Measurements of real-world vehicle CO and NOx fleet average emissions in urban tunnels of two cities in China

•In-use fleet average CO and NOX emission factors of urban China were measured.•CO emission has been far lower than the control standard.•NOX should be taken as the key point in the control of tunnels pollutants in China.•Vehicle emission is correlated with roadway conditions and traffic characteristics.•Emission factors increase with roadway gradient and decrease with vehicle speed.

Urban tunnels located in the city center areas, can alleviate traffic pressure and provide more convenient traffic for people. Vehicles emit pollutants that are significant contributors to air pollution inside and at the outlet of tunnels. Ventilation is the most widely used method to dilute pollutants in tunnels. To calculate the design required air volume flow accurately, vehicle emissions should be exactly determined. Emission factors are important parameters to estimate vehicle emissions. To characterize carbon monoxide (CO) and nitrogen oxides (NOX) emission factors for a mixed vehicle fleet under real-world driving conditions of urban China, we measured CO and NOX concentrations in Shanghai East Yan'an Road tunnel and Changsha Yingpan Road tunnel in 2012 and 2013. In-use fleet average CO and NOX emission factors were calculated according to tunnel pollutants mass balance models.The results showed that the maximum CO concentration in August was 86 ppm, while in October it was 45 ppm in Shanghai East Yan'an Road tunnel. The maximum concentrations of CO and NOX were 33 ppm and 2 ppm in Changsha Yingpan Road tunnel, respectively. In-use fleet average CO emission factors of East Yan'an Road tunnel, with gradient of −3% ∼ 3%, were 1.266 (±0.889) ∼ 3.974 (±2.189) g km−1 vehicle−1. In-use fleet average CO and NOX emission factors of Yingpan Road tunnel with gradient of −6% ∼ 6% amounted to 0.754 (±0.561) ∼ 6.050 (±5.940) g km−1 vehicle−1 and 0.121 (±0.022) ∼ 0.818 (±0.755) g km−1 vehicle−1, respectively. The dependences of CO and NOX emission on roadway gradient and vehicle speed were found. The average CO and NOX emission factors increased with the ascending of roadway gradient as well as reverse with vehicle speed. These findings provide meaningful reference for ventilation design and environmental assessment of urban tunnels, and further help provide basic data to formulate relevant standards and norms.