Plume-in-Grid Modeling for PM & Mercury

1. Plume-in-Grid Modeling for PM & Mercury Prakash Karamchandani, Krish Vijayaraghavan, Shu-Yun Chen & Christian Seigneur AER San Ramon, CA 5th Annual CMAS Conference October 16–18, 2006 Chapel Hill, NC

2. Why Use Plume-in-Grid Approach? Plume Size vs Grid Size (from Godowitch, 2004) Limitations of Purely Grid-Based Approach • Artificial dilution of stack emissions • Unrealistic near-stack plume concentrations • Incorrect representation of plume chemistry • Incorrect representation of plume transport

3. Plume Chemistry & Relevance to PM and Mercury Modeling 3 2 Early Plume Dispersion Long-range Plume Dispersion Mid-range Plume Dispersion NO/NO2/O3 chemistry 1 Reduced VOC/NOx/O3 chemistry — acid formation from OH and NO3/N2O5 chemistry Possible reduction of HgII to Hg0 Full VOC/NOx/O3 chemistry — acid and O3 formation

4. Mercury Chemistry in Power Plant Plumes • Evidence of HgII reduction in power plant plumes (Edgerton et al., ES&T, 2006; Lohman et al., ES&T, 2006) • Reduction of HgII by SO2 (possibly via heterogeneous reaction on particles) is compatible with the global Hg cycling budget (Seigneur et al., J. Geophys. Res., in press)

5. CMAQ-MADRID-APT-Hg • Based on CMAQ v 4.5.1, March 2006 release • MADRID: Model of Aerosol Dynamics, Reaction, Ionization and Dissolution • APT: Advanced Plume Treatment with embedded plume model SCICHEM (state-of-the science treatment of stack plumes at the sub-grid scale) • Mercury treatment included • Consistent treatments for chemical transformations (gas- and aqueous-phase) and PM in the host model and the embedded plume model

6. Model Components CMAQ v. 4.5.1 MADRID PM Treatment with Mercury CMAQ-MADRID-Hg SCICHEM-MADRID-Hg PM and Hg Treatment based on CMAQ-MADRID-Hg CMAQ-MADRID-APT-Hg

7. 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

8. Atmospheric Mercury • Mercury is present mostly as three “species” in the atmosphere • Elemental mercury (Hg0) • Divalent gaseous mercury: • HgCl2, Hg(OH)2, HgO, etc. • referred to collectively as HgII or reactive gaseous mercury (RGM) • Particulate-bound mercury: • HgII or Hg0 adsorbed on PM • mostly divalent • referred to collectively as Hgp

9. Atmospheric Chemistry of Mercury

10. Application to Southeastern U.S. • Simulation period: 2002 • Grid resolution: 12 km x 12 km, 19 layers (up to ~15 km) • Meteorology and emissions inventory from Georgia EPD and VISTAS • Non-Hg ICs/BCs from Georgia EPD • 5 day model spinup for each quarter • Two annual simulations with CMAQ-MADRID-APT-Hg • With SO2 + HgII reduction reaction • Without this reaction

11. Modeling Domain and Locations of APT sources

12. Boundary Conditions for Mercury Species • Boundary conditions (BCs) for mercury were obtained from a 2001 simulation conducted over the United States with the Trace Element Analysis Model (TEAM) • Spatially and temporally (hourly) varying BCs of Hg0, HgII, and Hgp

13. Preliminary Results from Plume Event Evaluations • Several power plant plume events observed at SEARCH monitoring locations (Edgerton et al., ES&T, 2006) • To compare the modeled plume events with observations, the plume information in the embedded plume model is used to calculate subgrid-scale concentrations downwind of the power plant impacting a SEARCH monitoring location • Plume concentrations are sampled at an array of receptors along an arc; the center of the arc is the power plant of interest and the arc extends to 30o on each side of the monitoring location • The receptor location with the closest match of modeled SO2 peak increment to the observed peak increment is used for comparison purposes

14. Placement of Receptors for Plume Event Evaluations

15. Monitoring Stations in SEARCH network http://www.atmospheric-research.com/studies/SEARCH/index.html operated by Atmospheric Research & Analysis, Inc. (ARA)

16. Comparison of Measured and Simulated Peak SO2 Increments

17. Comparison of Measured and Simulated Peak SO2 Increments

18. Plume Event on July 5, 2002Hg Plume Increments Source: Plant Bowen; Monitoring Location: Yorkville

19. Plume Event on July 21, 2002Hg Plume Increments Source: Plant Bowen; Monitoring Location: Yorkville

20. Power-Plant Contributions to 24 hr Average Sulfate Concentrations on July 5 CMAQ-MADRID-Hg CMAQ-MADRID-APT-Hg

21. Power-Plant Contributions to 24 hr Hg Total Deposition on July 5 CMAQ-MADRID-Hg CMAQ-MADRID-APT-Hg

22. Conclusions • Observed plume events are better captured in the plume-in-grid approach than in the purely gridded approach • Preliminary evaluation results suggest that observed RGM to TGM ratios during plume events are well simulated only when a plume-in-grid approach is used and a pathway for reducing HgII to Hg0 by SO2 is included • A purely gridded approach typically overestimates power plant contributions to PM2.5 because SO2to sulfate and NOx to nitrate conversion rates are overestimated (Karamchandani et al., Atmos. Environ., in press) • A purely gridded approach will also overestimate power plant contributions to RGM concentrations and depositions if a mechanism exists to reduce HgII to Hg0 in power plant plumes

23. Ongoing Work • Complete simulations for entire calendar year • Complete model performance evaluation: • SEARCH: Continuous gas, PM mass and components, Hg • Other air quality networks: AQS, IMPROVE, CASTNET • Wet deposition: NADP, MDN • Control scenario simulations

24. Acknowledgements • Funding: • EPRI (Eladio Knipping, Leonard Levin) • Southern Company (John Jansen) • Input Files: • Georgia Environmental Protection Division (James Boylan, Maudood Khan) • VISTAS (Pat Brewer) • SEARCH Plume Measurements: • Atmospheric Research & Analysis, Inc. (Eric Edgerton)