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

Category: Urban Air Quality

Volume: 16 | Issue: 10 | Pages: 2396-2404
DOI: 10.4209/aaqr.2015.09.0554
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Hiroaki Minoura 1, Judith C. Chow2, John G. Watson2, Joshua S. Fu3, Xinyi Dong3, Cheng-En Yang3

  • 1 Toyota Central R&D Labs. Inc., Nagakute, Aichi 480-1192, Japan
  • 2 Division of Atmospheric Science, Desert Research Institute, Reno, NV 89512, USA
  • 3 Department of Civil and Environmental Engineering, University of Tennessee, Knoxville, TN 37996, USA

Highlights

Elevated O3 concentrations were found near the top of PBL and in free troposphere.
HNO3 concentrations peaked later from source affects different O3 formation sensitivity.
Biogenic emissions contribute to O3 formation, both in urban areas and at top of PBL.
Vehicular contributions were found far from urban areas and in lower free troposphere.


Abstract

This study investigates the air pollutant interactions and emission source contributions to ozone (O3) formation within a complex terrain. Air quality simulations using the Community Multiscale Air Quality (CMAQ) Model focused on vertical distributions of O3 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 O3 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 O3 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 O3 upward. Formation of O3 is sensitive to the H2O2/HNO3 ratio, depending on fresh vs. aged pollutant mixtures. Biogenic emissions were important contributors to O3 formation, both in the SoCAB and at the top of the PBL. In contrast, the highest vehicle contributions to O3 were found far from urban areas and in the lower free troposphere. Vertical cross-sectional analysis provided some insights into the O3 formation and mixing processes present in the SoCAB.

Keywords

Ozone formation Source contribution Vertical circulation Model simulation


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