The CO2 emission factors should be within a range of ±5 percent, as they are dependent only on the carbon content of the fuel and fraction oxidised. However, considerable uncertainty is inherent in the computation of CO2 based on the uncertainties in activity data discussed below. For Tier 1, the uncertainty of the CH4 emission factor may range between -57 and +100 percent. The uncertainty of the N2O emission factor may range between -70 and +150 percent Moreover, CH4 and N2O emission factors vary with technology and using a single emission factor for aviation in general is a considerable simplification.
Information to assist in computing uncertainties associated with LTO emission factors found in Table 3.6.9 can be found in Lister and Norman, 2003; and ICAO, 1993. Information to assist in computing the uncertainties associated with cruise emission factors found in Table 3.6.10 data can be found in: Baughcum et al, 1996. Sutkus, et al, 2001; Eyers et al, 2004; Kim, 2005 a and b; Malwitz, 2005. If resources are not available to compute uncertainties, uncertainty bands can be used as defined as default factors in Section 126.96.36.199.
Special attention should be taken with the cruise NOx emission factors for Tier 2 found in Table 3.6.10. These emission factors, have been updated from the 1996 Guidelines to reflect the fact that climb phase emissions are substantially different from those in cruise. The calculation of the NOx emission factors is based on two sets of data, one from 1 km to 9 km, and the second from 9 km to 13 km., and the differences in the amount of NOx calculated during that phase could be in the range of approximately 15 to 20 percent, due to the thrust/power required in that phase, and its relation with the higher production of NOx. If Tier 2 is used, care should be taken to report a consistent time series (see Section 188.8.131.52 and Volume 1, Chapter 5).
The uncertainty in the reporting will be strongly influenced by the accuracy of the data collected on domestic aviation separately from international aviation. With complete survey data, the uncertainty may be very low (less than 5 percent) while for estimates or incomplete surveys the uncertainties may become large, perhaps a factor of two for the domestic share. The uncertainty ranges cited represent an informal polling of experts aiming to approximate the 95 percent confidence interval around the central estimate. The uncertainty will vary widely from country to country and is difficult to generalise. The use of global data sets, supported by radar, may be helpful in this area, and it is expected that reporting will improve for this category in the future.
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