Methodological issues 8321 Choice of method

The good practice method is to use either consumption data from users of SF6 or PFCs or top-down import, export and consumption data from national SF6 producers and distributors, disaggregated by major type of SF6 or PFC application. Acquiring this data will entail a survey of all producers and distributors of SF6 and PFCs to identify total net SF6 and PFC consumption. Once the data are obtained, the amount of SF6 and PFC consumed by application in this source category should be estimated.



SF6 is used as an insulating medium in the radar systems of military reconnaissance planes of the Boeing E-3A type, commonly known as AWACS. The purpose of the SF6 is to prevent electric flashovers in the hollow conductors of the antenna, in which high voltages of more than 135 kV prevail. When the plane ascends, SF6 is automatically released from the system and into the atmosphere to maintain the appropriate pressure difference between the system and the outside air. When the plane descends, SF6 is automatically charged into the system from an SF6 container on board. Most emissions occur during the pressure-balancing process on ascent, but emissions from system leakage can also occur during other phases of flight or during time on the ground. Annual emissions per plane have been estimated to be 740 kg, while the charge of each system is approximately 13 kg.

Figure 8.2 Decision tree for SF6 from AWACS

Figure 8.2 Decision tree for SF6 from AWACS


1. See Volume 1 Chapter 4, Methodological Choice and Identification of Key Categories (noting Section 4.1.2 on limited resources), for discussion of key categories and use of decision trees.


1. See Volume 1 Chapter 4, Methodological Choice and Identification of Key Categories (noting Section 4.1.2 on limited resources), for discussion of key categories and use of decision trees.

Tier 1 method - SF6 emissions per plane

If a country does not have data on SF6 consumption by its AWACS, it may use a per-plane emission factor to estimate emissions. An emission factor of 740 kg per plane per year is presented in Table 8.7 below; this figure is based on estimates of SF6 emissions from NATO Boeing E-3As. Note that actual emissions per plane are strongly influenced by the average number of sorties (take-offs) per plane per year. More frequent sorties will raise the emission rate above 740 kg/plane; less frequent sorties will lower it. Leakage rates during flight or during time on the ground will also affect the emission rate.

Equation 8.12 Emissions from AWACS (default emission actor)

User Emissions = 740 kg • Number of planes in AWACS fleet

Table 8.7

SF6 emissions per plane per year

Emissions per plane per year (kg SF6)


740 kg

±100 kg

Source: Schwarz (2005)

Table 8.8 includes information on national AWACS fleets world wide (Boeing, 2005); like other activity data, it may quickly go out of date. Countries are in the best position to know the numbers of planes in their AWACS fleets.

Table 8.8

National AWACS fleets




















Source: Boeing (2005)

Tier 2 method - user mass-balance method

The most accurate method for estimating SF6 emissions from AW ACS is to track SF6 consumption by the systems. To do so, the following equations, which are similar to the utility-level variant of the Tier 3 method for electrical equipment, may be used. Note that for AWACS, acquisitions and disbursements of SF6 containers are likely to be considerably more important to the result than acquisitions and retirements of operating systems.

Equation 8.13 Emissions from AWACS (user mass-balance)

User Emissions = Decrease in SF6 Inventory + Acquisitions of SF6 - Disbursements of SF6 - Net

Increase in AWACS Fleet Charge


Decrease in SF6 Inventory = SF6 stored in containers at the beginning of the year - SF6 stored in containers at the end of the year

Acquisitions of SF6 = SF6 purchased from chemical producers or distributors in bulk + SF6 purchased from AWACS manufacturers or distributors with or inside of new planes + SF6 returned to site after off-site recycling

Disbursements of SF6 = SF6 contained in AWACS that are transferred to other entities + SF6 returned to suppliers + SF6 sent off-site for recycling + SF6 destroyed

Net Increase in AWACS Fleet Charge = 13 kg • (New AWACS - Retiring AWACS)

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