CRC Report No. A-34 - Coordinating Research Council

April 2005
EXECUTIVE SUMMARY
Receptor models, such as the Chemical Mass Balance (CMB) model, are used to estimate what
sources contribute to ambient levels of volatile organic compounds (VOCs). Reliable source
contribution estimates (SCEs) derived from ambient data are valuable for evaluating and refining
emission inventories. Emission inventories are central to air quality planning activities to
comply with clean air regulations.
Important issues to understand when using the results of VOC receptor modeling are: (1) The
ability of the receptor model to accurately quantify individual source contributions from ambient
air samples impacted by many different sources; (2) Relationships between source contributions
to air concentrations vs. emission inventories, and; (3) How to associate source signatures
identified from air samples (e.g., gasoline engine exhaust) to activity categories used in emission
inventories (e.g., on-road mobile vs. off-road mobile vs. stationary gasoline engines). The
Coordinating Research Council (CRC) sponsored Project A-34 to evaluate the performance of
VOC receptor models under realistic conditions where the actual source contributions are
known.
A photochemical grid model was used to simulate the detailed VOC composition of an urban
atmosphere and to keep track of the contributions of 22 source categories to 55 VOC species.
The 55 VOC species were those routinely monitored by the Environmental Protection Agency’s
(EPA’s) Photochemical Assessment Monitoring Stations (PAMS). The modeled PAMS species
concentrations at several receptor locations were analyzed by VOC receptor modeling using the
CMB model. The CMB SCEs were compared to the known source contributions from the
photochemical grid model to permit a rigorous analysis of how well the CMB model performed.
Receptor modeling was performed in several “rounds” of analysis with progressively more
information being provided to the receptor modelers in each round. The receptor modelers
analyzed tens of thousands of “ambient samples” using a combination of manual and automated
fitting techniques.
The study 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. This is the first study to use a grid model to quantitatively evaluate
CMB, to our knowledge. The study results provide a unique opportunity to investigate how well
CMB performs for VOC and what assumptions are important. Nevertheless, the results are
limited by being based on a single modeling scenario.
Study Results
Example results from two “rounds” of analysis are shown in Figure ES-1. Figure ES-1(a) shows
results for 8 receptors analyzed in Round 2b whereas Figure ES-1(b) shows the receptors
analyzed again in Round 4. The difference between Rounds 2b and 4 was in the amount of
information provided to the group performing the CMB analysis. The Round 2b CMB analysis
was conducted with information typically available to a well-organized receptor modeling study;
sample location and time, source profile information from a tunnel study and liquid gasoline
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ES-1