Air Pollutant Concentrations near Texas Roadways: 

Chemical Transformation of Pollutants 

 

Extended Abstract #395 

Andrea Clements1

, Yuling Jia1, Matthew P. Fraser1,2, Yifang Zhu3, Jayanth Pudota3, 

Allison DenBleyker4, Edward Michel4, Donald R. Collins5, Elena McDonald-Buller4, and 

David T. Allen4

1 Rice University, Department of Civil and Environmental Engineering, Houston, TX 77005

2

Arizona State University, School of Sustainability, PO Box 873211, Tempe, AZ 873211 

3 Texas A&M University-Kingsville, Department 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 necessitates knowledge of 

pollutant concentration profiles near roadways. Studies in Los Angeles1 and other cities around 

the world2,3 found elevated levels of pollutants near roadways which fell to background levels 

exponentially with increased distance from the roadway. The Texas Commission on 

Environmental Quality sponsored a team led by the University of Texas at Austin, and including 

researchers at Rice University, Texas A&M University, College Station, and Texas A&M 

University, Kingsville, to examine the concentrations of vehicular emitted pollutants as a 

function of distance from Texas roadways. Air pollutant concentrations were studied near three 

types of roadways in Austin, Texas: (1) an arterial highway, dominated by passenger vehicles 

(Highway 71), (2) a limited access highway (I-35), and (3) a surface highway dominated by 

truck traffic (FM-973). Air pollutants examined include ultrafine particles (UFPs, diameter < 

100 nm), carbon monoxide (CO), oxides of nitrogen (NOx), volatile organic compounds (VOCs), 

several particle-bound organics (PAHs, alkanes, hopanes), several carbonyl species, and PM2.5 

mass and carbon content. 

The companion paper for the study focused on the observed behavior of UFP decay in the 

vicinity of the three roadways. This paper examines a more comprehensive set of pollutant data 

for one of the three roadways, Highway 71, shown in Figure 1, and examines the chemical 

interactions associated with the observed fall-off measurements for gas phase pollutants and 

particle-bound organic species. 

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Figure 1. Location and configuration of the Highway 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 three fixed-location measurements and one mobile 

trailer, shown in Figure 1. Stationary trailers were located 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 during a 1-hour video sample between 4:15 and 

5:15 pm on April 13, 2007; cars/jeeps and light-duty 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 measured during the study but one observational pass by a 

research team member at 11:35 a.m. on April 13, 2007 recorded outbound speeds of 70 mph and 

inbound speeds of 62 mph. During this observation 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 back-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 

2
tires on the rear axle) were counted as medium-duty trucks. Heavy-duty trucks were identified 

as trucks with three or more axles, or other industrial trucks. 

Figure 2. Afternoon fleet composition on Highway 71. 

401

1

12 8 5

405

0 7 15 13

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, with the exception of 

CO and NOx mobile measurements. Stationary integrated particle and gas phase measurements 

were collected at three stationary trailer locations. Low volume PM2.5 samples were collected in 

four-hour increments, representing morning, mid-day, and evening rush hour, onto Teflon filters 

using R&P sequential air samplers sampling at 16.7 liters min-1. Samples were used to obtain 

PM2.5 mass concentrations and were subsequently extracted to obtain ion concentration data. 

During the same time intervals, high volume PM2.5 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 extracted to obtain detailed information about particle 

bound organic species concentrations. Low volume gas phase samples were collected at 2 liters 

min-1 in six hour increments roughly representing morning and afternoon on 2,4-

dinitrophenylhydrazine (DNPH) coated 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/NO2/NOx monitor measured 

NO and NOx, and by calculation NO2. The mobile measurements of CO and NOx were collected 

between 4:30 and 6:30 pm on June 11, 2007. The CO and NOx 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 NOx. A steel grate was attached to the vehicle trailer hitch to carry a small generator, 

which powered an uninterruptible power supply (UPS) inside the Suburban. A Zeno data logger 

unit and CO and NOx 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 passenger 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 and generator output on the back platform were 

further downwind than the sample intakes as the magnetic mount was slid forward on the vehicle 

roof and the front of the vehicle faced the roadway, leaving the 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 monitored continuously and recorded as five-minute 

averages. For this reason, the data logger’s internal clock was synchronized with the time-keeper 

who noted times when the mobile platform changed 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 during 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, NO2, and 

NOx with increasing distance from Highway 71. These data have been normalized by 

subtracting the upwind concentration, but not segregated for wind direction. Each data point 

represents an average concentration during a 10-minute sampling period at each location from 

Highway 71. 

Carbon monoxide concentrations (Figure 3) decreased comparably to total NOx concentrations 

(Figure 4) with increasing distance from the roadway. Concentrations of NO and NO2 indicate 

that NO is converting to NO2 via reaction with ozone (O3) on the length scale of total NOx

falloff. Both NO and total NOx appear to decay exponentially, but NO2 decays less rapidly 

because it is chemically generated. Consistent with studies of vehicle engine emissions4, NO 

was the dominant component of NOx near Highway 71. 

4
Figure 3. Normalized CO concentration downwind of Highway 71 

y = 191.6e-0.0278x

R2 = 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, NO2, and NOx 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 trailers are shown in Figure 

5 for Highway 71. These data show falloff behavior similar to the gas-phase species CO and 

NOx. Error bars represent a 95% confidence interval for the figures shown below. Although a 

full profile curve cannot be produced from the three stationary measurements as it was in the 

mobile sampling of CO and NOx, the trend is still 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. Concentration profiles for all PAH compounds generally showed classic falloff 

behavior with low concentrations upwind of the roadway, the greatest concentration immediately 

downwind of the source, and decreasing concentrations 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. 

5
Figure 5. Concentration of PM2.5 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 profile 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 immediately 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 explanations of a reduced concentration at the nearest 

downwind sample point could be increased oxidation from other constituents of vehicular 

exhaust or dilution from increased mixing due to vehicle turbulence. 

Ambient concentrations of two of the measured carbonyls are presented in Figure 7. Near 

Highway 71, formaldehyde shows decreasing concentrations with increasing distance downwind 

of the roadway. Acetaldehyde (C2 aldehyde) increases in concentration at the far downwind site. 

One possible explanation might be the formation of acetaldehyde by atmospheric chemistry from 

the reaction of primary emissions from motor vehicles. 

SUMMARY 

This on-going analysis examines concentration profiles 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. 

6
Figure 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 carbonyls: (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) 

7
0.0

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 Sioutas, 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., Martuzevicius, D., Wang, Z., LeMasters, G., 

Lockey, J.E. and Biswas, P. Concentration gradient patterns of aerosol particles near 

interstate highways in the Greater 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 NOx

catalysis. Applied Catalysis B: Environmental. 1997, 13, 1-25.

ACKNOWLEDGEMENTS 

This work was supported by Texas Commission on Environmental Quality under contract 

number 582-4-56385. 

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