| With the rapid development of urbanization, energy consumption from transportation section has been increasing, and the carbon emission from urban traffic has become an important part of urban carbon emission. CO2 and CH4 are two kinds of the most important greenhouse gases from road traffic emission. In recent years, the increasing number of natural gas fueled vehicles, greatly enhances the CH4 emission, which raises the difficulties to evaluate the carbon emission from transportation section. In this study, we conducted experiments to analyze the features of CO2 and CH4 emission and its influential factors, which took the temporal and spatial variation of urban traffic into consideration. The atmospheric CO2 and CH4 observation were conducted on urban main roads and in tunnels in Nanjing. The relationship between CO2 and CH4 emissions (the ?CH4:?CO2 value) was established based on atmospheric observation data. And the CH4:CO2 emission ratio was also calculated by IPCC methods based on 2010-2014 vehicle statistics. Finally,these two methods were compared and analyzed. This research aimed to apply robust results of CH4 emission from transportation section. The results showed as follows:(1) During the observation period, the diurnal variation of CO2 concentration in the main road in Nanjing showed bimodal distribution, and the two peaks appeared in 07:30 and 17:30. The diurnal variation of CH4 concentration in the morning and evening was higher than at noon, and the high value appeared in 17:30.The CO2 and CH4 in workday were higher than those in weekend. The spatial variation of CH4 concentration was higher than the spatial difference of CO2 concentration on the road. Due to the "piston wind" in the tunnel, the CH4 concentration in tunnel was gradually increased from the inlet to the outlet.(2) Traffic volume was the main factors for the increase of CO2 and CH4 concentration on road. The concentration of CO2 and CH4 on the road can be also significantly increased due to the road traffic jamn. Road slope also can lead to elevated concentrations of CO2 and CH4 on roads.(3) There was a significant linear correlation between CH4 concentration and CO2 concentration on the road. The atmospheric ?CH4:?CO2 value on the main road was 0.0090. The diurnal variation showed "W" type. The range of atmospheric?CH4:?CO2 value in Nanjing experiment tunnels was 0.0006-0.0062. The atmospheric ?CH4: ?CO2 values in different tunnels showed a large difference.(4) There was a significant positive correlation between the atmospheric?CH4:?CO2 value and the proportion of natural gas taxis, but the traffic volume was not. The results showed natural gas vehicles could greatly enhance the intensity of the CH4 emissions on urban roads.(5) In 2010-2014, the CH4:CO2 emission ratio of vehicles was 0.00090,0.00089, 0.00090, 0.00091, and 0.00092, respectively. No matter what kind of fuel type vehicle,the CH4:CO2 emission ratio emitted from vehicle with the higher emission standards was smaller than with lower emission standards. For vehicle with same emission standards,the CH4:CO2 emission ratio of natural gas vehicle was maximum, the CH4:CO2 emission ratio from gasoline type vehicles was higher than diesel type vehicle.(6) The estimated value of CH4 emission from passenger vehicle based on atmospheric concentration observation method in Nanjing urban area was higher than that applied on IPCC method. The difference analysis of the two methods shows that the estimation of the IPCC method was underestimated, and the estimation of the atmospheric concentration observation method was credible. |
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