdev 5.0e4 2.3e4 3.4e4 2.4e4

SMPS Mass Concentration(mg m�3) (10e225 nm)

Average 6.4 6.7 7.5 7.2Stdev 2.4 1.5 3.1 1.7

a The traffic volume and composition data were obtained from California Freeway Performance Measurement System (PeMS) by CalTrans.b Wind directions perpendicular to roadway are 270 and 225� true N for I-710 and I-5 sites, respectively.

Z. Ning et al. / Atmospheric Environment 44 (2010) 3118e3127 3121

locations of the mobile platform in the direct downwind vicinity ofthe selected roadway region. The time periods that were influencedby non-freeway emissions (i.e. a truck passing by the mobile plat-form) were excluded from the data analysis. GPS tracking datawere exported and converted to determine the mobile platform’sperpendicular downwind distance to the freeway. The particle sizedistribution and other pollutants concentration data at variousdownwind distances were segregated into several distance ranges,and the distance values presented in the X-axis of the figures here-after represent the midpoint of each distance range. Additionalinformation is listed in detail in the Supporting information file.The concentration ratios of NO2, CO, and BC at different downwinddistances of the freeway were determined by normalizing thedownwind data frommobile platformwith their corresponding datacollected from the stationary sampling station. For CO, the data forthe I-5 freeway at the stationary station were not available due toa malfunctioning instrument, therefore absolute concentrations arepresented. The particle size distribution datawere further segregatedinto several size groups to derive their respective number and massconcentrations at various distances. The particlemass concentrationswere determined by their corresponding particle volume concen-trations and particle density of 1.2 g cm�3 (Geller et al., 2006).

3. Results and discussion

3.1. Overview of the sampling campaign

Table 2 shows the summary of the meteorological conditions,traffic volumes and average freeway pollutants concentrationsmeasured in the immediate proximity of the freeways during thesampling campaign. Similar temperature and humidity conditionswere observed among different sites. The average wind directionswere in the range of 225 and 233� to the north, placing the selectedsampling sites directly downwindof the freeways, and themeanwind

speed was in the range of 0.9e3.1 m s�1. Traffic data were obtainedfrom the California Department of Transportation ‘California FreewayPerformance Measurement System’ (PeMS) corresponding to thesampling periods, and the vehicles on both north and south bounddirections have been counted. The total traffic flows measured onI-710 and I-5werew12 200 andw8500 vehicles hour�1, respectively,with a lower truck flow, e.g. heavy duty diesel vehicles, on I-710(w500 trucks hour�1) than I-5 (w640 trucks hour�1). For the twosites at the same freeway, the traffic volume and truck compositionwere very consistent. The similarity of theirmeteorological and trafficconditions allows for a direct comparison between the results of thebarrier and non-barrier sites.

The average CO concentrations were 1.5� 0.4 and 1.4� 0.1 ppmat the two sites in the immediate proximity of I-170 freeway. Theirconcentrations near I-5 were not reported in Table 2 due to a mal-functioning of the instrument. The BC concentrations measured inthe two sites at I-710 freewaywere 11.0� 6.3 and 11.6� 1.4 mgm�3,comparable to 10.6 � 4.2 and 9.5 � 1.5 mg m�3 measured at I-5. Incomparison to the previous measurement on I-710 freeway(Westerdahl et al., 2005), in which the BC and CO concentrationswere 12 mgm�3 and 1.9 ppm asmeasuredwhile driving on the samefreeways using a mobile platform, the results from both studieswere very consistent. The average particle number concentrationsmeasured by SMPS (10e225 nm) at both I-710 sites (1.2� 0.5e5 and1.1 � 0.2e5 particles cm�3) are higher than those at I-5 sites(8.0� 3.4e4 and 7.5� 2.4e4 particles cm�3), due to the higher trafficvolume on I-710.

3.2. Evolution of particle size distributions downwind of freeway

Fig. 2a and b show the average particle size distributions atvarious distances downwind of I-710 (20, 40, 80, 200 and 450 m)and I-5 (20, 40, 90, 120, 400 m) freeways without roadside noisebarriers as measured by FMPS (6e523 nm). The size distributions

Fig. 2. Particle size distributions measured at various distances downwind of freeway using FMPS (6e523 nm) and in the immediate proximity of freeway using SMPS (10e225 nm)shown as subplot: (a) I-710 without roadside barrier; (b) I-5 without roadside barrier; (c) I-710 with roadside barrier; (d) I-5 with roadside barrier.

Z. Ning et al. / Atmospheric Environment 44 (2010) 3118e31273122

measured in the immediate proximity of the freeways using theSMPS (10e225 nm) are also included as subplots in the figures forcomparison. For all plots, the horizontal axis represents particlesize on a log scale while the vertical axis represents normalizedparticle number concentration. Data points represent averagesfrom multiple scans taken at a given sampling location.

As shown in Fig. 2a and b (subplots), the particle size distribu-tions in the immediate proximity of both freeways displayed a uni-modal shape, with a distinct peak at approximately 10 nm (e10 nm),indicating new particle formation by nucleation of supersaturatedsemi-volatile organic vapors in the exhaust (Alam et al., 2003).Shortly after highly concentrated vapors (i.e. semi-volatile organiccompounds) are emitted from the tailpipe of vehicles on freeway, therapid cooling in the atmosphere causes them to nucleate and formlarge numbers of nucleation mode particles (Zhang and Wexler,2004). The peak modal concentrations at w10 nm are 2.7e5 and2.0e5 particles cm�3 for I-710 and I-5 freeways, respectively.

As particles were transported away from the freeway, theparticle size distributions changed markedly, with a dramaticdecrease in the number concentrations. In Fig. 2a, the particlenumber concentrations at w10 nm were 1.7e5, 1.3e5 and 7.9e4particles cm�3, at 20 m, 40 m, and 80 m, respectively, whichaccounted for only 63%, 48%, and 29% of that measured in theimmediate proximity of freeway. Similar observations were alsoreported by Zhu et al. (2002b) near another freeway (I-405) inSouthern California. Several atmospheric processes may contributeto this significant change in size distribution and concentration,including particle evaporation and diffusion (Hinds, 1999) andsemi-volatile vapor condensation (Zhang et al., 2004). Due to theirsmall size, particles of 10 nm have much higher diffusion coeffi-cients than larger particles, and the dynamics of their evaporationare more pronounced due to the “Kelvin effect” (Hinds, 1999). Theevaporated organic vapors, in addition to those from fresh vehicleemissions, may also condense onto pre-existing particles and form

larger size particles. Although it was hypothesized the collisionof small particles by coagulation may contribute to the deceasingparticle number concentration (Zhu et al., 2002b), previousexperimental measurement (Shi et al., 1999) and theoretical studies(Vignati et al., 1999; Zhang et al., 2004) have indicated that the roleof coagulation is rather negligible in affecting the overall decay inparticle concentration. At 200 m, the particle size distributiondisplays a broad shoulder between 20 and 50 nm, similar to thatmeasured at 450 m, a clear indication that the particle concentra-tions have reached background levels at 200 m where the sizedistributions no longer change significantly with distance.

On the other hand, Fig. 2b shows the particle size distributions atvarious distances downwind of the I-5 freeway. In contrast to theobservations near I-710, the particle size distributions displayeda consistent bimodal trend. The first mode appeared at w10 nm,similar to the measurement near I-710 freeway, while there existeda distinct second mode in the larger size range of 30e50 nm. At20m, the modal concentration atw10 nmwas 8.3e4 particles cm�3,nearly half of that measured in the immediate proximity of thefreeway, indicating rapid dilution and the associated particle evap-oration and diffusional loss. Compared with the correspondingdownwind distance of I-710 freeway, the w10 nm modal concen-tration is significantly lower at the I-5 site, whereas the second peak,observed at around 35 nm, has similar concentrations at sites nearboth freeways, resulting in a pronounced dip at w25 nm in thebimodal distributions at the I-5 sites. The lower nucleation modeparticle concentrations indicate a lower strength of fresh trafficemissions at the I-5 freeway, withmuch lower traffic flow than the I-710 (w8500 vehicles hour�1 on I-5 versusw12 200 vehicles hour�1

on I-710). As the particles are further transported away from thefreeway, particle number concentrations gradually decrease at bothmodes, and the second mode is shifted from 34 nm (at 20 m) toabout 40 nm (at 120 m), indicating the possibility of small particleevaporation and vapor condensation onto pre-existing particles

Z. Ning et al. / Atmospheric Environment 44 (2010) 3118e3127 3123

(Shi et al., 1999; Zhang et al., 2004) as well as possible coagulationat different dilution conditions (Hinds, 1999; Zhu et al., 2002b).At 120 m, the nucleation mode particles at w10 nm reacheda concentration of 3.6e4 particles cm�3, similar to the backgroundlevel at 450 m; however, number concentration of particles at thelarger size mode of 39 nm continued to drop after 120 m until itreached background levels at 400 m. Nucleation mode particleshave a shorter residence time in the atmosphere, so they decaymuch faster and reach background levels at a shorter distance thanlarger particles (Raes et al., 2000).

Fig. 2c and d show the average particle size distributions atvarious distances downwind of the sections of I-710 (15, 40, 100,200, 400 m) and I-5 (15, 40, 80, 150, 250 m) freeways with roadsidenoise barriers. Compared with the non-barrier sites, the particlesize distributions and number concentrations were consistent atthe same freeway when the traffic volume and fleet compositionwere relatively consistent during the days of sampling. As shownin both figures, with increasing downwind distance from thefreeways, the evolution of particle size distributions displayeda dramatically different pattern with those at the non-barrier sites:the size distribution at distances close to the freeway (15 m) in thebarrier sites showed significantly lower number concentrationsfor both freeways, while the peak size distribution and particleconcentrations appeared at further downwind distances of80e100 m, followed by a gradual decrease in concentrations untilthey reached background levels at beyond 250 m.

As shown in Fig. 2c, at 15 m downwind of I-710 freeway, particlesize distributions displayed a uni-modal trend, with a slight peakat 10 nm and a broad shoulder at 16e19 nm. The 10 nm modalconcentration is 5.7e4 particles cm�3, i.e. significantly lower thanthat (1.7e5 particles cm�3)measured at a similar downwind distance(20m)without roadside noise barriers, as shown in Fig. 2a. A similartrend was also observed at 15 m downwind of I-5 with a roadsidebarrier (shown in Fig. 2d) where the peak modal number concen-tration at 10 nm was 3.1e4 particles cm�3, 64% lower than that(8.5e4 particles cm�3) measured at 20m fromnon-noise barrier site.Modeling results have shown that the presence of a roadsidenoise barrier alters the dispersion patterns of traffic emissions andthe trajectory of particles traveling downwind from their source(Bowker et al., 2007). The freeway vehicular emissions travel upwardfrom the road level due to the presence of the roadside barrier,effectively inducing an “elevated” source. In the lee of the roadwaynoise barrier, vertical mixing occurs due to strong turbulence (Finnet al., 2010) creating a well-mixed downwind side zone with rela-tively lower pollutant concentrations. Depending on the roadconfiguration andmeteorological conditions, the recirculation cavitymayextend from3 to 20 times thenoise barrier height (Baldauf et al.,2008b; Heist et al., 2009). The particle number concentration deficitsobserved in the downwind vicinity of the noise barrier stronglysupport the existence of a recirculation cavity as reported in litera-ture (Bowker et al., 2007; Baldauf et al., 2008b; Finn et al., 2010).

With increasing downwind distance from freeways, particlenumber concentrations gradually increased and the nucleationmode at w10 nm became more pronounced, indicating thediminishing of the recirculation cavity and the increasing influenceof freeway emissions. At 40 m, the modal concentration atw10 nmwas 1.2e5 and 6.8e4 particles cm�3 for I-710 and I-5 freeway,respectively, about two times higher than those measured at10e15 m. As the particles are further transported away from therecirculation cavity, the elevated source follows its trajectory andthe plume gradually reattaches to the ground (Bowker et al., 2007).The peak size distribution and number concentration wereobserved at 100 m and 80 m for the I-710 and I-5 freeway, respec-tively. For the I-710, the modal concentration of 10 nm at 100 m is2.2e5 particles cm�3, 55% of that measured in the immediate

proximity of freeway (4.0e5 particles cm�3). The uni-modal particlesize distribution at 100 m also resembles that observed at 20 mnear non-barrier I-710 freeway (Fig. 2a). It displays a similar level ofnucleation mode peak concentrations, thus indicating similardilution -induced particle evaporation and a more pronouncedshoulder at 19 nm, which may be attributed to re-condensationof the evaporated semi-volatile species, as argued by Zhang et al.(2004). This is also confirmed by similar observations at the I-5freeway, where the particle size distribution at 80 mwith roadsidebarriers (Fig. 2d) has a broader second mode ate30 nm (7.0e4 par-ticles cm�3), in contrast to the corresponding peak at 20 m(5.0e4 particles cm�3) near non-barrier I-5 (Fig. 2b). In comparisonto similar downwind distance between the sites, the total numberconcentration at 100 m downwind of freewaywith roadside barrierwas 1.9 and 2.2 times of that at 80e90 m for non-barrier I-710 andI-5 freeway, respectively. Previous investigations have shown thatparticle number concentrations reach the background levels ata distance of 100e200m downwind of freeway, depending onwindspeed and meteorological conditions (Zhu et al., 2002b).The observations made in this study indicate that the presence ofroadside noise barriers dramatically changes particles dispersionprofiles and affects their concentrations distribution downwind ofthe freeway. At 200 m downwind of I-710 with roadside barriers,particle size distribution still displays a sharp uni-modal shape,with peak concentrations of 1.4e5 particles cm�3 at 10 nm, which is2.5 times the background level measured at 200 m for the I-710 sitewithout roadside barrier. A similar observation can also be madefor the I-5 freeway at 150 m downwind of the barrier, where thew10 nm modal concentration is 1.6 times that measured at corre-sponding distance of 120 m for the non-barrier I-5 sites.

As the particles traveled further downwind at 400 m near I-710,the particle size distribution and number concentration reachedthe background level as shown in Fig. 2c. The much longer down-wind distance (400 m versus 200 m) needed to reach backgroundlevels indicates a larger impact zone from traffic emission sourcesfor freeway sections featured with roadside noise barriers. For I-5freeway with roadside barrier, the background level of particle sizedistribution and concentrations was reached at 250 m, as shown inFig. 2d. The difference of the needed distances to reach backgroundlevels between the two freeways may be explained by the higherinitial particle number concentration at the I-710, thus the longertime necessary for it to decrease to background concentrations.

3.3. Particle number and concentrations at differentdownwind distances

Fig. 3 shows the particle number (primary Y axis) and massconcentrations (secondary Y axis) with increasing downwinddistances of the freeways without (Fig. 3a and b) and with (Fig. 3cand d) roadside noise barriers. The data points at x ¼ 0 m representthe average number/mass concentrations in the immediateproximity of the freeways derived from SMPS (10e225 nm) whiledata points beyond x ¼ 0 m represent average number/massconcentration downwind of the freeways as derived from FMPSmeasurement (6e523 nm). The corresponding error bars denoteone standard deviation of measured particle number or massconcentration. Due to the different size ranges of the measure-ments, the data points derived from FMPS and SMPS were plottedseparately in the figures for clarity.

As shown in Fig. 3a and b, total particle number concentrationsmeasured in the immediate proximity of I-710 (1.2e5 particles cm�3)were higher than I-5 freeway (8.0e4 particles cm�3) due to the highertotal traffic volume on the I-710. As the downwind distanceincreased, the number concentrations decayed exponentially dueto the evaporation of semi-volatile particles and the diffusion of

Fig. 3. Particle number and mass concentrations at different distance downwind of the freeway (a) I-710 no noise barrier (b) I-5 no noise barrier; (c) I-710 with noise barrier; (d) I-5with noise barrier. Note: The total particle number and mass concentrations at X ¼ 0 are derived from SMPS (10e225 nm) data at stationary sampling station; the concentrations atX > 0 are derived from FMPS (6e523 nm) data.

Z. Ning et al. / Atmospheric Environment 44 (2010) 3118e31273124

nucleation mode particles, as discussed in detail in the previoussection. The concentrations reached background levels within 200mand 120 m for I-710 and I-5, respectively, where the measurednumber concentration was only 45% and 64% of that measuredat 20 m near its corresponding freeway. For both sites, the observedbackground particle number concentrations were similar at3.0e4 particles cm�3, which is a very typical urban background levelacross the Los Angeles Basin during that time period (Westerdahlet al., 2005; Moore et al., 2009). The faster decay trend for I-710freeway may be attributed to the higher initial particle numberconcentration observed inside the I-710. Similar to the overall decaytrend of number concentration, the total mass concentration (for therange of 6e523 nm) also decreased with increasing downwinddistance in both freeways. However, the decay ofmass concentrationis much slower than number concentrations, as shown in Fig. 3a andb. The rapid dilution next to the freeway not only lowers particleconcentration by dispersion, but also causes evaporation of semi-volatile species off the particle surface, which is more pronouncedwith decreasing particle size, due to the Kelvin effect (Hinds, 1999).The overall result is an accelerated decrease in number concentra-tions of smaller than larger particles. The evaporated vapors from thesmaller range of ultrafine PM, combined with those from freshvehicle emissions, may further condense onto pre-existing particles,resulting in a slower decay of particle mass concentration. At 200 m,the particle mass concentrations were 3.7 and 5.6 mg m�3 for I-710and I-5, respectively, and are comparable to those measured at400 m, suggesting that PM mass concentration also reached back-ground levels within 200 m.

Fig. 3c and d show the particle number andmass concentrationsin the immediate proximity and downwind of I-710 and I-5with roadside noise barriers. Compared with those at non-barrierfreeway sites, the particle concentration distribution displayeda significantly different trend, exemplifying the impact of a road-side barrier on the particle dispersion profiles. At 15 m, particlenumber concentrations were 4.8e4 and 3.1e4 particle cm�3 at I-710and I-5 sites, respectively, 43% and 45% lower than those measured

at 20 m downwind of corresponding freeway without roadsidebarrier. A similar trend was also observed for particle massconcentrations at the same sites. The particle concentration deficitobserved at 15 m strongly underscores the existence of the recir-culation cavity in the lee of the roadway barrier (Bowker et al.,2007; Baldauf et al., 2008b). As the downwind distance increased,the particle concentrations increased and reached a maximum at100 m and 80 m for I-710 and I-5, respectively. The peak particlenumber concentrations are 2.4 and 2.2 times higher than thoseobserved at corresponding distance for non-barrier sites at theI-710 and I-5 freeways. As the aerosols traveled further downwind,the particle number concentrations gradually decreased andreached background levels at 400 m; while the particle massconcentrations became stabilized to background levels earlier, at200 m. The differential trend of particle mass and numberconcentrations highlights the role of semi-volatile species evapo-ration and re-condensation in the dynamics of particles evolutionin the atmosphere (Vignati et al., 1999; Zhang et al., 2004; Ning andSioutas, 2010). It is noted that a similar trend is also observed nextto the freeways without roadside barriers (Fig. 3a and b), whereparticle mass concentration decayed much slower than numberconcentrations during the rapid dilution.

3.4. Size-segregated particle number concentrationsat different downwind distances

Fig. 4 shows the average size-segregated particle numberconcentrations at different distances downwind of the freeways.The particle size distributions measured by FMPS (6e523 nm) weresegregated into five different mobility size ranges of 6e25, 25e50,50e100, 100e300 and 300e550 nm, respectively. The measurednumber concentrations in the size bins within each size range wereadded to derive the concentration of each size group.

As shown in Fig. 4a and b, particles in different size rangesbehaved quite differently as the distance from freeways increased.The number concentration of particles in the size range of 6e25 nm

Fig. 4. Size-segregated particle number concentrations at different distances downwind of the freeway (a) I-710 no noise barrier (b) I-5 no noise barrier; (c) I-710 with noise barrier;(d) I-5 with noise barrier. Note: Total PNC is the total particle number concentration derived from FMPS.

Z. Ning et al. / Atmospheric Environment 44 (2010) 3118e3127 3125

accounted for 73% and 57% of the total number concentration at15 m next to I-710 and I-5 freeways, respectively. The large fractionof <25 nm particles is consistent with the observation ofNtziachristos et al. (2007) and Zhu et al. (2002b) conducted next tothe same freeway. As the particles are advected away from freeway,the 6e25 nm particles number concentrations decreased by halffrom 15 m to 80e90 m for I-710 and I-5, respectively, and graduallyleveled off beyond 200 m. This observation is also reflected in therapid drop of the nucleation peak displayed in the particle sizedistribution plots (Fig. 2a and b) from nearby the freeway to 100 mdownwind. With increasing particle size, number concentrationsdecayed more slowly, illustrating the effect of size-dependentparticle evaporation due to the Kelvin effect, and diffusion duringrapid dilution. On the other hand, particles above 100 nm did notexperience a substantial change in their number concentrationwith increasing downwind distance to the freeway, suggesting theinsignificant contribution of freeway emissions to their concen-trations in the atmosphere. Fig. 4c and d show size-segregatedparticle number concentrations for the barrier sites for I-710 and I-5 at various downwind distances. Close to the freeways at 15 m, theparticle number concentrations in all size ranges were significantlylower than those at similar distances in non-barrier conditions andeven comparable to those measured at background, a clear indi-cation that the presence of roadside barrier induces a recirculationcavity in the lee of the barrier with deficit of pollutants concen-trations. An evident peak of particle number concentrations in theranges of 6e25 nm, 25e50 nm and 50e100 nm is observed at100 m and 80 m for I-710 and I-5, where the reattachment ofpollutants plume from traffic emissions occurs. Compared to thenon-barrier freeway conditions, the number concentrations ofparticles in different size ranges near barrier freeway reachedbackground levels at a farther distance of 400 m and 250 m for I-710 and I-5 freeways.

3.5. Co-pollutants concentrations at different downwind distances

Fig. 5 shows the concentrations ratios of carbon monoxide (CO),nitrogen dioxide (NO2) and black carbon (BC) at various distancesdownwind of the freeways with no roadside noise barrier (3a:I-710; 3b: I-5) and with barrier (3c: I-710; 3d: I-5). The concen-tration ratios were calculated by dividing the average concentra-tions measured at different downwind distances by the averageconcentrations in the immediate proximity of the freeways (x ¼ 0).Error bars represent one standard deviation of the average ratios.CO concentrations for I-5 were expressed as absolute downwindconcentrations in the secondary Y axis due to a malfunction of theCO monitor used at the site in the immediate proximity of I-5.These species were selected because their concentrations in urbanenvironments are closely related to traffic emissions. For theconcentration ratios in Fig. 5a and b, exponential decay curves wereused to fit the decreasing ratios with increasing downwinddistances. The best fitting decay equations and their correspondingR2 values are listed in Table 3.

As shown in Fig. 5a and b, all pollutants concentration ratiosdecreased exponentially with increasing downwind distances ofthe freeway. For the gaseous species of CO and NO2, their concen-trations decreased by 70e80% within the first 100 m for bothfreeways. Particle-bound BC concentration dropped by 60% and 80%in the first 100 m for I-710 and I-5, respectively. Within 150 m, allpollutants concentrations reach asymptotically background levels.Similar observations have been reported extensively in the recentliterature (Zhu et al., 2002b; Baldauf et al., 2008a; Clements et al.,2009). As shown in Table 2, the decay coefficients of NO2 and BCfor I-710 were consistently higher than that for I-5, suggestinga faster decay of their concentrations near the I-710 freeway. Thismay be explained by their higher initial concentrations of thesepollutants at the I-710 freeway, which has a roughly 50% higher

Fig. 5. BC and gaseous pollutants normalized concentrations at different distance downwind of the freeway (a) I-710 no noise barrier (b) I-5 no noise barrier; (c) I-710 with noisebarrier; (d) I-5 with noise barrier.

Z. Ning et al. / Atmospheric Environment 44 (2010) 3118e31273126

traffic volume than the I-5 freeway. Other meteorological condi-tions and the local topography may also contribute to the decaycurves of air pollutants from the freeway (Zhu et al., 2004), butgiven the overall similarity in both of these sets of parametersbetween the two freeways, we attribute the faster decrease in theI-710 to the higher traffic volume in that freeway.

In contrast, the concentration ratios downwind of the freewaysections with roadside noise barriers displayed a different trend, asshown in Fig. 5c and d. At downwind distance of 15 m, the closestlocation downwind of the I-710, the pollutants concentration ratioswere 0.36, 0.28 and 0.22 for CO, NO2 and BC, respectively, compa-rable to the background levels measured at 400 m (0.33, 0.26, and0.28 for CO, NO2 and BC, respectively). The low concentration ratiosare consistent with the observations of particle number and massconcentrations, due to the strong turbulence that exists in therecirculation cavity of the roadside noise barrier (Finn et al., 2010).At 80e100 m, where the concentrations have dropped to back-ground levels for the non-barrier sites (Fig. 5a), the pollutantsdisplayed a peak ratio of 0.57, 0.59, and 0.67 for CO, NO2, and BC,respectively, as shown in Fig. 5c for the I-710 with barrier. Thedramatic difference of the pollutant concentration profiles down-wind of the freeway underscores the impact of roadside noisebarrier on pollutant dispersion. As the pollutants are transported

Table 3Concentration decay curve equation and coefficients (“x” is the distance downwindof the freeway from the stationary sampling station; “y” is the normalizedconcentration ratio).

NO2 BC CO

I-710 y ¼ 0.17 þ 0.83e(�0.16x)R2 ¼ 0.99

y ¼ 0.36 þ 0.63e(�0.05x) R2 ¼ 0.97

y ¼ 0.35 þ 0.65e(�0.12x) R2 ¼ 0.99

I-5 y ¼ 0.24 þ 0.76e(�0.11x); R2 ¼ 0.99

y ¼ 0.21 þ 0.77e(�0.03x) R2 ¼ 0.98

e

further away from freeways, their concentrations graduallydecrease and reach background concentrations at 400m and 250mfor I-710 and I-5, respectively. The results suggest that the freewayroadside features, such as noise barriers, should also be taken intoconsideration in assessing public exposure to ambient pollutantsfrom traffic emissions in the community nearby busy freeways.

4. Summary and conclusions

The present study investigated the evolution of particle sizedistributions and pollutants concentrations downwind of twomajor freeways (I-710 and I-5) sections in Southern California, bothfeatured with and without roadside noise barriers. The resultscorroborate those of earlier studies by showing that the particlenumber and pollutants concentrations decay exponentially nearfreeways without the roadside noise barriers. Particle numberconcentrations decrease sharply with distance from the freeway,especially for smaller (<30 nm) particles due to evaporation anddiffusion. The background pollutant concentrations are reachedwithin 150 m downwind of freeway without roadside barriers.

With the presence of roadside barrier, the dynamics of particleand co-pollutants dispersion change dramatically. A recirculationcavity is formed in the close vicinity downwind of the barrier, asobserved at 15 m in the present study, resulting in a concentrationdeficit zone in the lee of the barrier, where the particle numberconcentrations are 45e50% of those measured at similar downwinddistances of freeways without roadside barrier. The particle sizedistributions and co-pollutants concentrations were comparable tobackground levels. With the increasing downwind distance, parti-cles and gaseous co-pollutant concentrations increase and peak at80e100 m, where the plume of elevated traffic emissions sourcesreattaches to the ground. The particle size distribution displayeda sharp nucleation mode peak, with total number concentrations

Z. Ning et al. / Atmospheric Environment 44 (2010) 3118e3127 3127

1.9e2.2 times of those at similar distance near non-barrier freeways.Particle mass, CO, NO2 and BC also reached maximum concentra-tions ratios. The background particle and co-pollutants concentra-tions were reached at distances of 250e450m, farther than the sitesnear non-barrier freeways.

The much longer downwind distance needed to reach back-ground levels indicates a larger impact zone of traffic emissionsources near the freeways with roadside noise barriers. Our resultssuggest that freeway roadside features, such as noise barriers andplantation, should also be taken into consideration in assessingpopulation exposure to ambient particles and co-pollutants fromtraffic emissions.

Acknowledgements

We would like to acknowledge Dr. Katharine Moore for hersignificant contributions in the implementation of experiment.Also, we would like to thank Ali Attar, Kalam Cheung, for theirassistance in the field and data work, and the support of Universityof Southern California Provost’s Ph.D. fellowship. This study wasfunded by EPA under the STAR program through grant RD-8324-1301-0 and by California Air Resources Board through ARB Contact05-317 to the University of Southern California.

Appendix. Supporting information

Supporting information associated with this paper can be found,in the online version, at doi:10.1016/j.atmosenv.2010.05.033

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