Application of the CMAQ-UCD Aerosol Model to a Coastal Urban Site Chris Nolte

1. Application of the CMAQ-UCD Aerosol Model to a Coastal Urban Site Chris Nolte NOAA Atmospheric Sciences Modeling Division Research Triangle Park, NC 6th CMAS Conference Chapel Hill, North Carolina October 3, 2007

2. Coauthors Prakash Bhave NOAA/EPA Robin Dennis NOAA/EPA Jeff Arnold EPA Max Zhang Cornell University Tony Wexler UC Davis

3. Motivation • Certain ecosystems very sensitive to nitrate deposition • In coastal environments, coarse sea salt particles may act as sinks for nitric acid: NaCl + HNO3 NaNO3 + HCl • Most regional air quality models either neglect sea salt aerosol dynamics entirely or have crude estimates of sea salt emissions/boundary conditions

4. CMAQ-UCD • Sectional aerosol module coupled to CMAQ 4.4 • 9 size bins, 0.039 – 20 mm. • SO4, NO3, NH4, Na, Cl, EC, POA, SOAa, SOAb, dust, H+, H2O • Dynamic mass transfer between gas/aerosol phases (non-equilibrium) • Simplified thermodynamics derived from AIM • SOA treatment similar to CMAQ • No coagulation, no heterogeneous reaction of N2O5

5. Application to BRACE • Bay Region Atmospheric Chemistry Experiment – assess sources of nitrogen deposition to Tampa Bay • Extensive measurements in May 2002—rich data set for this application • This evaluation is not intended as a comparison against the CMAQ modal aerosol model!

6. Modeling Configuration • 32 km continental U.S. domain, windowed to 8 km, then 2 km. • Will present results from 8 km only. • MM5: 30 layers, Pleim-Xiu PBL and land-surface schemes, Grell microphysics. • Used sea surface temperature from GOES satellite. • US EPA 1999 National Emissions Inventory, projected to 2002. • CMAQ 4.4, SAPRC99, 21 layers • April 21 – June 3, 2002

7. Sea Salt Emissions • Size-dependent fluxes of sea salt Na+, Cl-, SO42- over open ocean and surf zone from parameterizations of Gong et al. and de Leeuw et al. • see K.M. Zhang et al., 2005 • Initial tests with 100 m surf zone width resulted in too much sea salt • Reduced surf zone width to 50 m

8. Modeling Domain 8 km domain (158 x 158) Observational Sites

9. Observations Compare against two surface data sets • 10- or 12-stage MOUDI (Evans et al., 2004). • 23-h samples at 3 sites, 15 sampling days • Semi-continuous parallel plate wetted denuder (Dasgupta et al., 2007) • Inlet 50% cut point ~ 12.5 mm. See Jeff Arnold’s poster for comparisons against aircraft measurements!

12. Two co-located MOUDIs at Sydney; 2nd measurement is shown in red. Nitrate is consistently underpredicted

13. Size distributions: May 14

14. Size distributions: May 14

15. Size distributions: May 15

16. Size distributions: May 15

17. Chloride Displacement by NitrateNO3 / (NO3 + Cl)

18. Conclusions • Modeled size distributions of SO4, NH4, Na, and Cl generally in good agreement with obs, but NO3 too low by a factor of 2. • Missing source of NO3? • Too rapid dry deposition? • Though NO3 is too low, model correctly predicts it is predominantly in the coarse size sections. • Cl and Na concentrations are reasonably unbiased, indicating sea salt emissions and transport are correct on average. • Frequent overpredictions in early morning Cl (not shown) and underprediction of extent of chloride displacement by nitrate. • Underestimated rate of mass transfer? • Inaccurate thermodynamics? • Other processes?

19. Acknowledgments Met modeling Lara Reynolds, Nancy Hwang Emissions Charles Chang, Lucille Bender, George Pouliot Observational data Sandy Dasgupta, Ben Hartsell Disclaimer: A portion of the research presented here was performed under the Memorandum of Understanding between the U.S. Environmental Protection Agency (EPA) and the U.S. Department of Commerce's National Oceanic and Atmospheric Administration (NOAA) and under agreement number DW13921548. This work constitutes a contribution to the NOAA Air Quality Program. Although it has been reviewed by EPA and NOAA and approved for publication, it does not necessarily reflect their policies or views.