Air Pollutant Concentrations Near Texas Roadways and Evaluation of MOBILE6

1. Air Pollutant Concentrations Near Texas Roadways andEvaluation of MOBILE6 Andrea Clements1, Yuling Jia1, Allison Denbleyker2, Elena McDonald-Buller2*, Matt Fraser1,3, Yifang Zhu4, Jayanth Pudota4, David Allen2, Don Collins5, and Edward Michel2 1 Arizona State University 2 The University of Texas at Austin 3 Rice University 4 Texas A&M University-Kingsville 5 Texas A&M University *corresponding author: ecmb@mail.utexas.edu; 512-471-2891

2. Objectives Examine spatial gradients and chemical transformation vehicular-emitted pollutants near Texas Roadways Gas-phase species: CO, NOx and aldehydes Downwind distance zone of impact Decay rate compared with previous studies UFP number, surface, and volume Particle bound organic species: PAHs, hopanes, elemental and organic carbon Compare predicted CO/NOx ratios from MOBILE6 with observations

3. Texas Roadway Study Sampling Sites

4. Site Configuration Mobile platform CO, NO-NO2-NOx, UFP Wind speed and direction Stationary trailers Aldehydes Particle-bound organics: PAHs, hopanes, elemental and organic carbon Wind speed and direction

5. Gas-Phase Species: CO Decay Segregated by Wind Direction Strong exponential decay relationship within 100 m under perpendicular wind conditions that leveled off around 150 -200 m downwind of the roadways. CO decay rate for the Texas study was similar to the decay rates measured by Zhu et al. (2002) for Los Angeles. Parallel Wind Perpendicular Wind

6. Gas-Phase Species: NOx Decay Segregated by Wind Direction Parallel Wind Perpendicular Wind • Strong exponential decay relationship for NO and NOx under perpendicular wind conditions similar to CO. • NO2 exhibited little downwind decay. • More rapid decay of NO than NOx downwind of the roadways consistent with chemical loss.

7. Gas-Phase Species: Aldehydes Upwind and downwind of SH-71. Concentrations of most aldehydes were higher downwind than upwind and decreased over 65 m. Acetaldehyde and acrolein increased with distance from the roadway, suggesting chemical generation.

8. Gas-Phase Species: Comparison of the Fall-Off of CO and Aldehydes Percent change in concentrations 35 - 65 m downwind of SH-71. CO decreased by nearly 50%. Benzaldehyde, valeraldehyde, and hexaldehyde may decrease more rapidly. In contrast, formaldehyde and propionaldehyde decreased more slowly than CO, and acetaldehyde and acrolein increased by 30% and 59%, respectively at the far downwind site at 65 m.

9. UFP Number Concentrations • Total particle number concentrations decayed exponentially. • Concentrations in 6 to 25 nm range decayed sharply within 100-200m. • Similar to studies in Los Angeles, concentrations in 25 to 50 nm range experienced a shoulder between 100 - 200 m likely due to condensation and coagulation of 6 to 25 nm particles. • Smaller gradients in concentration of particles larger than 100 nm.

10. Particle-Bound Species: PAHs Upwind and downwind of SH-71. PAH concentrations were higher upwind than downwind with nearly complete fall-off over 65 m.

11. Particle-Bound Species: Hopanes Upwind and downwind of SH-71 (* indicates hopane was below detection at 65 m). Concentrations of hopanes were higher upwind than downwind with complete fall-off over 65 m. Results were similar to those of Phuleria et al. (2007) for Los Angeles.

12. Particle-Bound Species: EC and OC Upwind and downwind of SH-71. EC exhibited classic fall-off behavior. OC increased with distance downwind of the roadway, consistent with condensation of semi-volatiles.

13. Conclusions: Texas Roadway Study CO, NO, and NOx concentrations exhibit exponential decay and return to ambient concentrations by 150-200 m downwind of the roadways, similar to UFP number concentrations. Evidence of chemical processing of gas-phase species within 100 m of the roadways. Production of NO2 from the oxidation of vehicular-emitted NO. Increase in acetaldehyde and acrolein concentrations with downwind distance. Behavior of particle-bound organics is complex PAHs, hopanes, EC exhibited classic fall-off. OC concentrations increased with distance downwind of SH-71, consistent with the condensation of semi-volatiles.

14. MOBILE6 Evaluation • Further analysis of the Texas Roadway Study data was used to evaluate the MOBILE6 emissions model • Ratios of CO/NOx predicted using MOBILE6.2 were compared with observations from the Texas Roadway Study • MOBILE6.2 used to predicted CO and NOx emission factors • Hourly emission factors and meteorological data were used as inputs in CALINE4, a roadway plume dispersion model, to predict hourly averaged CO and NOx concentrations and CO/NOx ratios • Because winds were perpendicular to FM-973 during sampling, continuous stationary measurements from 8 sampling periods with a range of traffic volumes (e.g., rush hour, midday) were selected for comparison with MOBILE6 predictions

15. Modeling Results • Predicted CO/NOx values were higher than observed for 7 out of 8 time periods • Average ratio of ratios was 1.96 • Nearly all over-predictions were within factor of 3, similar to a previous analysis by ENVIRON (2004).

16. Acknowledgements Texas Commission on Environmental Quality Contract # 582-4-56385 Houston Advanced Research Center Contract # 20-23016-UT0707