CRC Report No. A-34 - Coordinating Research Council

April 2005
1.4 OVERVIEW OF PROJECT APPROACH
Project A-34 used the CAMx photochemical grid model (ENVIRON, 2004) to simulate an urban
atmosphere with known contributions from 22 source categories to 55 individual VOC species.
The VOC species selected were those routinely monitored by the Environmental Protection
Agency’s (EPA’s) Photochemical Assessment Monitoring Stations (PAMS). The PAMS species
profiles simulated for specific receptor locations were analyzed by receptor modeling using the
Chemical Mass Balance (CMB) model to identify source contributions. The source
contributions estimated by CMB were compared to the known source contributions simulated by
the photochemical grid model to permit a rigorous analysis of how well the CMB model
performed. This approach required two teams working collaboratively, but independently. The
photochemical grid modeling was designed and executed by ENVIRON whereas the Desert
Research Institute (DRI) carried out the CMB receptor modeling. Dr. Warren White of the
University of California at Davis provided independent assistance in the study design and the
interpretation of results.
The receptor model performance was evaluated in four rounds of tests. In Rounds 1, 2, and 4,
the receptor modeling team analyzed the same set of 8 experiments (from Round 1) but with
increasing levels of knowledge about how the ambient samples were simulated. Round 3 was
designed after Round 2 had been completed to follow up on specific uncertainties in the CMB
receptor modeling analysis.
Round 1: Blind Test. In the first round, ENVIRON provided DRI with virtual PAMS samples
for 8 receptors, four days (August 4-7, 1997) and 8 experiments for a total of 6144
simulated air samples. DRI had no supporting information other than the sample
identification number. The 8 experiments differed in modeling assumptions used to
prepare the simulated air samples (e.g., relative source contributions, atmospheric
reactivity, sampling noise, source profiles).
Round 2: Available Information. In Round 2, DRI reanalyzed the same set of 6144 simulated air
samples as in Round 1. The difference was that for Round 2 ENVIRON provided DRI
with sample location, date, time and experiment number and additional information that a
receptor modeler would typically have available to support a “real-world” receptor model
application. For example, a virtual tunnel study was performed to provide DRI with
information on the mobile source emissions profiles.
Round 3: Uncertainty Analysis. New photochemical modeling experiments were performed in
Round 3 to follow up in issues identified after Rounds 1 and 2. Four new experiments
were designed (3072 simulated ambient samples) to understand how chemical decay and
sampling noise influence CMB performance in situations where the receptor model
should do well
Round 4: Full Information. In the final round, DRI was provided with detailed information on
how the experiments in Rounds 1-3 had been performed. This included VOC speciation
profiles used in the photochemical modeling emissions inventory for each major source
category. This round assessed the ability of the CMB receptor model to estimate source
contributions given an ideal situation in which all emissions speciation information is
known.
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