Environmental Engineering Reference
In-Depth Information
8.3 Factors Controlling the Formation of Ozone
in the San Francisco Bay Area
Su-Tzai Soong and Saffet Tanrikulu
Bay Area Air Quality Management District, San Francisco, CA, USA
1. Introduction
Factors controlling the production of ozone in the San Francisco Bay Area (SFBA)
were studied using the process analysis module of the CAMx model. This study is
a continuation of previous modeling work that was performed as part of the
Central California Ozone Study (CCOS). During that study, meteorology of one of
the captured ozone episodes (July 29-August 3, 2000) was simulated by Wilczak
et al. (2004) using the MM5 model and ozone concentrations were simulated by
Soong et al. (2004) using the CAMx model. Both MM5 and CAMx performed well
in simulating this episode.
The modeling domain had a 185 × 185 horizontal grid with 4 km resolution,
covering most of Central California. A stretching vertical grid spacing was employed
in both models with 50 layers in MM5 and 20 layers in CAMx. The thickness of
the layer near the surface was 24 m in both models. Initial and boundary conditions
of CAMx were specified using surface and upper air observations. A day-specific
emissions inventory was generated for the modeling period.
To study the factors controlling the production of ozone in the SFBA, we focused
on July 31, 2000, when the 1-h observed maximum ozone concentration reached
126 ppb in Livermore. We hypothesized and investigated three factors that may be
controlling ozone production in the SFBA: the VOC-NOx ratio (V/N ratio hereafter),
the magnitude of VOC and NOx concentrations, and ambient temperatures.
2. Results
The left panel of
Fig. 1
shows simulated 1-h ozone concentrations in the SFBA at
1400 PDT on July 31, 2000, when simulated ozone production reached a maximum
there. The highest ozone production rate of 20-30 ppb/h was simulated in portions
of four counties, Alameda, Contra Costa, Santa Clara and Sonoma (dark areas of
right panel of
Fig. 1
). In the remaining portions of these counties, the production
rate was mostly 10-20 ppb/h. Along the northern and western boundaries of the
SFBA, the production rate was under 10 ppb/h. In the core areas of San Francisco,
Oakland and San Jose, and along some freeways, however, the production rate
was negative (white areas).
