The research team of EMISIA and the Environmental Protection Department of Hong Kong have published their recent study:
Papadopoulos G, Keramydas C, Ntziachristos L, Lo T-S, Ng K-L, Wong H-LA and Wong CK-L (2018). Emission Factors for a Taxi Fleet Operating on Liquefied Petroleum Gas (LPG) as a Function of Speed and Road Slope. Frontiers in Mechanical Engineering 4:19. doi: 10.3389/fmech.2018.00019.
The article may be accessed from here.