Carbon Dioxide and Methane Measurement on Urban Roads in Nanjing, China

In recent years, cities have become more and more reliant on natural gas as a main source of clean energy to reduce air pollution. One unintended consequence, however, is increase in CH4 emissions which contribute to global warming. In Nanjing, the capital city of Jiangsu Province in the Yangtze River Delta, China, almost all taxis and about 30 percent buses are now powered by natural gas, and an increasing number of trucks are switching to natural gas as energy source. However, CH4 emissions from road vehicles have so far been ignored by the Intergovernmental Panel on Climate Change (IPCC) inventory method. In this study, we determined the CH4:CO2 emissions ratio for taxi and other vehicles, using the atmospheric CH4 and CO2 concentrations measured on three main streets in the city of Nanjing. The CH4:CO2 emissions ratio for all vehicles was 0.0088 mol mol-1 on average, a little higher than the ratio measured at the city scale (0.0079 mol mol-1). But the ratio for taxi was much high, with a mean and a median value of 0.014 and 0.0094 mol mol-1, respectively, and a maximum of 0.070 mol mol-1. This atmospheric estimate of the CH4:CO2 emissions ratio for vehicles is in broad agreement with measurement on board of vehicles observing directly tailpipe emission found in the literature. Omission of on road vehicle emission is likely one reason for the 67.3% underestimation Nanjing's methane emissions using the IPCC method (Shen et al. 2014. Advances in Atmospheric Sciences, 31: 1343-1352).