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

Hiroaki Minoura,John G Watson,Judith C Chow,Joshua S Fu,Xinyi Dong,Cheng-En Yang

doi:10.4209/aaqr.2015.09.0554

Abstract

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.

Highlights

The South Coast Air Basin (SoCAB) in California, including the city of Los Angeles (LA), is located in complex terrain surrounded by mountains with ridges of more than 2000 m above mean sea level
Finer vertical coordinate grids of Community Multiscale Air Quality (CMAQ) are used at the altitude close to the surface according to the vertical coordinate parameter (VGLVLS)
This modeling study of O3 near mountains in the eastern part of the South Coast Air Basin (SoCAB) identifies vertical wind velocity profiles with counter clockwise air flow on the west side of San Bernardino mountain range that enhances the up-slope mountain chimney effect with some circulation found on the east side

Summary

Introduction

The South Coast Air Basin (SoCAB) in California, including the city of Los Angeles (LA), is located in complex terrain surrounded by mountains with ridges of more than 2000 m above mean sea level (asl). Anthropogenic air pollutants from urban areas, transported by sea breezes, contribute to high ozone (O3) concentrations on the mountain slopes of the eastern SoCAB during summer. Insufficient ventilation and accumulation of photochemical air pollutants in the SoCAB have been studied since HaagenSmit (1952). Updrafts generated by solar radiation on mountain slopes result in vertical transport of air pollutants, called the “mountain chimney effect”, (e.g., Lu and Turco, 1996; Langford et al, 2010). To understand air pollutant evolution in complex terrain, it is important to consider O3 formation through atmospheric transport and photochemical reactions of pollutants

Methods

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Discussion

Conclusion