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

Coauthors Prakash BhaveNOAA/EPA Robin DennisNOAA/EPA Jeff ArnoldEPA Max ZhangCornell University Tony WexlerUC Davis

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

CMAQ-UCD Sectional aerosol module coupled to CMAQ 4.4  9 size bins, – 20  m.  SO 4, NO 3, NH 4, Na, Cl, EC, POA, SOAa, SOAb, dust, H +, H 2 O 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 N 2 O 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!

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 CMAQ 4.4, SAPRC99, 21 layers April 21 – June 3, 2002

Sea Salt Emissions Size-dependent fluxes of sea salt Na +, Cl -, SO 4 2- 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 km domain (158 x 158) Observational Sites Modeling Domain

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  m. See Jeff Arnold’s poster for comparisons against aircraft measurements!

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

Size distributions: May 14

Size distributions: May 15

Chloride Displacement by Nitrate NO 3 / (NO 3 + Cl)

Conclusions Modeled size distributions of SO 4, NH 4, Na, and Cl generally in good agreement with obs, but NO 3 too low by a factor of 2.  Missing source of NO 3 ?  Too rapid dry deposition? Though NO 3 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?

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 DW 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.