Environmental Engineering Reference
In-Depth Information
2. Model Description
The UCD/CIT air quality model is used in this study with the Caltech Atmospheric
Chemistry Mechanism (CACM) that has been expanded to perform source
apportionment of SOA [2]. Semi-volatile organic compounds predicted by CACM
are allowed to partition to both the organic and aqueous phases of particles to form
SOA. The modeling domain covers the Central Valley of California and is hori-
zontally divided into 8 × 8 km cells, with 95 grid cells in each direction.
Meteorological fields were generated using the objective analysis method. Hourly
emissions used in the study are based on the emission inventory of 4 km spatial
resolution generated by the California Air Resources Board (CARB) that were
aggregated spatially to be consistent with the 8 km modeling grid. In order to
perform SOA source apportionment, VOCs emissions were grouped into nine
source categories: solvent use, wood smoke, diesel engines, catalyst gasoline
engines, non-catalyst gasoline engines, gasoline disposal and storage, high sulfur
fuel combustion, other anthropogenic sources, and the biogenic source.
15
Unknown OA
Predicted SOA
10
5
0
SAC Day
MOD Day
BAC Day
SAC Night
MOD Night
BAC Night
-5
-10
Fig. 1.
Comparison of the average predicted SOA with the average “unknown” OA for daytime
(day, 1000-1800PST) and nighttime hours (night, 2000-0800PST) at Sacramento (SAC), Modesto
(MOD), and Bakersfield (BAC)
3. Model Results
Figure 1
compares SOA concentrations predicted by the UCD/CIT/CACM model
to the concentration of “unknown” OA described by Kleeman et al. [3]. The
“unknown” OA was calculated as the difference between the measured total OA
and the primary OA identified by a molecular marker source apportionment
technique. Comparisons are made at the urban locations of Sacramento, Modesto,
and Bakersfield spanning a north-south transect of the SJV for both daytime
(1000-1800 PST) and nighttime (2000-800 PST) hours. Error bars for “unknown”
OA concentrations reflect the uncertainty in the molecular marker source
