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Modifying Link-Level Emissions Modeling Procedures for Applications within the MOVES Framework

5.0 Model Outputs

MOVES outputs from the model runs performed during this study are summarized in this section. These include the TDM-based and HPMS-based model runs originally performed for each of the eight counties in the Houston-Galveston non-attainment area, as well as the model runs conducted for the same region and VMT bases using the alternate Kansas City drive cycles described above in Section 4. These outputs are described in additional detail below.

5.1 Comparison of MOVES Outputs to MOBILE-6 Based Inventory

First, ERG compared the two sets of MOVES runs conducted using the default drive cycles to the original TTI MOBILE6 based inventory 29, which was previously described in detail in Section 2.0. Table 5-1 presents ERG's modeled TDM-based and HPMS-based MOVES model outputs, along with TTI's MOBILE6 inventory emissions in tons, by county, for a Non-School ozone season weekday. Table 5-2 presents the percent difference between both sets of MOVES runs and the original TTI inventory. Note that according to these tables, MOVES is calculating significantly higher NOx emissions across the board, regardless of the VMT basis used for model inputs. There is a wider variance in emissions for CO and VOC; counties with relatively few VMT return a decrease in CO and VOC, while counties with more VMT are associated with a significant increase in CO and VOC. The largest county (in terms of both VMT and population), Harris county, demonstrates much higher emissions of all three pollutants of interest relative to the MOBILE6 inventory.

Note that generally, as shown in Table 5-1, the HPMS VMT being modeled is somewhat higher (for most counties) relative to the modeled MOBILE6 inventory, which in turn leads to higher emissions relative to that same inventory. We conclude here that it's very important, regardless of the VMT basis chosen for calculation, for users to be sure that their activity data is representative, since the model outputs are highly sensitive to it.

Table 5-1. Summary of VMT and Tons of Emissions, by County, Non-school Ozone Season Weekday

Table 5-2. Percent Difference between MOVES and MOBILE6 Inventory Results

Tables 5-3 and 5-4 present emissions and VMT split by light-duty and heavy duty vehicle classification. Consistent with summaries found in TTI's July 2010 Technical Note, vehicle classes in the MOBILE6 summaries are defined as follows:

• Light duty vehicles include those classified as LDGV, LDGT1, LDGT2, LDGT3, LDGT4, LDDV, LDDT12, MC, and LDDT34.
• Heavy duty vehiclesinclude all other classes.

Similarly, TTI defined light- and heavy-duty vehicles in MOVES output summaries as follows:

• Light duty vehicles include the source types Motorcycles, Passenger Cars, Passenger Trucks, and Light commercial Trucks.
• Heavy duty vehiclesinclude all MOVES source type.

For consistency we have duplicated this classification scheme in the following tables. The reader will immediately note in Table 5-4 the large discrepancy in VMT when comparing heavy duty VMT modeled in MOVES to the heavy duty VMT originally modeled by TTI in MOBILE6. For example, the HD VMT modeled under the TDM-based VMT scenario is between 35 and 40 percent less than that modeled in MOBILE6. This is a result of attempting to directly compare emissions aggregated under MOBILE6 vehicle types (which classify vehicles on a GVWR weight basis) versus emissions aggregated under MOVES source types (which classify vehicles based on their expected mode of operation). In particular, vehicle types HDDV2B through HDDV5 inclusive (along with their gasoline counterparts), considered heavy-duty in MOBILE6, map to MOVES source types 31 (passenger trucks) and 32 (light commercial trucks). These are considered "light-duty" under the TTI classification. Because of this, it is difficult to draw conclusions about emissions when differentiating by vehicle classification, but we include the tables below for reference.

Table 5-3. Summary of VMT and Tons of Emissions, by County and Vehicle Classification, Non-school Ozone Season Weekday

Table 5-4. Percent Difference between MOVES and MOBILE6 Inventory Results, by Vehicle Classification

Tables 5-5 and 5-6 present emissions and VMT split by on-network and off-network process classification. Processes associated with running exhaust and evaporative emissions were classified as on-network, while off-network processes included emissions due to vehicle starts and extended idling. This classification is consistent with that proposed in TTI's July 2010 Technical Note.

Table 5-5. Summary of VMT and Tons of Emissions, by County and Emissions Process Classification, Non-school Ozone Season Weekday

Table 5-6. Percent Difference between MOVES and MOBILE6 Inventory Results, by Emissions Process Classification

Complete summaries of emissions and VMT output by county for each model run performed, by VMT-basis (TDM or HPMS), drive cycle (default or Kansas City derived), vehicle classification (light- or heavy-duty), emissions process, and process classification (on- or off-network) are presented in Appendix C. Raw model outputs are also available in Appendix A.

5.2 Comparison of MOVES Default Drive Cycle Output to Alternate Drive Cycle Outputs

In performing comparisons of the modeling performed using MOVES default drive cycles to the modeling conducted using the Kansas City drive cycles developed by ERG, we limited our summaries to only those source types affected by the drive cycle changes. These included motorcycles, passenger cars, passenger trucks, and light commercial trucks (source types IDs 11, 21, 31, and 32, respectively).
Table 5-7 presents a comparison of modeled light duty emissions for the combined 8-county Houston-Galveston area. Note that the Kansas City drive cycles produce slightly lower emissions for both VMT bases, in the range of approximately 1-3%. This difference is dominated by emissions from Harris county; in Appendix D, it can be seen that percent differences range from 1-17% on a by-county basis, with emissions from counties with fewer VMT showing the widest variance. It can also be seen that VOC tends to vary less than the other two pollutants modeled.

Table 5-7. Comparison of Eight County Light Duty Emissions for Both Drive Cycles and VMT Bases (Overall tons)

Table 5-8 presents a comparison of combined light duty emissions by MOVES source type. The largest differences between the modeled drive cycles are seen in motorcycles, which are a relatively small portion of the light-duty fleet. Overall differences across source types and counties range from approximately +1% to -3%. As before, these emissions are dominated by results from Harris county, and other more rural counties exhibit wider variance in emissions between modeled drive cycles, as shown Appendix D.

Table 5-8. Comparison of Eight County Light Duty Emissions for Both Drive Cycles and VMT Bases (Tons, by Source Type)

Table 5-9 presents combined light-duty emissions by MOVES road type. Variations in this table are fairly pronounced. Although there is virtually no change in off-network emissions (which is expected, as no drive cycles for that road type was modified), the Kansas City restricted road types (freeways) have approximately 3-8% higher emissions, while unrestricted road types (local roads) have emissions that are about 5-15% lower. These differences are independent from the rural/urban distinction, which is expected, since as described above we did not have enough microtrips in the Kansas City Data to make that distinction in the development of our final drive cycles. The variation also seems to be independent of VMT calculation basis.

Table 5-9. Comparison of Eight County Light Duty Emissions for Both Drive Cycles and VMT Bases (Tons, by Road Type)

Table 5-10 presents combined light duty emissions by MOVES emission process ID. For CO and NOx, the only difference between modeled drive cycles is attributable to running exhaust and crankcase running exhaust, and is on the order of about 1-4% less, depending on VMT basis. There is no difference at all in start emissions. For VOC, there is also no difference in start emissions, and further, no difference in evaporative emissions either. This is expected, since start emissions and evaporative emissions are both off-network processes, and as such, should be unaffected by changes to on-network drive cycles. However, there is larger variance in both exhaust and refueling emissions, approximately 4-6% less, depending on the VMT basis modeled.

Table 5-10. Comparison of Eight County Light Duty Emissions for Both Drive Cycles and VMT Bases (Tons, by Emission Process ID)

The information presented in Tables 7 through 10 above is also available on a by-county basis in Appendix D. As shown in these tables, the effects of implementing the drive cycles developed by ERG in MOVES varied considerably depending on which county was being modeled. The drive cycles we developed were based on largely urban driving data as discussed in Section 4, and applying these drive cycles to more rural counties in the Houston-Galveston area (as exhibited by fewer VMT driven in those counties) produced emissions that may not be representative. On the other hand, emissions from Harris County, the most urban of the counties modeled (with the corresponding largest daily VMT modeled), did not vary much at all. We believe that any alternate drive cycles developed and used in MOVES should closely match the type of driving activity for the area being modeled for emissions to be considered representative.

29 All references to the MOBILE6-based TTI inventory in this section are based on TTI's Production of MOVES On-Road Mobile, Link-Based Emissions Estimates and Document Preparation Technical Note, submitted to TCEQ in July 2010, in which TTI assessed the 2005 and 2006 years using both MOVES and MOBILE6. This is the most recently available document from TTI that discusses summaries of their MOBILE6 -based inventory for 2005 and 2006. This document was produced subsequent to TTI's 2005/2006 On-Road Mobile Source, Ozone Episode Modeling Emission Inventories for the HGB Eight-Hour Nonattainment Area, which was the source of the data we based this analysis upon originally.