Coastal Thermal Circulation and its Effect on Photochemical Modeling

The South Coast air basin that includes the greater Los Angeles, its surrounding mountains and neighboring ocean, is known for the worst air pollution in the nation. Complex topography and unique climate promotes photochemical production while mountainous areas in the east and the north of the basin inhibits ventilation and creates a unique multi-layered vertical structure. A special field campaign utilizing an aircraft, rawin and ozone sondes, vertical remote sensing instruments was deployed to characterize the local thermal circulation associated with the complex terrain and the land-sea contrast. Weather Research and Forecast (WRF) mesoscale meteorological model was employed to simulate airflow and the circulation pattern. Community Multiscale Air Quality Model (CMAQ), a photochemical transport model was simulated to re-produce ozone, PM2.5 and photo-oxidants. The measurements and numerical simulations indicated a layer of aged photo-oxidants located at the top of the marine boundary layer where the return component of the thermal circulation formed. The pollutants from this elevated layer were entrained down to the surface layer during convective mixing. The vertical entrainment process turned out to be a critical factor to simulate surface concentration appropriately.