Nitrogen and Sea Salt Interaction in Coastal Environments: Results from CMAQ Applications in the Tampa Bay Airshed

CMAQ v4.4 was run with 32 km, 8 km, and 2 km grids to simulate urban and regional conditions in the southeastern U.S. for 30 days in May 2002. We report here results of the first application of this version of the model to the complex meteorological and chemical regimes of urbanized subtropical coastal climates. CMAQ v4.4 contains substantial aerosol chemistry updates, including the addition of sea salt, thought to provide significant surface area for nitrogen transformation reactions. The model version for these runs also included a multi-sectional aerosol module in place of the standard modal one. In addition, changes in MM5 significantly improved the meteorology used to drive CMAQ, improvements essential for capturing the sea-land effects at coastal interfaces. Output from MM5 and CMAQ is compared here to elements of the extensive sets of surface and aloft data collected in the May 2002 Tampa Bay Regional Atmospheric Chemistry (BRACE) field experiment. More than 100 gas- and aerosol-phase pollutants were continuously sampled at five ground sites with a chemistry supersite having multiple instruments for intensive analysis of aerosol mass distribution and particle composition. In addition, the NOAA-ARL Twin Otter aircraft was instrumented for continuous collection of multiple gas-phase components along with integrated collection of volatile organic compounds and aerosols on bulk and fine scales. Meteorology variables measured as part of BRACE included standard elements on 10 m towers at the surface and vertical winds and temperatures from three radar profiler+RASS installations. Results indicate that the MM5 and CMAQ improvements in this model version have worked very well for correctly partitioning atmospheric nitrogen among the many gas-phase species and across gas- and aerosol-phase pollutants in this highly complex airshed.