ghg-inventory-1990-2013

Figure 3.3.15 Splicing method decision tree for gasoline emissions
For all fuels, interpolation was considered inappropriate due to the size of the block of
unavailable data and the lack of data earlier than the missing block (1990–2000).
For emission estimates from diesel and liquefied petroleum gas (LPG), the relationship
between Tier 1 and Tier 2 appears nearly constant for both N 2 O and CH 4 from 2001 until
2004. As a result, the overlap method was used (IPCC, 2000), with:
Where:
/
‒ y t is the recalculated emission estimate computed using the overlap method
‒ x t is the estimate developed using the previous method
‒ y i and x i are the estimates prepared using the new and previously used
methods during the period of overlap, as denoted by years m through n.
However, for gasoline vehicles the ratio Tier 2/Tier 1 appears to change approximately
linearly with time. While surrogates for Ministry of Transport data were available (fuel
consumption), their use resulted in a step-change that is likely not representative of road
transport emissions for the period. While the trend in emissions was not consistent over
time, the trend of the Tier 2/Tier 1 ratio emission estimates showed a strong linear
relationship with time. As a result, a hybrid method of overlap and trend extrapolation
was chosen with:
Where:
‒ t is the year for which a new estimate is required
New Zealand’s Greenhouse Gas Inventory 1990–2013 73