Mobile Measurement of CO2 and CH4 on Ring Roads and Expressways in Xi’an, China: Spatial Patterns and Influencing Factors

Carbon dioxide (CO2) and methane (CH4) are important greenhouse gases emitted by vehicles, yet their spatial pattern within urban road networks remains poorly characterized. This study conducts mobile measurements of CO2 and CH4 along the 1st Ring Road (mean speed of the monitoring vehicle: 22 km h−1), 2nd Ring Road (22 km h−1), and Ring Expressway (90 km h−1) in Xi’an, China, to investigate their spatial distribution and influencing factors. Average CO2 and CH4 concentrations on the three ring roads exhibited a clear decline with increasing distance from the city center. Both CO2 and CH4 showed higher concentrations at lower vehicle speeds and at higher traffic volumes, but CH4 hotspots differed spatially from CO2 and exhibited weaker dependence on vehicle speed, indicating additional off-road CH4 sources (e.g., natural gas leaks, nearby livestock farms). In a tunnel case study, CO2, CH4, and CO concentrations increased concurrently from entrance to exit, resulting in strong correlations between CH4 and CO2 (R2 = 0.86) and between CO and CO2 (R2 = 0.66). This produced emission ratios (ΔCH4/ΔCO2 = 0.160 ppb ppm−1; ΔCO/ΔCO2 = 3.151 ppb ppm−1), representative of incomplete combustion under congestion. Our results provide direct observational evidence linking urban traffic dynamics to greenhouse gas emissions and emphasize the need for congestion mitigation to reduce on-road climate-relevant emissions.