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18 JFK Emissions fied even if a passenger was dropped off at the airport. This could also apply to airport operator and tenant employees who DEN JFK travel to or from the airport, as well as the movement of con- Emissions ORD struction equipment. Ideally, multipurpose trips (e.g., involv- DEN ing travel to a store before returning to the origin) should only SFO account for the direct distance from the origin to the airport ORD and vice versa. Such side trips should not be included in the in- Emissions ventory as they are not a part of the purpose for the airport trip. Figure 2-2. Attribution of aircraft emissions to the However, if the side trips are already embedded as part of the departure airport based on single flight legs. data available (e.g., total VMT data), it may be difficult--and resource intensive--to attempt to remove the side-trip contri- butions. Therefore, side trips could be left in the data and prop- erly documented to reflect this, especially since the overall All of the emissions from aircraft flights are allo- effect on the inventory will likely be small. cated to the departure airport. 2.6 Data Availability As an option, the flights (and hence, emissions) can be As with any new evaluation process, an airport operator further categorized into domestic and international flights, may not have data collected in a format and depth necessary as follows: to undertake some of the preferred approaches to quantify- ing emissions from a specific source. Alternatively, less de- A domestic flight is from a U.S. airport to another U.S. tailed data may be available requiring the use of alternative airport. quantification approaches. Additionally, many airports may An international flight is from a U.S. airport to a non-U.S. not have any data concerning a specific source, particularly airport; for non-U.S. airports, an international flight is those associated with a tenant. The Guidebook recommends from a non-U.S. airport to a U.S. airport. that the documentation accompanying the inventory clearly identify the data that were available (and their source), how It should be noted that when all of a flight's emissions are the data were used, and any issues associated with that data. attributed to the departure airport, it results in a geograph- Such documentation should strive for the greatest clarity and ically distorting effect--the location of the point of actual ability to replicate the results outside of the report. Depend- emissions do not align with the geographic location of the air- ing on data availability, airport operators may not be able to port as they do with the traditional assessment of criteria pol- perform any quantification of emissions for certain sources. lutants within the LTO cycle. Also, the total departure flight All such cases should be clearly documented. emissions attributed to an airport could be different than the total emissions derived from summing half the emissions 2.7 Reporting Units for Pollutants from arriving flights and half the emissions from departing GHG emissions are typically reported in metric tons. How- flights. Notwithstanding the technical difficulties (e.g., poten- ever, depending on the units of the source data (e.g., emission tial for overlaps with other airport inventories) associated with factors), emissions calculations can directly result in either such an approach, this could at least geographically "center" English (e.g., lbs) or metric (e.g., g, kg) units. These should all the emissions at the airport. These issues need to be carefully be converted to a common unit, preferably the metric ton. considered, especially as policies are made regarding attribu- Using this common unit allows easy reviews and compar- tion of GHG emissions to airports. Similar concerns can also isons with other inventories. The following conversions may be raised for GAVs, construction equipment, etc., that oper- be useful in preparing an inventory: ate outside of the airport property. For publicly owned and controlled GAV travel, emissions 1 lb = 0.0004536 metric ton, resulting from full round-trips should be captured. An exam- 1 g = .000001 metric ton, ple of a round-trip would be for a passenger who travels from 1 kg = 0.001 metric ton, his/her home to the airport, and upon completion of the air 1 Mg = 1 metric ton, travel, returns home. Thus, for GAVs, the emissions from ori- 1 Gg = 1,000 metric tons, and gin to the airport and the return segment should be quanti- 1 Tg = 1,000,000 metric tons.