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28 The following is a sample calculation for one round trip: 3.4.3 GAV Method 3 CO2 emissions = (1.67 gal fuel ) (19.564 lbs CO2 gal fuel ) As an alternative to the use of these fuel-based emission fac- = 32.67 lbs CO2 . This equates to 0.0148 metric tors in Methods 1 and 2, the USEPA's MOBILE6.2 model can tons which is derived using a conversion be used to generate CO2 emission factors for different vehicle factor of 0.0004536 metric tons s lb. types (USEPA 2002). Similarly, the USEPA's MOVES2004 (USEPAb 2004) can be used to estimate fuel consumption (and hence, CO2 emissions) as well as emissions of CH4 and N2O. 3.4.2 GAV Method 2 These provide another option in calculating higher resolution Method 2 is similar to Method 1 except that the VMT data emissions (e.g., by specific vehicle types, age, mileage, emis- sions controls, etc.) to better track emissions over time. An are expanded to show a range of vehicle types (e.g., cars, trucks, overview of Method 3 is provided in Figure 3-9. motorcycles, etc.) and potentially other specific categorizations This method is contingent upon having the VMT data for the including vehicle age, mileage, emissions controls, etc. This is specific vehicle categories. If those data are not available, there indicated in Figure 3-8. would be no fidelity gained from trying to use the specific emis- These specific categorizations would allow better tracking sion factors from a model like MOBILE6.2. In such a situation, of emissions over time. The inventory developer must deter- it would be more appropriate to simply use the MOBILE6.2 mine the range of specific categorizations that would be ap- composite emission factor. In that case, the fidelity and resolu- propriate for the airport. In each of these cases, because the tion would essentially be the same as Method 1, and hence, no data are still in a general form, fuel economy data are sug- advantage would be gained from using MOBILE6.2. gested to quantify the fuel consumed by these vehicles. Some examples of more specific fuel economy data than that shown in Section 3.4.1 are as follows: Method 3 for GAVs is contingent upon the availability of VMT data for specific vehicle cate- Light truck = 17.4 mpg (USEPAb 2005), gories; no fidelity is gained from using MOBILE6.2 Light truck = 17.6 mpg (FHWA 2002), without such data. Two-axle, four-tire truck = 16.2 mpg (DOE 2007), Medium truck (10,000 to 26,000 lbs) = 8.0 mpg (DOE 2007), and The following example calculation is for one round trip Heavy truck (more than 26,000 lbs) = 5.8 mpg (DOE 2007). using an emission factor from MOBILE6.2: CO2 emissions = ( 40 mi ) ( 0.25 kg CO2 mi ) = 10 kg CO2 . This equates to 0.01 metric tons VMTs for Each Specific CO2 when using a conversion factor of Category of Vehicle Type, Vehicle Age, Mileage, Vehicle Fuel 0.001 me etric ton kg. Emissions Control, etc. Economy Data This example calculation would correspond to a specific cat- egory of vehicle type, age, mileage, emissions control, etc.-- again, depending on the needs of the airport. Calculate VMTs for Each Specific Specific Emission Factors GAV Fuel Emission Category of Vehicle Type, from MOBILE6.2 and Other Consumption Factors Vehicle Age, Mileage, Models as Necessary Emissions Control, etc. Calculate Calculate GAV GHG GAV GHG Emissions Emissions Figure 3-8. Overview of GAV Method 2. Figure 3-9. Overview of GAV Method 3.