Original Article | Published:

Ultrafine particle levels at an international port of entry between the US and Mexico: Exposure implications for users, workers, and neighbors

Journal of Exposure Science and Environmental Epidemiology volume 23, pages 289298 (2013) | Download Citation


Exposure to diesel-emitted particles has been linked to increased cancer risk and cardiopulmonary diseases. Because of their size (<100 nm), exposure to ultrafine particles (UFPs) emitted from heavy-duty diesel vehicles (HDDV) might result in greater health risks than those associated with larger particles. Seasonal UFP levels at the International Bridge of the Americas, which connects the US and Mexico and has high HDDV traffic demands, were characterized. Hourly average UFP concentrations ranged between 1.7 × 103/cc and 2.9 × 105/cc with a mean of 3.5 × 104/cc. Wind speeds <2 m s−1 and temperatures <15 °C were associated with particle number concentrations above normal conditions. The presence of HDDV had the strongest impact on local UFP levels. Varying particle size distributions were associated with south- and northbound HDDV traffic. Peak exposure occurred on weekday afternoons. Although in winter, high exposure episodes were also observed in the morning. Particle number concentrations were estimated to reach background levels at 400 m away from traffic. The populations exposed to UFP above background levels include law enforcement officers, street vendors, private commuters, and commercial vehicle drivers as well as neighbors on both sides of the border, including a church and several schools.


Exposure to diesel-emitted particles has been linked to pulmonary inflammation, increased susceptibility to respiratory infections, chronic obstructive pulmonary diseases, exacerbation of asthma, and increased risk of cancer.1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 In this regard, the US Environmental Protection Agency has diesel-emitted particles listed as a likely carcinogen, while the World Health Organization considers diesel-engine exhaust carcinogenic.12, 13, 14 Although, diesel-emitted particles denote particles of all sizes, there is reason to believe that exposure to ultrafine particles (UFPs) emitted from heavy-duty diesel vehicles (HDDV) might result in higher health risks than those associated with coarser particles.15, 16, 17 Because of their small size (<100 nm), UFP can evade human defense mechanisms, penetrate deep into the body, reach the bloodstream, and be distributed to potentially sensitive sites, such as bone marrow, lymph nodes, spleen, and heart.18, 19, 20, 21, 22 Particularly, UFP have been shown to impact the cardiovascular, pulmonary, and central nervous systems, even more so in compromised individuals.15, 23, 24, 25

Accurate characterizations of exposure conditions at both occupational and urban environments are necessary for the advancement of UFP health risk assessments. Particularly critical is the identification of settings of exposure of large populations to extreme UFP levels. Such scenarios are plausible in close proximity to dense traffic conditions. Especially near dense HDDV traffic as UFP emissions from these vehicles have been observed to be considerably greater than from light-duty gasoline vehicles.26 The International Bridge of the Americas (BOTA), as one of the busiest ports of entry between US and Mexico, has an elevated traffic demand and stringent security inspections, which result in long queues of idling vehicles on both sides of the border. A peculiarity of the BOTA, as compared with other ports of entry on the US/Mexico border, is that it has the largest combined traffic demand of privately owned (mostly light-duty gasoline) and commercially operated (mostly HDDV) vehicles.27 The combined traffic conditions at the BOTA are expected to induce UFP exposure on large numbers of private commuters and law enforcement officers. Furthermore, exposure to combined gasoline and diesel-engine emissions might produce amplified impacts to the cardiovascular system as compared with gasoline or diesel only exposures.28, 29, 30

In this study, UFP number concentrations at the BOTA were characterized. Specifically, the temporal variations of particle number concentrations (PNCs) and their associations with traffic and meteorological conditions were assessed, and exposure scenarios and populations at risk identified. Also, the specific size fractions associated with HDDV traffic were determined. The measurements for this study were performed as part of a comprehensive air quality characterization at the BOTA.31


Study Site

The BOTA is located near the geographic center of the border separating the El Paso, Texas, USA and Ciudad Juarez, Chihuahua, Mexico urban region (Figure 1a). Customs and immigration inspection areas as well as administrative offices are located at both the US and Mexican sides of the BOTA (Figure 1b). Five additional ports of entry operate within the urban region. The BOTA traffic demands of both commercial and private vehicles account for >50% of the regional total.27 It has been reported that 89% of the northbound commercial vehicle fleet at the bridge is composed of HDDV, whereas private traffic is mostly composed of light-duty gasoline-fueled vehicles.32 Considering that traffic is mostly composed of local commuters and drayage trucks, similar traffic fleet characteristics are expected for both north- and southbound traffic. The bridge is permanently open to private vehicle and pedestrian traffic. Northbound commercial traffic services are limited to 0600 hours to 1800 hours from Monday to Friday and from 0600 hours to 1400 hours on Saturdays. Southbound commercial lanes are open from 0800 hours to 2100 hours on weekdays and Saturdays. The bridge is closed for commercial traffic on Sundays.

Figure 1
Figure 1

Study site; (a) El Paso regional map with shaded areas representing mountains, (b) study site at the International Bridge of the Americas, with arrows indicating commercial traffic routes and shaded areas indicating inspection areas.

Study Period

Four measurement campaigns were conducted between December 2008 and September 2009. Each campaign lasted 2 weeks. The seasonal 2-week monitoring scheme has been shown to produce good estimates of annual averages for urban air pollutants.33, 34, 35 The monitoring dates and corresponding meteorological variables are listed in Table 1.

Table 1: Monitoring periods and corresponding meteorological summary.

Measurement Equipment

The monitoring site was located within a storm pumping station operated by the El Paso Water Utilities, at approximately 30 m from the traffic centerline and 80 m from the US customs inspection station (Figure 1b). Particle size distributions and number concentrations were measured with a Scanning Mobility Particle Sizer (SMPS) Model 3936-L75 (TSI, Shoreham, MN, USA) and an Aerodynamic Particle Sizer (APS) Model 3321 (TSI, Shoreham, MN, USA). The SMPS produced size distributions composed of 102 size bins for particle diameters between 6 nm and 225 nm. The APS produced size distributions composed of 52 size bins for particle diameters between 500 nm and 20 μm. The SMPS scan time was 120 s with a retrace of 30 s performed at 10-min intervals. The APS produced real-time measurements for 2 min at 10-min intervals. The instruments operated continuously during the measurement campaigns and were stopped periodically for quick maintenance (e.g., nozzle and impactor cleaning). Meteorological information recorded at a monitoring station (CAMS 41) located approximately 400 m (1/4 mile) from the BOTA was obtained from the Texas Commission on Environmental Quality website. Wind speed and direction were also measured on site with a portable AutoMet Model 466A (MetOne Instruments, Grants Pass, OR, USA). Yearly northbound traffic data were obtained from the University of Texas at El Paso Border Modeling Database. Daytime hourly crossing rates for both north- and southbound traffic were determined via manual counts from video recordings performed during 4 days per monitoring campaign. Nighttime crossing rates were not determined. Reduced video quality during the nighttime hours impeded identification of crossing vehicles. Furthermore, traffic queues were short during nighttime hours and were outside the recording angle. Video recordings did not include the area near inspection stations for security purposes.

Data Analysis

PNCs are reported in number of particles per cubic centimeter (No./cc). PNCs were processed as both 10-minute and 1-h averages. Some analyses were performed exclusively for downwind or upwind conditions relative to traffic. Traffic queues near the monitoring site are approximately parallel to the north–south orientation as shown in Figure 2b, allowing downwind conditions to be defined by an east wind direction (90°±45°) and upwind conditions by a west wind direction (270°±45°). The Pearson correlation coefficient was used to evaluate associations between variables. Time-dependent graphs were used to study pollutant peaks and diurnal trends. Principal component analysis was used to synthetize the 154-bin size-resolved particle data set into a reduced set of variables (principal components (PCs)) that capture independent variation between particle size ranges.36 The PCs were subsequently used to study the associations between specific particle size distributions, traffic, and meteorological variables. The analysis was done on the varimax rotated matrix.

Figure 2
Figure 2

Wind distribution during the study; (left) wind rose, (right) wind rose over study site.

Quality Assurance

The SMPS and the APS were calibrated by the manufacturer previous to the start of the study. Sampling flows and equipment performance parameters (e.g., voltage and laser intensity) were checked on a daily basis. The SMPS inlet impactor and the APS inlet nozzles were cleaned on a daily basis. Data was corrected for diffusion losses inside the SMPS by the instrument software.37


Traffic Characteristics

In 2009, the total northbound crossings at the BOTA were 4.7 million vehicles, of which 7.3% were commercial vehicles (Figure 3a). Ten years before, in 1999, the total northbound crossings were 8.5 million vehicles, of which 4.2% were commercial vehicles. The decrease of private vehicle crossings during the 10-year period was 45%, compared with a 4% decrease of commercial vehicles. Private traffic crossing rates decreased after 2001, coinciding with the implementation of stringent security measures by US law enforcement agencies. Private crossings increased after 2003 but decreased considerably again after 2008. Commercial traffic increased gradually from the mid 1990s until a noticeable decrease also in 2008. The 2008 total traffic decrease coincides with the start of a national economic recession, which impacted the region’s industry and commercial activity. Figure 3b shows the monthly northbound crossing rates for 2009. During that year, private traffic crossing rates were highest in August and lowest in November. Commercial traffic was lowest in February, increased gradually from June to October, and decreased afterwards. The percentage of total crossings represented by commercial vehicles was lowest in August (6.6%) and highest in October (8.3%). Future traffic trends at the BOTA cannot be determined from these results. Still, based on the substantial industrial activity of the region, elevated commercial traffic demands can be reasonably expected at the BOTA in the upcoming years.

Figure 3
Figure 3

Northbound traffic; (a) yearly crossing rates, (b) monthly crossing rates. Private traffic is plotted on right axis and commercial traffic on left axis.

Daytime hourly crossing rates by vehicle type and traffic direction are presented in Figure 4. Northbound private traffic was highest in the morning at around 0900 hours and remained above 700 vehicles per hour during the day. Southbound private traffic crossing rates increased from 0800 hours to 1200 hours and varied minimally until peaking at 1800 hours. South- and northbound private vehicle crossing rates were comparable between 1200 hours and 1600 hours. However, due to stringent inspections by US customs, northbound private traffic queues were constantly present, whereas southbound traffic moved rapidly and queue formation was intermittently observed. During daytime hours, routine inspections of southbound traffic by US law enforcement officials were observed to induced traffic queues towards the north of the study site. Private traffic weekend patterns were similar to those observed during weekdays, with the exception that on weekends northbound private traffic peaked in the afternoon at later hours than on weekdays.

Figure 4
Figure 4

Hourly vehicle crossing rates; (a) private traffic, (b) commercial traffic.

Commercial northbound traffic crossing rates peaked at 0800 hours and again at 1500 hours. Southbound commercial traffic crossing rates increased from 0800 hours to 1200 hours and peaked in the afternoon at 1900 hours. During weekdays, long southbound queues were common between 1700 hours and 1900 hours.

UFP Levels

Total PNC represents the measured size range between 6 nm and 20 μm. The UFP range (<100 nm) represented 93.9% (SD 5.4) of the total particle concentrations. The hourly average PNC at the BOTA ranged between 1.7 × 103/cc and 2.9 × 105/cc with a mean of 3.5 × 104/cc (SD 3.5 × 104). Seasonal and daily PNC variations are presented in Figure 5. Seasonally, particle concentrations were highest in winter and lowest in summer independent of wind direction (Figure 5a). Stable atmospheric conditions, common in winter, have been shown to inhibit dilution and affect the particle concentration gradients away from traffic.38 During the week, PNC peaked on Wednesdays, with comparable levels observed on Thursdays and Fridays. The lowest concentrations were observed on Sundays (Figure 5b). Bearing in mind that PNCs are strongly influenced by nearby sources,39 and that commercial traffic was absent on Sundays when lowest concentrations were observed, PNC appears to be strongly associated with commercial traffic.

Figure 5
Figure 5

Temporal variation of particle number concentrations; (a) by season, (b) by day.

Hourly PNC variations by season are shown in Figure 6. Overall, during weekdays, the average PNC increased rapidly in the morning between 1700 hours and 1900 hours (Figure 6). During the day, the overall averaged PNC varied minimally and peaked above 5 × 104/cc at 1800 hours. During winter, the hourly PNC variation had clear morning (0800 hours) and evening (1800 hours) peaks above 7 × 104/cc. During spring, the morning and evening peaks were also observed but at lower concentrations. The hourly variations during the fall were comparable with the overall average. The summer PNC was consistently lower than the overall average.

Figure 6
Figure 6

Hourly variation of particle number concentration by season.

The local background PNC was estimated as the average number concentration between 0200 hours and 0300 hours under upwind conditions. Local background estimates were considered a good approximation of actual values considering that: (a) between 0200 hours and 0300 hours traffic was minimal or absent, (b) the BOTA is mostly surrounded by parks, (c) the nearest major highway is more than a 1.3 km away, and (d) contributions from other sources are unlikely as UFP levels decay sharply away from sources.40 During the study, the estimated local background levels averaged 1.0 × 104/cc. Background levels varied minimally by season ranging between 1.3 × 104/cc and 0.9 × 104/cc, with the highest level observed in winter and lowest in fall (Figure 6). Figure 7 shows the hourly variations of PNC categorized by meteorological parameters. The impact of meteorology on PNC levels between 0200 hours and 0300 hours was minimal (Figure 7). Wind direction had the smallest effect on nighttime PNC, confirming the absence of a meaningful source at that time (Figure 7b).

Figure 7
Figure 7

Impact of meteorological parameters on hourly particle number concentrations; (a) wind speed, (b) wind direction, (c) temperature, and (d) relative humidity.

Wind Effects

During this study, downwind and upwind conditions represented 31.8% and 33.4% of the measurements, respectively (Figure 2). Calm conditions were observed during 0.18% of the measurements. High wind speeds were predominately associated with upwind conditions. Both wind speed and wind direction impacted PNC (Figure 7). As shown on Figure 2b, most particle measurements under downwind conditions (90°±45°) would be associated with emissions from vehicles in the north end of the BOTA, rather than those in the queue towards the south. Considering the low frequency of winds from the south and the minimal percentage of calm conditions (0.18%), the impact of queue length on the measurements was considered minimal. Wind speeds <2 m/s were associated with PNC above the average (Figure 7a). Expectedly, PNC were lowest under upwind conditions (west) and highest under downwind conditions (east) as shown in Figure 7b. To isolate the effects of wind speed from the effects of traffic, PNC averages were calculated for categorized wind direction and time period as shown in Figure 8. The daytime period was selected based on the presence of traffic (0600 hours– 0900 hours) while the nighttime period included the complementary hours. Independent of wind direction and the presence of traffic, PNC decreased as wind speed increased (Figure 8). Between 2005 and 2009, the wind speed measured near the BOTA at CAMS 41 was <2 m/s during 36% of the time.

Figure 8
Figure 8

Wind effect on particle number concentrations.

Temperature Effects

Previous studies have shown that ambient temperature affects particle concentrations.41, 42 During the study, ambient temperature varied between −1 °C (30 F) and 35 °C (95.5 F). Temperature impacted PNC considerably as shown in Figure 7c. Temperatures <15 °C (60 F) were usually associated with PNC above the average. The impact of temperature on PNC was comparable with that of wind speed, whereas relative humidity had a reduced impact on PNC (Figure 7d). Figure 9a shows the temperature variation by hour and season. The consistent diurnal temperature pattern across seasons facilitated the standardization of PNC by time segment and the isolation of the temperature effect on PNC (Figure 9b). The standardization consisted of subtracting the averaged PNC, for a specific time segment, from each PNC value and dividing over the corresponding SD. Hourly standardized PNC averages were categorized by temperature range as shown in Figure 9b. Again, a PNC above the mean was associated with temperatures <15 °C. Particle concentrations increase sharply as temperature decreases <15 °C but vary slightly at higher temperatures. It has been suggested that lower exhaust temperatures favor new particle formation particularly in the nuclei mode (<40 nm).42 Also low ambient temperatures have been observed to inhibit particle agglomeration and limit the decay of the particle plume.42 Furthermore, stable atmospheric conditions common during colder periods dampen dilution and extend the concentration gradients away from traffic. The temperature effect explains the higher averaged PNC observed in winter (Figure 5a). Between 2005 and 2009, the temperature at CAMS 41 was <15 °C during 42% of the time, but mostly during nighttime hours. During daytime hours, the temperature was <15 °C during 15% of the time.

Figure 9
Figure 9

Temperature effect on particle number concentrations; (a) diurnal temperature profile, (b) standardized PNC summarized by temperature category.

Particle Size Distributions

The average size distributions shown in Figure 10 were obtained by averaging PNCs by size bin for the respective time periods. The size distributions reflect the seasonal variation already observed in Figure 5a with highest levels observed in winter and lowest in summer. Overall the size distributions had one distinct mode with geometric mean diameter ranging between 15 nm and 30 nm. Single-mode size distributions were consistent throughout the year. The size distribution during nighttime hours was comparable among seasons, suggesting a minimal impact of traffic at this time. Within each season, the size distribution change minimally throughout the day. Between seasons, the size distributions had some noticeable differences. The size distributions in the spring and summer show higher fractions of the smallest particles (<15 nm). Such increase was more pronounced in the summer. This could be attributed to vehicle fleet characteristics as the percentage of commercial vehicles varied by season as previously discussed (Figure 3b). Also a decreased rate of coagulation due to a smaller particle size difference could have influenced the higher fraction of the smallest particles.43, 44 Considering the average temperature and humidity values presented in Table 1, the seasonal size distribution variation shown in Figure 10 agrees with the impacts of humidity and temperature on size distributions assessed by Zhu et al.45 in Los Angeles. However, because humidity is predominately lower in the semi-arid climate of El Paso, in this study temperature had a greater impact on particle size distributions, as compared with humidity in Los Angeles where the climate is sub-tropical.

Figure 10
Figure 10

Average particle size distributions by season and diurnal time periods.

Traffic Effects

The impact of diesel versus gasoline traffic was evaluated by studying the mean differences of PNCs between weekdays and Sundays. The comparisons were reasonable, because on Sundays commercial traffic was absent and private traffic patterns were similar to those observed on weekdays. The daily variation shown in Figure 5b illustrates the considerable drop of PNCs on Sundays as compared with weekdays. Averaged particle concentrations for weekdays and Sundays were 39,217/cc and 17,363/cc, respectively. For daytime hours (0600 hours–2100 hours), when commercial traffic is present during weekdays, averaged particle concentrations for weekdays and Sundays were 49,217/cc and 17,699/cc, respectively. The ratio of average PNC over local background levels for weekdays and Sundays were 4.8 and 1.7, respectively. Considering the independent increase of PNCs above local background levels (10,362/cc) induced by the presence of each type of traffic, the impact of commercial traffic is 4.3 times greater than that of private traffic. Exposure to UFP at the BOTA is considerably higher when commercial traffic is present.

By means of principal component analysis, the data set composed of 154 size bins was reduced to four PCs that explained 79.5% of the variability (Table 2). The factor loads and the reconstructed particle size distributions are shown in Figure 11. The factor loads represent the correlation between each variable (size bin) and the corresponding component. The size distributions associated with each PC were reconstructed by multiplying factor loads >0.6 by the SD of the PNC of the corresponding size bin.46 The PCs are ordered by percentage of explained variation according to statistical convention (see Table 2). Based on the reconstructed size distributions, the four components approximate nucleation (PC2; from 6 nm to 30 nm), ultrafine (PC4; from 15 nm to 100 nm), accumulation (PC3; from 50 nm to 450 nm), and fine (PC1; from 800 nm to 20 μm) particle size ranges (Figure 11b). The gaps between the four size distributions in Figure 11b represent the particle sizes that did not correlate strongly (load <0.6) with any component or were due to a measurement gap between 225 nm and 500 nm associated with the instrument’s detection limits. To determine the temporal variation of the components, factor scores were estimated using a linear regression approach.47 PC2 and PC4 cover the size range of the size distributions shown in Figure 10. By definition PCs are independent of each other. The principal component analysis captured the independent temporal variation of the particle size ranges represented by each component. Therefore, the independence of PC2 and PC4 suggests that UFPs might have been affected by two or more distinctive sources and/or physical phenomena during the study. Identifying the source of this distinctive variation is relevant if exposure reduction is to be undertaken via emission reduction strategies.

Table 2: Principal component analysis of the size-resolved particle number concentrations.
Figure 11
Figure 11

Size distributions of principal components; (a) factor loads, (b) reconstructed size distributions.

To further investigate the associations of the PCs against traffic, each component was characterized by averaging all measured values of a specific variable (e.g., traffic, number concentration) for which the factor score was above its 90th percentile and then normalizing by the overall average of that variable (see Table 3).46 Within each column, the variable with the highest value was considered to have the best association with the corresponding component.46 Southbound private traffic showed a slight association with both PC2 and PC4 components. Note that the lack of association of northbound private traffic with the PCs indicates that vehicle-crossing rates were not a proper surrogate of private vehicle emissions rather than the lack of an actual physical association. Northbound private vehicle crossings had minimal variation during daytime hours when heavy traffic was constantly present at the BOTA. Total commercial traffic associated with all for components but more strongly with PC2, which represents particles in the nuclei size range. The association of northbound commercial traffic with PC2 was also strong, whereas southbound commercial traffic associated strongly with PC4. Overall, the results indicate that PNCs at the BOTA are strongly associated with the presence of commercial traffic. However, it appears that emissions from northbound commercial traffic specifically have a strong and distinctive impact on number concentrations of the smallest particles. Distinctive UFP emission characteristics between commercial traffic might be associated with the vehicle load. Northbound commercial vehicles haul loaded trailers while southbound vehicles bring back a greater number of empty trailers.32

Table 3: Associations of principal components with traffic and particle concentrations.

Table 4 shows reported PNCs near dense traffic conditions in other US cities. Average particle concentrations at the BOTA were lower than those observed near two major highways in Los Angeles, CA.40, 48 The distances between the monitoring sites and traffic were comparable between most studies (30 m). Traffic flows at the BOTA were at least eight times less than the 12,000 vehicles per hour observed in Los Angeles.40, 48 However, PNCs at the BOTA were 4–5 times less than those observed in Los Angeles. Note that the water-based particle counters without a sheath flow design, as those used in Los Angeles, have been shown to underestimate vehicle-emitted PNCs, particularly for particles <20 nm.49 In Los Angeles, higher relative humidity was associated with higher PNCs.45 Also, traffic speeds were considerably distinct between studies. A drop in UFP concentrations with traffic slowdown conditions, indicating that fewer UFPs are emitted under such conditions, have been previously reported.40 Specifically, higher particle number emission rates from diesel engines under cruise driving cycles as compared with idling conditions have been measured.50 The driving cycles at the BOTA are mostly under idling and creep idling (<5 mph) conditions.32 Furthermore, at the BOTA the commercial vehicle fleet is mostly composed of older models used exclusively for drayage transport between Juarez and El Paso.32 Differences between PNCs and traffic flows measured at the BOTA and the highways in Los Angeles might be associated with distinctive traffic flows, driving conditions, fleet characteristics, and ambient conditions between studies.

Table 4: Reported ultrafine particle concentrations near traffic.

Local Impact

The customs and immigration workforce might be at highest risk as their occupational exposure extends through their work shifts, which have been reported to commonly exceed 12 h. The UFP concentrations observed at the monitoring site are a conservative estimate of the exposure levels expected at the inspection areas, which are closer to traffic. Higher exposure is expected at the commercial traffic inspection areas on both sides of the border. Private vehicle crossing times commonly extend beyond an hour. Commuters driving with open windows would be exposed to in-cabin levels at least as high as those observed at the monitoring site. Lower in-cabin exposures would be expected for those commuters driving with close windows. The filtering efficiency for UFPs of vehicle air conditioning fans has been observed to be approximately 50% and increased to 85% when operated in recirculation mode.51 Because north- and southbound sidewalks are closest (<10 m) to commercial traffic lanes and particle concentrations increase exponentially near traffic,40 pedestrian commuters might be exposed to particle levels considerably greater than those observed during this study. In 2009, northbound pedestrian crossings were above 2500 per day. Furthermore, street vendors might be exposed to the highest levels as they usually move in between vehicles in close proximity to vehicle exhaust systems.

Peak 10-minute exposures at the BOTA were observed above 7.0 × 105/cc, which are comparable to the peak exposures above 5.0 × 105/cc reported in settings where soldering, welding, and plasma-spraying processes occurred.52, 53, 54 The health impact of severe acute exposure to UFP levels remains undetermined. Still peak UFP exposures near dense urban traffic at the BOTA are comparable with the severest occupational exposures.

Neighborhood Impact

Ambient UFP levels measured in 1999 at CAMS 41, which is located approximately 400 m away from the BOTA, were of 14,600/cc.55 Supplemental, daytime measurements were performed from August 14 through August 16, 2012 at CAMS 41. The hourly PNC averages ranged between 0.7 × 104/cc and 1.7 × 104/cc, and averaged 1.2 × 104/cc. The averaged PNC under upwind conditions, which constituted 67% of the measurements, was 1.1 × 104/cc. The daytime UFP concentrations are in the same range as that of the estimated local background levels and are comparable with the values measured at this site in 1999. Considering that traffic was and still is the major source of UFP near the BOTA, that PNC subside rapidly in short distances from dense traffic,40 and that background concentrations are not expected to vary drastically over time, the concentrations observed at CAMS 41 in 1999 are considered to be close to the local background. In this regard, UFP exposures above background levels can be realistically expected within distances of 400 m from the traffic centerline. Figure 12 shows the region near the BOTA, where particle number concentrations above background levels are expected. On the US side, a public park (<50 m), an elementary school (<50 m), a church (<50 m), and a high school (<300 m) are all within the 400 m buffer zone of the BOTA traffic queues. On the Mexican side, a public park (<50 m), a sports recreation facility (<50 m), a high school (<100 m), and a university campus (<300 m) are also within the buffer zone. The UFP exposure of the populations at the above-mentioned locations might be considerably above background levels.

Figure 12
Figure 12

Areas of expected ultrafine particle exposure above background levels.


The hourly average UFP number concentrations at the BOTA ranged between 1.7 × 103/cc and 2.9 × 105/cc with a mean of 3.5 × 104/cc. During the study, the estimated background levels were 1.0 × 103/cc. Meteorological conditions had a significant impact on particle concentrations. PNCs increased during colder weather periods and decreased as wind speed increased. More specifically, PNCs increased for temperatures <15 °C and wind speeds <2 m/s. Between 2005 and 2009, daytime temperature near the BOTA was <15 °C during 15% of the time, while wind speed was <2 m/s during 36% of the time. Commercial traffic, which is mostly composed of HDDV, strongly influenced UFP concentrations in the vicinity of the BOTA. On Sundays when commercial traffic was absent, the UFP number concentrations were the lowest. Northbound commercial traffic had a strong and distinctive impact on number concentrations for particles in the nucleation size range. Southbound commercial traffic was also associated with UFP concentrations but with a size distribution dominated by larger particles. At the BOTA, traffic flows were at least eight times less than those observed near highways in Los Angeles. Yet, PNCs at the BOTA were only 4–5 times less than those observed in Los Angeles. Exposures to UFPs near dense idling traffic conditions, such as those at the BOTA, and in semi-arid conditions such as those in El Paso are different than those near highways in Los Angeles. Published UFP concentration gradients near highways and under dense traffic conditions are useful as part of exposure assessment protocols. However, exposure assessments to UFPs near dense traffic should take into consideration differences in: (a) total traffic flows, (b) fractions of heavy-duty diesel truck, (c) average vehicle speed, (d) fleet characteristics, and (e) ambient meteorological conditions.

The populations in close proximity of the BOTA-induced traffic buffer zone (including immigration, customs and law enforcement officers, street vendors, private commuters, and commercial vehicle drivers) are exposed to UFPs considerably above the background level. In addition, neighbors at a local church and several schools on both sides of the border are susceptible to UFP exposures well above the background level.


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This project was supported by Award Number A-08-4 from the Southwest Consortium For Environmental Research and Policy, by Award Number 3P20MD002287-05S1 from the National Institute on Minority Health and Health Disparities and the Environmental Protection Agency, and by Award Number S11 ES013339 from the National Institute of Environmental Health Sciences (NIEHS). The content is solely the responsibility of the authors and does not necessarily represent the official views of the Hispanic Health Disparities Research Center, the National Institute on Minority Health and Health Disparities or the National Institutes of Health, or the Environmental Protection Agency.

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  1. Hispanic Health Disparities Research Center, Center for Environmental Resource Management, University of Texas at El Paso, El Paso, TX, USA

    • Hector A Olvera
  2. Civil Engineering Department, University of Texas at El Paso, El Paso, TX, USA

    • Mario Lopez
  3. University of Texas at El Paso, Civil Engineering Department, El Paso, TX, USA

    • Veronica Guerrero
  4. Instituto Tecnologico de Estudios Superiores de Monterrey, Ciudad Juarez, Mexico

    • Humberto Garcia
  5. University of Texas at El Paso, Civil Engineering Department, El Paso, TX, USA

    • Wen-Whai Li


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The authors declare no conflict of interest.

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Correspondence to Hector A Olvera.

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