CRC Report No. A-34 - Coordinating Research Council

April 2005
2.0 PHOTOCHEMICAL MODELING METHODOLOGY
This section of the report describes how the photochemical grid modeling was performed to
prepare synthetic air samples for receptor locations in the Los Angeles area. An overview of A-
34 study design was presented in Section 1. The procedures described below were developed in
modeling protocol at the outset of the study. The modeling protocol was reviewed by CRC, but
not by DRI since this would have revealed critical information.
2.1 MODELS, DOMAIN, EPISODE AND RECEPTORS
The Comprehensive Air-quality Model with extensions (CAMx) photochemical modeling
database for the August 3-7, 1997 Southern California Ozone Study (SCOS) episode was used
for this study. This is the same database as was used in the CRC A-36-A1 Weekday/Weekend
Proximate Modeling (Yarwood et al., 2002 and 2003a) and CRC A-38 EMFAC on-road vehicle
emission factor model evaluation (Yarwood et al., 2003b) studies. The previous studies have
found good model performance for ozone and ozone precursors in the Los Angeles are with this
modeling system.
Ambient Conditions During the SCOS Episode
The August 3–7, 1997 ozone episode began with warm temperatures at the surface and aloft and
weak pressure gradients directed offshore opposing onshore sea breezes. August 3 was the first
model spin-up day and is not discussed. Ozone concentrations were relatively low on August 4
in most locations with the highest ozone occurring inland in the mountains and passes consistent
with onshore flow. On August 5, temperatures increased reaching 29 °C at Los Angeles
International Airport (LAX) and 49 °C at Palm Springs. The offshore pressure gradient
increased in intensity and the episode maximum ozone concentration was 187 ppb at Riverside,
consistent with continued weak onshore flow. By August 6, the offshore pressure gradients had
weakened and inland temperatures cooled (43 °C at Palm Springs). The maximum ozone on
August 6 occurred in the mountains at Crestline. On August 7, pressure gradients turned onshore
and the onshore winds strengthened so that the highest ozone concentrations occurred far inland.
The meteorological patterns during the August 3–7, 1997 period fit a typical pattern for Los
Angeles ozone episodes. High ozone levels occurred because the period was relatively stagnant,
tending to trap ozone and precursors within the Los Angeles basin.
Models and Modeling Domain
Photochemical ozone modeling for the August 3-7, 1997 SCOS period was performed with
version 4.03 of the Comprehensive Air-quality Model with extensions (CAMx; ENVIRON,
2004). CAMx simulates the emission, dispersion, reaction, and removal of ozone precursors and
ozone in a Eulerian (grid) framework. The modeling domain covered 65 by 40, 5-km grid cells
as shown in Figure 2-1. This domain was selected to be consistent with past modeling for air
quality management plans in the LA area (SCAQMD 1997, 1999). CAMx was run with 10
layers extending between a surface layer of 60 m and a model top at 4 km.
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