## Air Pollutant Concentrations near Texas Roadways: Chemical Transformation of Pollutants

Extended Abstract #395

Andrea Clements 1 , Yuling Jia 1 , Matthew P. Fraser 1,2 , Yifang Zhu 3 , Jayanth Pudota 3
, Allison DenBleyker 4 , Edward Michel 4 , Donald R. Collins 5 , Elena McDonald-Buller 4 , and David T. Allen 4

1 Rice University, Department of Civil and Environmental Engineering, Houston, TX 77005
2 Arizona State University, School of Sust ainability, PO Box 873211, Tempe, AZ 873211
3 Texas A&M University-Kingsville, Depa rtment of Environmental Engineering, 700 University Blvd. MSC 213, Kingsville, TX 78363
4 University of Texas, Center for Energy & Environment Resources, 10100 Burnet Rd, Mail Code R7100, Austin, TX 78758 5 Texas A&M University, Department of Atmospheric Sciences, 1204 Eller O&M Bldg., 3150 TAMU, College Station, TX 77843



### INTRODUCTION

Understanding the public health impact of roadway emissions neces sitates knowledge of pollutant concentration pr ofiles near roadways. Studies in Los Angeles 1 and other cities around the world 2,3 found elevated levels of pollutants near roadways which fell to background levels exponentially with increased distance fr om the roadway. The Texas Commission on Environmental Quality sponsored a team led by the University of Texas at Austin, and including researchers at Rice Universi ty, Texas A&M University, Co llege Station, and Texas A&M University, Kingsville, to examine the concen trations of vehicular emitted pollutants as a function of distance from Texas roadways. Air po llutant concentrations we re studied near three types of roadways in Austin, Texas: (1) an ar terial highway, dominate d by passenger vehicles (Highway 71), (2) a limited access highway (I-35), and (3) a surface highway dominated by truck traffic (FM-973). Air pollu tants examined include ultrafine particles (UFPs, diameter < 100 nm), carbon monoxide (CO), oxides of nitrogen (NO x ), volatile organic compounds (VOCs), several particle-bound organics (PAHs, alkane s, hopanes), several car bonyl species, and PM 2.5 mass and carbon content.

The companion paper for the study focused on th e observed behavior of UFP decay in the vicinity of the three roadways. This paper exam ines a more comprehensive set of pollutant data for one of the three roadways, Highway 71, s hown in Figure 1, and examines the chemical interactions associated with the observed fall- off measurements for gas phase pollutants and particle-bound organic species.

1Figure 1. Location and configuration of the Hi ghway 71 sampling site, southeast of Austin, Texas.

Upwind Site Downwind Site 1 Downwind Site 2

Mobile Trailer



### SAMPLING METHODOLOGY

Site Description and Roadway Characterization Data collection on Highway 71 consisted of thre e fixed-location measurements and one mobile trailer, shown in Figure 1. St ationary trailers were locate d at 80 m upwind, 35 m downwind, and 65 m downwind of the center of the road. The surrounding terrain is flat , open grassland. The roadway at the sampling location had two lanes in each direction.

Fleet composition for Highway 71 was observed dur ing a 1-hour video sample between 4:15 and 5:15 pm on April 13, 2007; cars/jeeps and light-d uty trucks comprised approximately 53% and 45% of the traffic, respectively. Thus, Highway 71 can be classified as an arterial highway dominated by passenger vehicles. Figure 2 shows the fleet composition, separated by inbound/outbound direction during the sampling hour. The outbound direction is away from the Austin urban core (to the southeast in Figure 2). The number of inbound cars and jeeps is less than half of those traveling in the outbound direction.

Traffic speeds were not systematically measur ed during the study but one observational pass by a research team member at 11:35 a.m. on Apr il 13, 2007 recorded outbound speeds of 70 mph and inbound speeds of 62 mph. During this observati on the driver matched the speed of other vehicles passing the sampling location and noted the speedometer reading. The highway did not have any stop lights in close proximity to the sampling site to cause b ack-up queues. Several private driveways connected to this segment of Highway 71. Sport utility vehicles, vans and 4- wheel trucks were counted as light-duty trucks. Vehicles with two axles and six tires (i.e. four

2tires on the rear axle) we re counted as medium-duty trucks. Heavy-duty trucks were identified as trucks with three or more ax les, or other industrial trucks.

Figure 2. Afternoon fleet composition on Highway 71.

401

1 8 12 5 405
0 13 15 7 638 837

0 200 400 600 800 1000 1200 1400

Cars & Jeeps SUVs, Vans & 4-wheel trucks Motorcycle 6-wheel trucks 18-wheelers & industrial Dump trucks
Vehicle Type# Count Outbound

Inbound



### Measurements The majority of measurements were obtained on April 12 and 13, 2007, w ith the exception of CO and NO x mobile measurements. Stationary integr ated particle and gas phase measurements were collected at three stationary trailer locations. Low volume PM 2.5 samples were collected in four-hour increments, representing morning, mid- day, and evening rush hour, onto Teflon filters using R&P sequential air sample rs sampling at 16.7 liters min -1 . Samples were used to obtain PM 2.5 mass concentrations and were subsequently extracted to obtain ion concentration data. During the same time intervals, high volume PM 2.5 samples were collected at 1132 liters min -1 on quartz fiber filters. Samples were used to obtain total elemental and organic carbon concentrations and portions were also extracte d to obtain detailed info rmation about particle bound organic species concentrations. Low volume ga s phase samples were collected at 2 liters min -1 in six hour increments roughly representing mo rning and afternoon on 2,4- dinitrophenylhydrazine (DNPH) coat ed PUF cartridges which were extracted to obtain gas phase carbonyl concentrations.

Mobile semi-continuous measurements of carbon monoxide (CO) and oxides of nitrogen (NOx) were collected upwind and as a function of distance downwind of Highway 71. CO was measured using an Ecotech CO Trace Analyzer. A Teledyne NO/NO 2 /NO x monitor measured NO and NO x , and by calculation NO 2 . The mobile measurements of CO and NO x were collected between 4:30 and 6:30 pm on June 11, 2007. The CO and NO x monitors sampled continuously with data collection by a Zeno data logger that averaged the concentrations measurements over 5-minute intervals.

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### Configuration of the Mobile Trailer A 1999 Chevrolet Suburban was used as the mobile monitoring platform to gather fall-off data for CO and NO x . A steel grate was attached to the vehicle trailer hitch to carry a small generator, which powered an uninterruptible power supply (U PS) inside the Suburban. A Zeno data logger unit and CO and NO x analyzers drew power from this UPS and all devices were secured by placing them on the floor of the vehicle on foam pads, which prevented sliding and minimized vibrations when moving between mobile sample point locations. Tubing was run from the instrument inlets through a gap in the pa ssenger window up through a PVC mounting device. The base of the mounting device was a magnet that anchored to the roof of the front of the Suburban. Sample line ran up the length of the PVC pipe and was positioned such that it was open to ambient air. The tube openings faced the roadway and drew in air 3 meters above ground level. The tailpipe of the Suburban a nd generator output on the back platform were further downwind than the sample intakes as th e magnetic mount was slid forward on the vehicle roof and the front of the vehi cle faced the roadway, leaving th e emissions of the Suburban and generator to discharge downwind of these intakes. As with the stationary trailer measurements, air sampled by the mobile platform was monito red continuously and recorded as five-minute averages. For this reason, the data logger’s inte rnal clock was synchronized with the time-keeper who noted times when the mobile platform chan ged distances from the roadway. The mobile platform was stationary at each sampling point near the roadway for at least 10 minutes in order to obtain two measurements. Measurements duri ng the five minute intervals between transitions in sample locations were discarded, as these data could be influenced by vehicle exhaust entering the sampling inlets due to vehicle turbulence.



### RESULTS

Mobile measurements for gas phase pollutants illustrate that pollutants generally exhibit exponential decay characteristics. Figures 3 and 4 show the decay profiles of CO, NO, NO 2 , and NO x with increasing distance from Highway 71. These data have been normalized by subtracting the upwind concentration, but not se gregated for wind direction. Each data point represents an average concentration during a 10-minute sampling period at each location from Highway 71.

Carbon monoxide concentra tions (Figure 3) decreased comparably to total NO x concentrations (Figure 4) with increasing distance from the roadway. Concentrations of NO and NO 2 indicate that NO is converting to NO 2 via reaction with ozone (O 3 ) on the length scale of total NO x falloff. Both NO and total NO x appear to decay exponentially, but NO 2 decays less rapidly because it is chemically generated. Consiste nt with studies of ve hicle engine emissions 4 , NO was the dominant component of NOx near Highway 71.

4Figure 3. Normalized CO concentration downwind of Highway 71

y = 191.6e -0.0278x

R 2 = 0.9818

0.00 20.00 40.00 60.00 80.00 100.00 120.00 140.00

0 20 40 60 80 100 120 140 160
Downwind distance (m)CO (ppb)

Figure 4. Normalized concentrations of NO, NO 2 , and NO x downwind of Highway 71.

y = 15.89e -0.03x R2 = 0.985

y = 10.62e -0.0152x
R 2 = 0.816

0 2 4 6 8 10 12

0 20 40 60 80 100 120 140 160

Downwind distance (m)NOx (ppb) NO
NO2
NOx NO exponential NOx exponential

Fine particulate mass concentrations measured at the three stationary tra ilers are shown in Figure 5 for Highway 71. These data show falloff behavi or similar to the gas-phase species CO and NO x . Error bars represent a 95% confidence inte rval for the figures shown below. Although a full profile curve cannot be produced from the three stationary m easurements as it was in the mobile sampling of CO and NOx, the trend is st ill evident; the trailer 65 m downwind of the roadway exhibited lower concentrations than the trailer 35 m downwind for most pollutants.

Particle bound organic compounds were analyzed for 10 polycyclic aromatic hydrocarbon (PAH) compounds. C oncentration profiles for all PAH com pounds generally showed classic falloff behavior with low concentrations upwind of th e roadway, the greatest concentration immediately downwind of the source, and decreasing concentr ations further downwind from the roadway. For this class of compounds, it was clear that the roadway was the dominant source of the pollutants because upwind concentrations were so low.

5Figure 5. Concentration of PM 2.5 mass concentrations near Highway 71.

0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0

-100 -50 0 50 100
Distance from Roadway (m)Ambient Mass Concentration ( μg m-3)

Figure 6 shows the spatial concentration prof ile for C28, a representative normal alkane compound. Alkane behavior differs from the previous pollutants in Figures 3-5 in that the lowest concentration was observed imme diately downwind. The nearly equivalent upwind and far downwind concentrations of C28 on Highway 71 suggest that roadway emissions are not the only source of the species. Possible explanati ons of a reduced concen tration at the nearest downwind sample point could be increased oxidation from othe r constituents of vehicular exhaust or dilution from increased mixing due to vehicle turbulence.

Ambient concentrations of two of the measur ed carbonyls are presented in Figure 7. Near Highway 71, formaldehyde shows decreasing concen trations with increas ing distance downwind of the roadway. Acetaldehyde (C2 aldehyde) increases in concentration at the far downwind site. One possible explanation might be the formatio n of acetaldehyde by atmospheric chemistry from the reaction of primary emi ssions from motor vehicles.



### SUMMARY

This on-going analysis examines concentration prof iles in the vicinity of an arterial highway dominated by passenger vehicles. Falloff results imply that vehicular emitted pollutants have a range of influence within 150 m of the roadway.

6Figure 6. Concentration of C28 Alkane for Highway 71.

0.0 5.0 10.0 15.0 20.0 25.0

-100 -80 -60 -40 -20 0 20 40 60 80
Distance from the Roadway (m)C28 Concentration (ng m-3)

Figure 7. Concentration profiles of two representative carbo nyls: (a) formaldehyde and (b) acetylaldehyde.

0.0 200.0 400.0 600.0 800.0 1000.0 1200.0 1400.0 1600.0

-100 -80 -60 -40 -20 0 20 40 60 80
Distance from Roadway (m)Formaldehyde Concentration (ng m-3) (a)

70.0 500.0 1000.0 1500.0 2000.0 2500.0 3000.0 3500.0

-100 -80 -60 -40 -20 0 20 40 60 80
Distance from Roadway (m)Acetaldehyde Concentration (ng m-3) (b)



### REFERENCES

1. Zhu, Y.F., Hinds, W.C., Kim,S., Shen, S. and Siout as, C. Study of ultrafine particles near a major highway with heavy-duty diesel traffic. Atmospheric Environment 2002

, 36 , 4323- 4335. 2. Gramotnev, G. and Ristovski, Z. Experimental investigation of ultra-fine particle size distribution near a busy road. Atmospheric Environment 2004

, 38 , 1767-1776. 3. Reponen, T., Grinshpun, S.A., Trakumas, S., Ma rtuzevicius, D., Wang, Z., LeMasters, G., Lockey, J.E. and Biswas, P. Concentration gr adient patterns of aer osol particles near interstate highways in the Gr eater Cincinnati airshed. Journal of Environmental Monitoring 2003

, 5 , 557-562. 4. Fritz, A. and V. Pitchon. Review of the current state of research on automotive lean NO x catalysis. Applied Catalysis B: Environmental

. 1997

, 13 , 1-25.



### ACKNOWLEDGEMENTS

This work was supported by Texas Commissi on on Environmental Quality under contract number 582-4-56385.

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