Large methane emissions from natural gas vehicles in Chinese cities

In recent years, cities in developing countries have become more and more reliant on natural gas as a clean energy source for transportation to reduce air pollution. In this study, we used street-level measurement of atmospheric CH4 and CO2 concentrations to quantify CH4 emissions from natural gas vehicles (NGVs) in China. These measurements were made in eight cities (Chengdu, Urumqi, Jinan, Nanjing, Lanzhou, Harbin, Guangzhou) with varying sizes of NGV fleet. A traffic CH4:CO2 emission ratio (TER) was determined via linear regression of CH4 versus CO2 concentration data obtained from each street transect. The TER value was combined with the ratio of NGVs in the street traffic in a mathematical model to obtain the CH4 emission factor for NGVs. Results show that the TER increases with increasing NGV ratio and decreases with increasing traffic speed. Overall, the NGV CH4 emission factor in these cities is 0.022 +/- 0.0033 kg m(-3), about 8 times the Intergovernmental Panel on Climate Change (IPCC) default factor for NGVs and is more than 100% higher than the mean NGV tailpipe emission factor found in the published literature. That the overall emission factor is much larger than the tailpipe emission factor indicates that on-road vehicle gas leakage is a widespread problem. A business-as-usual scenario suggests that NGVs may emit 1.23 Tg CH4 yr(-1) in 2030, or about 3% of China's current total anthropogenic emission. Our study suggest that curbing the emissions from this sector should be a high priority for global climate mitigation efforts.