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Retrofitting older vehicles with diesel particulate filter (DPF) is a cost-effective measure to quickly and efficiently reduce particulate matter emissions.This study experimentally analyzes real-world performance of buses retrofitted with CRT DPFs.18 in-use Euro Ⅲ technology urban and intercity buses were investigated for a period of 12 months.The influence of the DPF and of the vehicle natural aging on buses fuel economy are analyzed and discussed.While the effect of natural deterioration is about 1.2%-1.3%,DPF contribution to fuel economy penalty is found to be 0.6% to 1.8%,depending on the bus type.DPF filtration efficiency is analyzed throughout the study and found to be in average 96% in the size range of 23-560 nm.Four different load and non-load engine operating modes are investigated on their appropriateness for roadworthiness tests.High idle is found to be the most suitable regime for PN diagnostics considering particle number filtration efficiency.