Sub-Grid Scale Modeling of Air Toxics Concentrations Near Roadways Prakash Karamchandani, Kristen Lohman & Christian

1. Sub-Grid Scale Modeling of Air Toxics Concentrations Near Roadways Prakash Karamchandani, Kristen Lohman & Christian Seigneur AER San Ramon, CA 6th Annual CMAS Conference October 1–3, 2007 Chapel Hill, NC

2. Background • Population exposure to hazardous air pollutants (HAPs) is an important health concern • Exposure levels near roadways are factors of 10 larger than in the background–models need to capture spatial variability in exposure levels • Many of the species of interest are chemically reactive–e.g., formaldehyde, 1,3-butadiene, acetaldehyde–models need to treat the chemistry of these species • Traditional modeling approaches are inadequate to provide both chemistry treatment and fine spatial resolution

3. Near-Roadway Modeling • Air toxics emitted from mobile sources: • diesel particles • benzene • butadiene • formaldehyde • ultrafine particles • etc.

4. Three Major Approaches • Parameterization of sub-grid variability • Hybrid modeling (near-field model + grid-based model) • Plume-in-grid modeling (this work)

5. Improving Spatial ResolutionParameterization of Spatial Variability Formaldehyde PDF PDFs: sub-grid variability Touma et al., J. Air Waste Manage. Assoc., 56, 547-558 (2006) Ching et al., Atmos. Environ., 40, 4935-4945 (2006)

6. Improving Spatial ResolutionHybrid Modeling Formaldehyde concentrations (mg/m3) • Plume and background are simulated separately, then added • Chemical reactions cannot be taken into account; only appropriate for chemically “inert” pollutants Touma et al., J. Air Waste Manage. Assoc., 56, 547-558 (2006) Isakov & Venkatram, J. Air Waste Manage. Assoc., 56, 559-568 (2006)

7. Improving Spatial ResolutionPlume-in-Grid Modeling • Combines 3-D grid-based modeling approach with a local scale modeling approach within a single model • Provides capability of both capturing near-source variability and treating chemical transformations of reactive species • Roadway emissions are treated as discrete sources and simulated with the embedded puff model • Concentrations can be calculated at discrete receptor locations by combining the incremental puff concentrations from the puff model with the grid-cell average background concentration from the host grid model

8. Plume-in-Grid (PiG) Model • Uses CMAQ as the host model and SCICHEM as the embedded puff model • Based on previously developed PiG model for ozone and PM (CMAQ-APT, available from CMAS) • Prototype version for this proof-of-concept study: • simulates near-source CO and benzene concentrations from roadway emissions • chemistry is switched off • roadway emissions are treated as a series of area sources along the roadway with initial size equal to the roadway width

9. SCICHEM • Three-dimensional puff-based model • Second-order closure approach for plume dispersion • Puff splitting and merging • Treatment of plume overlaps • Optional treatment of building downwash • Optional treatment of turbulent chemistry • PM, gas-phase and aqueous-phase chemistry treatments consistent with host model

10. Model Application • Busy interstate highway in New York City (I278) • July 11-15, 1999 period of NARSTO/Northeast Program • Grid model domain

11. Roadway Emissions • Selected section of I278 passing through all five boroughs of New York City (~ 50 km) • Section divided into small segments of 30 m length, with each segment representing an area source. Number of sources: ~1700 • Emissions for each segment based on • County emissions for highway traffic (from SMOKE) • Traffic count information from the National Highway Planning Network (NHPN) for counties and I278 • I278 emissions removed from 3-D CMAQ emissions file to avoid double counting of emissions

12. Receptor Locations • Located along busiest stretch of roadway in Queens and Manhattan (Triborough Bridge) • Located where roadway exhibited significant curvature, to increase the likelihood of capturing the maximum spatial variability in exposure levels • Placed along transects perpendicular to the roadway at 10, 20, 30, 40, 50, 100, 200, 300, 400 and 500 m from the center of the roadway in both directions Source for map: Google

13. Results for Transect 1 (near intersection of I278 with Queens Blvd) Source for map: Google

14. Results for Transect 8 (Intersection of I278 with Grand Central Parkway) Source for map: Google

15. Results for Transect 15 (Triborough Bridge near Wards Island Park) Source for map: Google

16. Results for Transect 29 (I278-I87 Interchange; Bruckner Expressway) Source for map: Google

17. Observations in Los Angeles Near I-405 and I-710 • Zhu et al., J. Air Waste Manage. Assoc., 52, 1032-1042 (2002) • Measurements in vicinity of Interstate 405 • May to July 2001 • CO, Black Carbon (BC), and ultrafine particles • At 30, 60, 90, 150 and 300 m downwind and at 300 m upwind from freeway • Zhu et al., Atmos. Environ., 36, 4323-4335 (2002) • Measurements in vicinity of Interstate 710 • August to October 2001 • CO, Black Carbon (BC), and ultrafine particles • At 17, 20, 30, 90, 150 and 300 m downwind and at 200 m upwind from freeway

18. Qualitative Comparison with L.A. Observations NYC Model Predictions LA Measurements

19. Summary and Future Work • Feasible to adapt available full-chemistry models to conduct sub-grid scale modeling of HAPs • Model captures observed sub-grid scale variability in concentrations near roadways • Future work should address: • Incorporate treatment of traffic-induced turbulence • Activate chemistry for reactive species • Improve computational efficiency of model • Application of model to region where data are available to evaluate the model (e.g., Los Angeles & North Carolina)