EMEP/EEA air pollutant emission inventory guidebook 2016, 1.A.4 Non road mobile machinery 2016, Table 3-2 'Tier 2 emission factors for off-road machinery', notes that emission factors for LPG are not available at Tier 2 technology level and refers to Tier 1 emission factors, in this case, for all technology levels.
LPG emission factors for Tier 1 are shown in Table 3-1. E.g. 1.A.2.g.vii, 1.A.4.a.ii, 1.A.4.b.ii and 1.A.4.c.ii: NOx emission factor = 61093 g/tonnes fuel.
This value is higher than any NOx emission factor, at any technology level, for diesel fuel listed on Table 3-2 for NFR sectors 1.A.2.g.vii and 1.A.4.a.ii. E.g. 1991-Stage I NOx EF = 43552 g/tonnes fuel.
As one of the known benefits of LPG vs. diesel is a potential reduction of NOx emissions, is there any explanation to these differences?
Many thanks in advance for your help.