Investigation of real-world emissions of China III and China IV buses in a high-altitude city: A case study of Lhasa

Limitations still remain among vehicle emissions standards and regulations about high-altitude areas at altitude over 2400 m. In view of high vehicle-use intensity for buses, on-road measurements were performed on four heavy-duty diesel vehicles by employing a portable emissions measurement system (PEMS) in Lhasa, with an average elevation of approximately 3650 m. The result indicated that under real-world driving conditions of Lhasa, compared with China III buses, China IV buses did not show excellent emission performance consistently. For example, carbon monoxide (CO) emission factor of China III-5.2L buses is 2.4 times of China IV-5.2L buses, while the nitrogen oxides (NO_x) is only 36% of China IV-5.2L. Furthermore, an operating mode binning and a micro-trips method are used to link real-world emissions of each vehicle to driving conditions. For China III-5.2L and China IV-5.2L buses, we found strong correlations between relative emission factors of carbon dioxide (CO₂) and NO_x and average speed. However, these correlations for China IV-7.8L bus were less strong. This phenomenon proves that the role of traffic conditions in affecting road driving emissions would be mitigated when selective catalytic reduction (SCR) functioning properly. This study have potential implications for policymakers concerning vehicle emissions management and control strategies, such as promotion of alternative fuel with in-used buses, aimed at emissions reduction.