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NARSTO Workshop on Innovative Methods for Emission Inventory Development and Evaluation University of Texas, Austin October 14-17, 2003 |
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Comparing the Point and Non-Point Source Emission Inventory to Ambient Data in Houston: A Fine-Resolution Approach
John Jolly, Fernando Mercado, and David Sullivan
Texas Commission on Environmental Quality
Analyses of data collected in the Texas Air Quality Study in 2000 (TexAQS) have revealed the important role of Houston´s industrial emissions on ozone levels. Atmospheric scientists have shown that some VOC compounds that are emitted in large amounts by industry are critically important to the rapid ozone formation and high ozone concentrations that occur there. As well, research has shown that these compounds are often dramatically underreported in the TCEQ point source emission inventory (EI).
However, few of these studies have looked at the role of non-point source EI categories in a detailed manner. In this project, we compare VOC:NOx ratios and individual species ratios measured at area monitors with the same ratios from several combinations of inventory categories, including point and non-point sources. We employ emissions data for four anthropogenic non-point source categories (area, non-road, mobile, and ships), drawn from a fully speciated version of an inventory used in recent photochemical modeling.
Our study both follows and expands upon an established methodology for comparing VOC:NOx and individual species ratios, which has been used in several studies in recent years. The methodology involves comparing ratios from a gridded emission inventory to those from one or more subject monitors located within the grid, for purposes such as inventory verification. Various combinations of grid cells are used for comparison; emissions from the cells composing the upwind quadrant of the grid are one common combination. While upwind quadrants are used in our study, their limitations are apparent in Houston, where the irregular geographic distribution of the numerous point sources makes such a coarse approach – grouping all sources in a 90-degree sector together – less than ideal. For that reason, we also employ a finer resolution approach, whereby VOC:NOx and species ratios are compared for 10-degree wind direction bins. Using this finer resolution approach for both point and non-point sources yields a very detailed look at the Houston EI, which will help in adjusting it to better reflect reality. Also, the accuracy of this approach should make it a useful improvement for future EI:Ambient reconciliation studies.