Methane emissions from on-road vehicles in China: a case study in an urban tunnel

Abstract

Reducing emissions of methane (CH4) in developed regions and urban areas is a practical way to curb the unexpected surge in global CH4 levels in recent decades. Traffic emissions are among the important anthropogenic CH4 emission sources in megacities, yet CH4 emissions from on-road vehicles are less characterized and not well addressed. Based on tunnel tests in an urban tunnel in south China, a real-world emission factor (EF) of CH4 was measured to be 0.26 ± 0.03 g·km−1 (mean ±95% C.I.) for on-road vehicle fleet which including gasoline vehicles, diesel vehicles, and liquefied petroleum gas vehicles, with an average CH4/CO2 mass ratio of 40.6E-5 g·g−1, and CH4 could account for 1.3% of vehicle CO2-equivalent emissions. Using the measured CH4/CO2 ratio and available automobile CO2 emission estimates, traffic CH4 emissions in 2014 could have reached 333 Gg and represented 0.6% of total anthropogenic CH4 emissions in China, approximately four times the previous reported value of 79 Gg. Our results indicate that improving energy efficiency would have co-benefits for reducing traffic emissions of CH4, as observed EFs of CH4 are positively correlated with that of CO2, and over 90% of traffic CH4 emissions in China could be avoided if the traffic CH4/CO2 ratio can be an order of magnitude lower as previously observed in a tunnel in Switzerland.