Refer to the Table 1.34 and 1.35, Road transportation, CO2, N2O in Part I of the Synthesis and Assessment (S&A) report to see for which of these Parties N2O emissions from road transportation is a key category and to see which methodological tier they reported to have applied.
For the Tier 1 approach, emission factors that are specific to each gas, fuel type, and sector are used. For Tier 2 and 3, the Detailed-Technology-Based Approach, emission factors are further specified by the type of combustion technology (and sometimes emission controls) used (e.g., four-stroke internal combustion engine with 3-way catalyst). The Tier 2 and 3 approaches provide more disaggregated emission estimates, but also require more data to apply.
Refer to Sections 2.2.1.1, p. 2.37 and 2.3.1.1, p. 2.44 of the IPCC Good Practice Guidance for a further description of Tier 1 and 2 (or higher) approaches for estimating non-CO2 emissions from fuel combustion (stationary and road vehicles) and for guidance on choosing an appropriate method for the different subcategories.