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

Abstract

Reducing emissions of methane (CH 4 ) in developed regions and urban areas is a practical way to curb the unexpected surge in global CH 4 levels in recent decades. Traffic emissions are among the important anthropogenic CH 4 emission sources in megacities, yet CH 4 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 CH 4 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 CH 4 /CO 2 mass ratio of 40.6E-5 g·g −1 , and CH 4 could account for 1.3% of vehicle CO 2 -equivalent emissions. Using the measured CH 4 /CO 2 ratio and available automobile CO 2 emission estimates, traffic CH 4 emissions in 2014 could have reached 333 Gg and represented 0.6% of total anthropogenic CH 4 emissions in China, approximately four times the previous reported value of 79 Gg. Our results indicate that improving energy efficiency would have cobenefits for reducing traffic emissions of CH 4 , as observed EFs of CH 4 are positively correlated with that of CO 2 , and over 90% of traffic CH 4 emissions in China could be avoided if the traffic CH 4 /CO 2 ratio can be an order of magnitude lower as previously observed in a tunnel in Switzerland.