The effects of roadside structures on the transport and dispersion of ultrafine particles from highways

TL;DR: In this paper, the effects of roadside barriers on the flow patterns and dispersion of pollutants from a high-traffic highway in Raleigh, North Carolina, USA were examined using the Quick Urban & Industrial Complex (QUIC) model.

About: This article is published in Atmospheric Environment.The article was published on 2007-12-01 and is currently open access. It has received 147 citations till now. The article focuses on the topics: Atmospheric dispersion modeling & Air pollutant concentrations.

Summary

1. Introduction

• Several air quality monitoring studies have measured elevated concentrations of a number of air pollutants near roadways including ultrafine particles (UFP, aerodynamic diameter o0.1 mm) (Zhu et al., 2002a, b; Sapkota and Buckley, 2003; Kittelson et al., 2004).
• These barriers are common features of high-traffic roadways, particularly those which run through populated areas.
• These models are simple Gaussian plume models, and do not explicitly simulate the complex flows around individual structures.

2. Modeling approach

• Characterizing the dispersion in the near road environment requires the use of modeling tools capable of resolving complex flow patterns induced by roadside barriers.
• Within QUIC, the complex geometry of the site was modeled using solid non-permeable simple shapes (blocks and cylinders).
• Upwind of the study domain consisted of relatively flat and uniform single-story buildings at a slightly lower elevation than the highway.
• Several line sources were used since QUIC does not simulate rectangular block volume sources, and does not include vehicleinduced turbulence.
• Within each grid cell, the authors computed the time-averaged concentration for 900 s (after a 300 s time period to reach an equilibrium state).

3. QUIC model results

• The QUIC simulations showed the influence of the roadside barriers on the airflow and pollutant dispersion patterns.
• This location was chosen because the concentrations were quite high, and should favorably compare with the mobile measurements since the emissions from the highway were just moving a short distance across flat, open terrain.
• Each vertical cross-section represents the median cross-wind concentration modeled for that portion of the domain.
• The concentrations for the barrier-only and fieldsite simulations were approximately the same as the base simulation in the open area near the road (from X ¼ 400 to 690m) (Fig. 3).
• Enhanced concentrations were predicted by QUIC where the noise barrier ends (at about X ¼ 350m), suggesting that plume material from the front of the barrier was moving laterally and being swept downwind at the edge of the barrier.

4. Comparison of model estimates with observations

• Observations from mobile measurements collected at the site were compared with QUIC model predictions to evaluate roadside structure effects and evaluate model results.
• The DMAs were operated at 10Lmin 1 sheath flow rate and 1Lmin 1 sample flow rate.
• The particle counts were converted to number concentrations using the charging efficiency for the particles at that size.
• Similar gradients were identified in the mobile measurements as in the QUIC results (Figs. 6b, 7b and c), though minor differences were seen in the absolute magnitudes and rates of decay with distance.
• In general, the measured concentrations were highest in the open area directly adjacent to I440, while the concentrations in the lee of the noise barrier at an equivalent distance from the roadway were 60% of the concentrations in the open area (Fig. 6b).

5. Summary

• At the local-scale, features such as noise barriers, trees, and buildings can have dramatic effects on the initial dispersion of pollutants from roadways, influencing concentrations up to several hundred meters from the road.
• For winds perpendicular to the roadway, under neutral stability atmospheric conditions, noise barriers appeared to influence dispersion patterns in three ways.
• Third, when the elevated plume encountered other downwind obstacles (e.g., trees or buildings), increased mixing occurred leading to decreased pollutant concentrations.
• The comparisons suggested that QUIC adequately reproduced the complex flow and dispersion patterns around the roadside structures, demonstrating potential value as a diagnostic tool for this application.
• Further evaluation of this model will likely be necessary before using this model in regulatory and urban planning applications.

Acknowledgments

• This work reflects the collaboration of many individuals working with EPAs near-road program.
• In particular, the authors appreciate the direction and support of Dr. Dan Costa, US EPA’s national program director for air, for leading the near-road ARTICLE IN PRESS G.E. Bowker et al. / Atmospheric Environment 41 (2007) 8128–81398138 research program.
• The research presented here was performed under the Memorandum of Understanding between the US Environmental Protection Agency (EPA) and the US Department of Commerce’s National Oceanic and Atmospheric Administration (NOAA) under Agreement number DW13921548.
• It does not necessarily reflect their policies or views.
• The US Government’s right to retain a nonexclusive royalty-free license in, and to any copyright is acknowledged.

Figures

Fig. 4. Elevation view showing flow streamlines for the ‘‘barrier only’’ c downwind of the sound barrier.

Fig. 5. Vertical alongwind sections showing the median cross-wind concentration behind the barrier as a function of downwind distance for (a) the base, (b) barrier-only, and (c) field site simulations. The line sources are at X ¼ 20 and 0m. The barrier is at X ¼ 12m.

Fig. 1. Aerial map of the near-road field study site along I-440 in Raleigh, North Carolina showing locations of mobile measurements (teal) using a GPS sensor.

Fig. 6. Normalized cross-wind concentration profiles (at 3-m) taken along the access road parallel to I-440 (median values from Y ¼ 14 to 34m) for (a) the three different QUIC simulations (base, sound barrier only, and field site), and (b) the mobile measurements for two classes of particulates (75 nm, black and 20 nm, yellow diamonds) over a 2-h period compared with the QUIC simulation of the field site.

Fig. 2. Plan view of the QUIC modeling domain. The legend indicates the height of the structures.

Fig. 3. Plan view showing horizontal concentration patterns simulated in QUIC at a height of 1m for the three simulations (a) base case, (b) sound barrier only, and (c) the field site.

Frequently asked questions

Q1. What contributions have the authors mentioned in the paper "The effects of roadside structures on the transport and dispersion of ultrafine particles from highways" ?

This study examined the effects of roadside barriers on the flow patterns and dispersion of pollutants from a high-traffic highway in Raleigh, North Carolina, USA. Model simulations were compared with the spatial distributions of ultrafine particles ( UFP ) from vehicular emissions measured using a passenger van equipped with a Differential Mobility Analyzer/Condensation Particle Counter. Comparison of the QUIC model with the mobile UFP measurements indicated that QUIC reasonably represented pollutant transport and dispersion for each of the study configurations. 

Q2. What is the effect of wind perpendicular to the barrier?

Wind perpendicular to the barrier may lead to an upward deflection of air flow caused by the structure, which could increase the apparent release height of the pollutant and increased vertical mixing due to the flow separation at the top of the barrier (Lidman, 1985). 

Q3. What was the effect of the elevated plume on the pollutant concentrations?

when the elevated plume encountered other downwind obstacles (e.g., trees or buildings), increased mixing occurred leading to decreased pollutant concentrations. 

Q4. What is the purpose of this study?

The objective of this study was to explore the effects of roadside obstacles on the near-field dispersion patterns of traffic emissions. 

Q5. How did the researchers study the effect of a barrier on air quality?

The authors used two independent methods to investigate the effect of a barrier on pollutant concentrations with wind perpendicular from the road: fine-scale numerical modeling and direct measurements of UFP using a mobile monitor. 

Q6. What is the number of particles emitted by gasoline and diesel vehicles?

The number of particles emitted by gasoline and diesel vehicles occurs primarily in the UFP size range, so the occurrence of high concentrations of these particles near the road likely represents primary combustion emissions from motor vehicles on that road. 

Q7. Where did the QUIC model predict the concentrations of plume material?

Enhanced concentrations were predicted by QUIC where the noise barrier ends (at about X ¼ 350m), suggesting that plume material from the front of the barrier was moving laterally and being swept downwind at the edge of the barrier. 

Q8. Why did the concentration field vary over the study period?

Because the concentration field varied not only spatially but temporally, the same route was traversed multiple times during the study period, with each route taking generally 10min to complete. 

Q9. How did the traffic activity on I-440 differ over the 10-min sampling period?

Since traffic activity on I-440 did not significantly vary over the 10-min sampling period for each route, the authors assumed that emission rates during the measurement time periods were relatively constant. 

Q10. What was done to ensure that the flow was not blocked?

Care was taken to leave openings between the vegetation blocks to ensure that flow was ‘‘disturbed’’ (with enhanced vertical and lateral mixing) rather than blocked. 

Q11. How long did it take to reach an equilibrium state?

Within each grid cell, the authors computed the time-averaged concentration for 900 s (after a 300 s time period to reach an equilibrium state). 

Q12. Why were the concentrations in the open terrain base simulation so high?

The highest and most-extensive concentrations were seen in the open terrain base simulation, due tothe lack of vertical mixing and dispersion of the plume. 

Q13. What is the average concentration of the three different QUIC simulations?

7. Normalized concentrations as a function of downwind distance (at 3m) for: (a) the three different QUIC simulations (base, sound barrier only, and field site); (b) the mobile measurements in the open area and the QUIC model for the base case; and, (c) comparison between mobile measurements and the QUIC model for the field site in the region downwind of the sound barrier in the residential neighborhood. 

Q14. What are the main sources of the QUIC simulations?

Several line sources were used since QUIC does not simulate rectangular block volume sources, and does not include vehicleinduced turbulence.