On-road CH4 emissions from NG taxis and buses
CH4 emissions from exhaust and leakage from NG buses and taxis in Baoding and Shijiazhuang were measured by our mobile laboratory equipped with fast-response sensors. We measured 26 h on-road, covering around 600 km in these two cities in June 2014 (details about instruments and spatial coverage can be found in Supplementary Table 3 and Supplementary Fig. 3). The fast-response sensors (10 Hz) allowed the use of the plume-chasing method to measure on-road emissions from vehicles. Several criteria, including sufficient CO2 and CH4 enhancements, correlations between CH4 and CO2 and videos recorded on the road, were developed to identify plumes from NGVs. Supplementary Movie 1 provides an example of the on-road measurements. A Gaussian puff model was used to investigate the effectiveness of our method to minimize the influence of the exhaust of nearby vehicles, and the results show our method can significantly reduce interferences caused by the emissions from other vehicles28. Using the plume-chasing method, we were able to capture emissions from 73 NG buses and 63 NG taxis during the field campaign. The observed CH4 and CO2 mixing ratios were used to derive CH4:CO2 enhancement and emission ratios. The emission ratios were then converted to fuel-specific CH4 emission factors. Similar methods have been used to estimate vehicular NH3 emissions29,30. More details and discussion about the uncertainty of the method can be found in the “Method” section and Supplementary Discussion. Figure 2 shows the on-road fuel-specific CH4 EFs (presented as % of NG consumed) derived from CH4:CO2 emission ratios measured in China as well as previously reported EFs.
Sixty-three NG taxis with clear NGV labels were sampled to represent light-duty NGVs in China, which had an average EF of 1.7 ± 0.5%. The EF is 16 times higher than the values reported for light-duty NGVs in the US and EU (0.10 ± 0.3%), but the EF agrees with the tailpipe CH4 EF measured in the exhaust of NG taxis by Hu et al.23 (1.7 ± 0.8%). The CH4 EF measured from 73 NG buses in China is 2.9 ± 0.5%, which is 90% higher than the CH4 limit of the China V standard for heavy-duty vehicles31. We were able to distinguish buses powered by liquified natural gas (LNG) and compressed natural gas (CNG) by checking the label of the buses. No statistically significant difference was found between the EFs of LNG buses (39 buses, 2.8 ± 0.4 %) and CNG buses (34 buses, 3.1 ± 0.5 %). The NG buses in these two cities were equipped with LB engine and OC, and they were certified for the China VI and China V standards, respectively. We also observed low NH3 emissions from NG buses (Supplementary Fig. 4), consistent with the reported pattern for NGVs with LB engine with OC32,33. The observed EF of NG buses is more consistent with the overall on-road CH4 EF measured by Hu et al.23 (3.0 ± 0.5%) than the observed EF of light-duty NGVs. To validate our method, we conducted additional measurements by following NG buses in Atlantic City, US, in the spring of 2015. The observed EF agrees with previously reported tailpipe CH4 emissions for NG buses in the US as well as the CH4 emissions used in the GREET model18.
Estimation of CH4 emissions from heavy-duty NG trucks
Identifying NG trucks in China was more difficult than NG buses since they were not labeled as clearly as the NG buses. Therefore, we could not derive CH4 EF for heavy-duty NG trucks using our observations. Our survey shows that NG trucks certified for China IV and V from the major manufacturers in China are equipped with similar LB engines and OC but with slightly larger displacements than the engines on NG buses (Supplementary Table 4). This type of engine is rarely used on trucks in other countries, and therefore, no CH4 EF have been reported for NG trucks equipped with LB engines. Previous studies suggested driving conditions of the vehicles may have larger impacts on CH4 emissions rather than the chassis2,19. Comparing CH4 EFs reported for NG buses and trucks equipped with similar SM engines and TWC, we did not find a significant difference for both the tailpipe and the crankcase CH4 emissions (Fig. 2 and Supplementary Table 2)33,34,35,36,37,38. Therefore, the measured CH4 EF of NG buses is used to estimate CH4 emissions from heavy-duty NG trucks. Since NG trucks may operate on the highway more frequently than NG buses, we assigned a larger error to the lower-bound uncertainty of EFs of NG trucks, which equals to the lower-bound uncertainty of the previously reported CH4 EF of LB engines with OC (Fig. 2 and Supplementary Table 2).
Venting-emission and seasonality adjustment
Because low CO2 enhancements and correlations between CH4 and CO2 mixing ratio enhancements are used to remove impacts from other CH4 sources, our method can capture operations related CH4 emissions from tailpipes and crankcases but may miss sporadic venting events directly from the on-board fuel tanks that are not fed to the engine. Clark et al.19 found these emissions are difficult to be characterized by in-field observations because of the large volume of methane vented in single events and their intermittent nature. Using tank pressure and liquid fuel level (%) differences before and after venting, they estimated the fuel-specific emission rate of these venting events is 0.1% of NG consumed in the US (about 8.4% of total pump-to-wheels CH4 emissions for NGVs in the US)19. The same emission rate is adopted in our study to account for the venting emissions. Our observations were made in June with an average ambient temperature of 30 °C, which may underestimate CH4 emissions during cold seasons, especially for the cold-start emissions. Among the studies reviewed, only two studies reported the cold-start CH4 emissions for heavy-duty NGVs at low temperatures. The ratio of cold- and hot-start for CH4 EFs at around 0 °C ranges from 1.08 for vehicles with a fuel-specific EF of 11.2% to 2.69 for vehicles with a fuel-specific EF of 0.2% (Supplementary Table 5)37,39. To account for the potential impact of cold-start emissions at low temperature, we adjusted the observed EFs using a cold-start/hot-start emission ratio of 1.5 and a weighting factor of 14% for cold-start emissions as listed in the testing procedure for the China VI standard (see “Method” section for details). The adjusted EFs are 1.9 [−0.7, +0.9] %, 3.2 [−0.8, +1.0] %, and 3.2 [−1.7, +1.0] % for NG taxies, heavy-duty NG buses, and heavy-duty NG trucks as shown by the red dots and bars in Fig. 2.
Technological pathways for the China VI standard
Figure 2 also shows the EFs for SM engines equipped with TWC and the high-pressure direct injection (HPDI) engines. Both have the potential to meet the CH4 limit of the China VI standard. However, high CH4 emissions from the crankcases of SM engines have been observed as NG could pass through the gaps between the piston rings and the cylinders19. When crankcase CH4 emissions are considered, it will be difficult for SM engines to meet the China VI standard unless a complicated, closed crankcase ventilation system (CCV) is installed2. No crankcase CH4 emission has been reported for the HPDI engines, but HPDI engines require venting of the high-pressure fuel to balance NG and diesel fueling pressures, leading to dynamic venting CH4 emissions19. The dynamic venting CH4 emissions could far outweigh the tailpipe CH4 emissions during urban operation and could be equivalent to tailpipe emissions during highway operation19.
Well-to-wheels GHG emissions of NGVs in China
Previous studies have estimated the WTW GHG emissions for NGVs in China with limited consideration of CH4 emissions from NGVs (see Supplementary Table 6 for studies reviewed)14,16,22. Ou et al.22 investigated multiple pathways of CNG and LNG in China and reported a WTP leakage rate about 0.6% of NG consumed in the Tsinghua Life Cycle Analysis Model. Huo et al. assumed the technologies in China for production and distribution of CNG and LNG are similar to the ones used in other regions and adopted the rates of 1.93% of NG consumed for extraction and production and 0.007% of NG transported per km via pipeline from the GREET model16,18. The difference of WTP GHG emissions between CNG and LNG (1%) is lower than the variation caused by the CH4 leakage from pipeline distribution (standard deviation of 7%) since the transport distance ranges from 200 to 4400 km for different provinces. Therefore, the same WTP GHG emission factor (28 ± 6 CO2eq MJ−1) and the same WTP CH4 leakage rate (1.65 ± 1.05% of NG consumed) are used for both LNG and CNG. The overall WTP leakage rate is about the same as the CH4 EF of light-duty NGVs and is 40% lower than the CH4 EF of heavy-duty NGVs (Fig. 2).
The distance-specific WTW GHG EFs for NGVs are derived in this study by combining previously reported upstream GHG EFs, distance-specific fuel consumption, and adjusted CH4 EFs of NGVs (shown in Fig. 3). The uncertainty of the national level WTW GHG EF for NGVs in China is large because of the variation in NG transport distance via pipeline (from 200 km to 4400 km). For provincial analysis, as demonstrated by Huo et al.16, the uncertainty could be reduced. With the observed CH4 emissions, both light-duty NGVs and NG buses are unlikely to reduce GHG emissions compared to their counterparts. For NG buses, the WTW GHG emissions are likely to be higher than diesel buses even if they satisfy the China VI standard CH4 limit because of their increased fuel consumption (Supplementary Table 7). Switching from diesel trucks to current generation NG trucks equipped with LB engines and OC as the measured NG buses is likely to increase GHG emissions by 160 [−200, +180] g CO2eq km−1. Only the ones operating mostly on the highways in the near-source regions may have lower WTW GHG EF compared to diesel trucks.
For trucks equipped with SM engines and TWC or HPDI engines, the WTW GHG emissions are similar to diesel trucks. It should be noted that the fuel consumption of trucks equipped with SM engines and TWC is assumed to be the same as trucks with LB engines. Operating at lean conditions is an effective way to improve fuel efficiency compared to a pure stoichiometric operation40. However, the fuel economy of SM engines can be significantly improved by operating the engine with diluted mixtures through exhaust gas recirculation (EGR) systems, which also can significantly reduce NOx emissions35,40. Hajbabaei et al.35 compared the fuel consumption of a SM engine with an EGR system and two LB engines. They found the SM engine with EGR had very similar fuel consumption compared to the LB engines. For the NG trucks to be certified for the China VI standard, SM engines are likely to be used with an EGR system to be competitive in the market in terms of fuel economy and to be in compliance with the China VI NOx emission limit and the China Stage 3 fuel consumption limits41. The same fuel consumption was scaled by 0.95 to approximate the fuel consumption of HPDI engines because Thiruvengadam et al.32 reported the fuel consumption of HPDI engines was 4% lower than that of SM engines with EGR systems.
If the China VI standard is stringently the enforced with the real-world emissions being the same as the CH4 emission limit, switching from diesel trucks to NG trucks will lead to a GHG reduction of 100 ± 150 g CO2eq km−1, and upstream CH4 leakages will become the limiting factor for lowering the WTW GHG emissions from NGVs in China. Although having real-world emissions in line with certified emission limits is challenging, it has been shown to be technically achievable at least for NOx emissions from Euro VI trucks, to which the China VI standard is equivalent26.
CH4 emissions from NGVs in China
NG consumption of the Transport, Storage, and Post sector reported in the China Statistical Yearbook (CSYB) does not have the detailed categorical information for estimating CH4 emissions from NGVs in China42. Therefore, we estimated NG consumption of NG taxis, light-duty NGVs (non-taxi), NG buses, and NG trucks in China as the product of vehicle population (Supplementary Table 1), distance-specific fuel consumption (Supplementary Table 7), and annual mileage traveled (Supplementary Table 8). The four categories are determined based on fuel consumption and emission characteristics and availability of the population data. Figure 4a shows the estimated NG consumption and reported NG consumption in the CSYB42. Personal light-duty NGVs (light-duty NGVs except for NG taxis) should be excluded when comparing the estimated NG consumption and the CSYB reported values since fuel consumed by personal vehicles are not included in the Transport, Storage, and Post sector in the CSYB43. The sum of NG consumption of NG taxis, buses, and trucks is slightly lower than the CSYB reported consumption because NG consumption of cargo ships is included in the CSYB but not included in our estimates. For 2017, our estimate is closer to the reported consumption of CSYB likely due to the NG shortage in China in the winter of 2017. In 2017, NG buses and trucks consumed about 70% of the total NG consumption of NGVs.
Total CH4 emissions and changes in WTW GHG emissions are calculated by multiplying the corresponding emission factors (venting-emission and seasonality adjusted) to the NG consumption (see “Method” section for more details). Figure 4b, c shows the estimated and the projected total CH4 emissions from NGVs in China and the changes in WTW GHG emissions of switching to NGVs from gasoline and diesel counterparts for 2000–2030. The annual CH4 emissions from NGVs in China increased from 0.0014 [−0.0004, +0.0004] Mt in 2000 to 0.77 [−0.28, +0.22] Mt in 2017. Switching to NGVs has increased the GHG emissions by 83 Mt CO2eq for 2000–2017. More than 80% of CH4 emissions from NGVs are emitted by NG buses and trucks in 2017 because of their high fuel consumption and high EFs. Therefore, the implementation of the CH4 limit of the China VI standard for heavy-duty vehicles is critical for mitigating future CH4 emissions from NGVs.
Three scenarios were designed to assess different pathways regarding the implementation of the China VI standard. Table 1 lists the major features of these scenarios. The population estimates are adapted from the projection by Wu et al.6, where aggressive electrification for applicable fleets was considered (see Supplementary Table 9 for projected vehicle population for the three scenarios). The fuel consumption of heavy-duty vehicles (both NGVs and conventional gasoline or diesel vehicles) purchased after 2021 is lowered by 15% assuming that the Stage 3 China Fuel Consumption Standard will be implemented successfully41.
The high-emission scenario represents the pathway that retrofitting light-duty vehicles is allowed. In addition, this scenario assumes that the CH4 limit of China VI standard is loosely enforced, which has been the case for previous standards as demonstrated here. Although LB engines with OC are considered the last generation technology, they could meet the NOx limit of China VI standard if SCR is implemented11. If the CH4 limit of the China VI standard is loosely implemented, LB engines may dominate the heavy-duty vehicle market because of their advantages in terms of upfront cost, since SM engines require precise air–fuel ratio control strategies and an exhaust gas recirculation system40. Under this scenario, annual CH4 emissions from NGVs in China would increase to 3.3 Mt, equivalent to 8% of the estimated total anthropogenic CH4 emissions and 17% of CH4 emissions related to fossil fuel production and consumption in China in 201013. Cumulatively, switching to NGVs from counterparts would increase the WTW GHG emissions by 432 Mt CO2eq from 2020 to 2030 under this scenario (the integrated area under the orange curve in Fig. 4b from 2020 to 2030).
The medium-emission scenario represents the pathway that retrofitting is prohibited, and heavy-duty NGVs sold after 2019 are equipped with SM or HPDI engines. Because of the increased cost, the penetration rate of NGVs is lower than the high-emission scenario. Under this scenario, CH4 emissions from NGVs in China would increase at a slower rate, reaching 1.3 Mt in 2030 and the cumulative changes in the WTW GHG emissions from 2020 to 2030 would increase by 117 Mt CO2eq.
The low-emission scenario assumes that the EF of the heavy-duty NGVs purchased after 2019 is the same as the CH4 limit of China VI standard. The growth of NGVs is assumed to be localized within source regions where NG price is low, and the leakage CH4 emissions related to NG distribution are lower than the medium- and high-emission scenarios. The annual CH4 emissions from NGVs in China would gradually decrease to 0.7 Mt in 2030 and reduce the WTW GHG emissions by 77 Mt CO2eq cumulatively from 2020 to 2030 under this scenario. Comparing the cumulative WTW GHG changes between the high- and the low-emission scenarios, we find that stringently enforcing the China VI standard for heavy-duty vehicles could generate a GHG reduction of 509 Mt CO2eq for 2020– 2030, equivalent to eliminating GHG emissions of 12 million passenger cars with the current GHG emission level.