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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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Poster 5: Use of MOBILE6 to Develop and Quality Assure “Link-Based” Inventories for the 8-County Houston/Galveston/Brazoria Nonattainment Area
Chris Kite
Texas Commission on Environmental Quality
Under contract to the Texas Commission on Environmental Quality (TCEQ), the Texas Transportation Institute (TTI) has utilized MOBILE6 to develop “link-based” inventories for the 8-County Houston/ Galveston nonattainment area. 2000 “base case” and 2007 “future case” inventories have been developed with MOBILE6 to photochemically model an ozone episode that originally occurred from August 22 to September 1, 2000. For each roadway “link” throughout the area, average speed and vehicle miles traveled (VMT) estimates per hour are coupled with MOBILE6 emission rates for each of the 28 vehicle types to obtain temporally and spatially allocated inventories for each episode day. These link-based MOBILE6 inventories were converted into a binary gridded format acceptable for photochemical modeling input with the 2x version of the Emissions Preprocessor System (EPS2x). Similar MOBILE6 linkbased inventories have also been received for the following other metropolitan areas in Texas: Austin, Beaumont/Port Arthur, Corpus Christi, Tyler/Longview/Marshall, San Antonio, and Victoria.
By running various scenarios for Harris County in both 2000 and 2007, TCEQ staff have been able to determine that NOX emission levels are most sensitive to VMT mix, local age distributions, and the relative humidity input. Due to significantly higher NOX emission rates from heavy-duty diesel vehicles (HDDVs), the amount of VMT assigned to each of the MOBILE6 HDDV classes has a significant impact on total NOX emission levels. Currently, MOBILE6 VMT mix defaults are based on annual nationwide mileage accumulation rate estimates by vehicle type and do not vary by roadway type, time-of-day, or day-of-week. In addition, MOBILE6 VMT mix defaults do not vary between urban and rural areas, which is an important distinction for the HDDV8A and HDDV8B classes that accumulate the overwhelming majority of their VMT in rural areas. Due in part to relatively “new” age distributions for the light-duty gasoline truck (LDGT) classes, little difference in MOBILE6 emission rates was found among the LDGV and LDGT1-4 vehicle types for Harris County in both 2000 and 2007. Consequently, the NOX, VOC, and CO emissions from passenger cars, pickups, and SUVs do not appear to be very sensitive to the distribution of VMT among the LDGV and LDGT1-4 classes. The relative humidity input (new with MOBILE6.2) was found to have a significant impact on estimated NOX emissions among just light-duty gasoline vehicles and motorcycles. MOBILE6 does not show any effect from changing the humidity input on heavy-duty diesel NOX, even though the NOX emission rates from HDDVs are relatively high.