Vertical Circulation of Atmospheric Pollutants near Mountains during a Southern California Ozone Episode

This study investigates the air pollutant interactions and emission source contributions to ozone (O_3) formation within a complex terrain. Air quality simulations using the Community Multiscale Air Quality (CMAQ) Model focused on vertical distributions of O_3 for the July 14-18, 2005 episode in the South Coast Air Basin (SoCAB). The Zero-Out method was applied in sensitivity tests for seven emission source categories. Elevated O_3 concentrations were found near the top of the planetary boundary layer (PBL, ~1200 m) and in the free troposphere (~3500 m) over the eastern SoCAB. Low O_3 concentrations were found near the surface at the center of the basin due to nitrogen oxide (NO) titration by fresh vehicle exhaust. Sea and land breezes, enhanced by up-slope flows (the ＂mountain chimney effect＂) transported O_3 upward. Formation of O_3 is sensitive to the H_2O_2/HNO_3 ratio, depending on fresh vs. aged pollutant mixtures. Biogenic emissions were important contributors to O_3 formation, both in the SoCAB and at the top of the PBL. In contrast, the highest vehicle contributions to O_3 were found far from urban areas and in the lower free troposphere. Vertical cross-sectional analysis provided some insights into the O_3 formation and mixing processes present in the SoCAB.

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