Tracing the regulated emissions of field-aged gasoline/natural gas bi-fuel taxis from new to 160,000 km: Deterioration and environmental implications

•NG fueling emitted initially lower CO but higher THC and NOx emissions than gasoline.•Emission deteriorated faster in NG mode due to poor calibration and regulation flaws.•NG fueling reduced 20 % CO2 but the GWP benefit was partially offset by fuel leakage.