From 2000 to 2017, China saw a dramatic increase in natural gas vehicles (NGVs), rising from 6,000 to 6.08 million. This surge was primarily driven by environmental concerns and economic incentives. Natural gas, perceived as a cleaner fuel with lower carbon content and air pollutants, offered NGVs a cost advantage in meeting stringent emission standards compared to gasoline and diesel vehicles. The financial benefits incentivized adoption; payback periods on the additional cost of purchasing an NGV were estimated at two to three years for taxi drivers and around one year for truck drivers. China aimed to further increase the NGV population to 10 million by 2020, with even more growth anticipated beyond 2020, particularly in heavy-duty applications (buses and trucks) where electrification presented greater challenges. In 2017, heavy-duty trucks comprised 3.1% of China's total vehicles but accounted for 53% and 60% of vehicular NOx and particulate matter (PM) emissions. To address this, the State Council of China set a goal to retire older heavy-duty vehicles and promote heavy-duty NGVs. The rapid expansion of NGVs, while aiming to reduce NOx, PM, and CO2 emissions, also introduced the unintended consequence of potentially significant methane (CH4) emissions, a potent greenhouse gas, thus jeopardizing the potential climate benefits of this transition. Although China boasts the world's largest NGV fleet, CH4 emission factors (EFs) for NGVs had not been accurately quantified, and NGV CH4 emissions were largely excluded from China's CH4 emission inventories. Previous life-cycle analyses (LCAs) mainly focused on upstream CH4 emissions (extraction, processing, distribution) but neglected operational vehicle emissions due to a lack of direct measurements within China. This paper aimed to fill this knowledge gap by providing a comprehensive analysis of the real-world methane emissions from NGVs in China and their impact on overall greenhouse gas emissions.

Existing studies on NGVs in China primarily focused on the upstream stages of natural gas production and distribution, utilizing LCA to assess CH4 emissions. However, these studies largely overlooked CH4 emissions from vehicle operation, due to limited on-road measurement data. Some research did attempt to incorporate vehicle emissions, but often relied on emission factors (EFs) derived from studies in other regions like the US, which may not accurately reflect the situation in China. One study, Hu et al. (2018), reported an overall CH4 EF for NGVs in China that was significantly higher than that reported by the IPCC. This difference is likely due to several factors. Firstly, a significant portion of NGVs in China were retrofitted from conventional vehicles. These vehicles often used engines and after-treatment systems not originally designed for natural gas, resulting in different efficiency levels and different rates of CH4 emission. Secondly, many heavy-duty NGVs in China utilize lean-burn (LB) engines with oxidation catalysts (OC), which, due to lower exhaust temperatures compared to the optimal temperature for CH4 removal, may contribute to higher real-world CH4 emissions than suggested by certification tests. The existing literature highlighted the need for direct measurements of CH4 emissions from NGVs in China to accurately assess their climate impact and improve the accuracy of life-cycle assessments and emission inventories.

This research employed on-road measurements of CH4 emissions from NGVs in Baoding and Shijiazhuang, China, during the 2014 CAREBEIJING North China Plain field campaign. A mobile laboratory equipped with fast-response sensors (10 Hz) enabled the use of the plume-chasing method, allowing researchers to directly measure emissions from individual vehicles while minimizing interference from nearby vehicles' exhaust. The Gaussian puff model was used to validate the method, showing significant reduction in interference from nearby vehicles. Data from 73 NG buses and 63 NG taxis were collected, using CH4 and CO2 mixing ratios to derive CH4:CO2 emission ratios, which were subsequently converted into fuel-specific CH4 emission factors. For heavy-duty NG trucks, direct measurements were challenging due to less clear labeling. Instead, researchers used the measured CH4 EF of NG buses as a proxy, accounting for potential differences in driving conditions and engine specifications. The emission factors were adjusted to account for venting emissions (CH4 released from onboard fuel tanks) and seasonal variations (accounting for higher emissions during cold starts). Well-to-wheels (WTW) GHG emissions for NGVs were estimated by combining upstream GHG emission factors from previous studies, distance-specific fuel consumption data, and the adjusted CH4 EFs. Three scenarios were developed to project future CH4 emissions and WTW GHG emissions under varying levels of China VI standard implementation and enforcement. These scenarios considered vehicle population projections, different engine technologies (lean-burn vs. stoichiometric), and levels of enforcement of the methane emission limits in the China VI standard. Finally, a bottom-up CH4 emission inventory for NGVs in China was constructed from 2000 to 2017 using vehicle population data, fuel consumption estimates, and the derived emission factors. The methodology carefully considered uncertainties inherent in the various data sources and the measurement methods.

The study's key findings indicate significantly higher CH4 emissions from NGVs in China than previously estimated. On-road measurements revealed a fuel-specific CH4 EF of 1.7 ± 0.5% for NG taxis and 2.9 ± 0.5% for NG buses. The bus emission factor was 90% higher than the China V standard limit for heavy-duty vehicles. The EF for NG taxis was substantially higher than those reported for light-duty NGVs in the US and EU. While the researchers couldn't directly measure emissions from NG trucks, they estimated the EF based on the bus data and driving conditions, accounting for uncertainties. After accounting for venting and seasonal variations, the adjusted EFs were 1.9%, 3.2%, and 3.2% for NG taxis, NG buses, and NG trucks, respectively. Life-cycle analyses incorporating these higher CH4 emissions demonstrated that switching to NGVs from conventional vehicles in China resulted in a net increase of 77 Mt CO2eq in GHG emissions between 2000 and 2017. Scenario analyses revealed that stringent enforcement of the China VI standard could reduce GHG emissions by 509 Mt CO2eq from 2020 to 2030. Conversely, a scenario with weak enforcement of the China VI standard would lead to a substantial increase in CH4 emissions and a cumulative increase in WTW GHG emissions of 432 Mt CO2eq from 2020 to 2030. The study also found that for cars and buses, using natural gas may not lead to GHG emission mitigation. However, for NG trucks meeting the China VI standard, GHG emissions were lower than for diesel trucks.

The findings highlight the critical need to consider real-world emissions rather than relying solely on certified emission limits when assessing the climate impact of NGVs. The discrepancy between real-world and certified emissions underscores the limitations of existing testing protocols, particularly the use of the European Transient Cycle (ETC), which might not accurately reflect real-world driving conditions. The study's comprehensive approach, combining on-road measurements with life-cycle analyses and scenario modeling, allows for a more accurate and nuanced assessment of the climate implications of NGV adoption in China. The results strongly suggest that stringent implementation and enforcement of the China VI standard is crucial for mitigating CH4 emissions from NGVs and achieving a net reduction in greenhouse gases. Failure to do so could lead to significant environmental consequences. The success of implementing the China VI standard will serve as a valuable case study and could influence NGV development and policy in other countries.

This study demonstrates that the widespread adoption of NGVs in China has, to date, resulted in a net increase in greenhouse gas emissions due to significant and previously unaccounted-for methane emissions from heavy-duty vehicles. Strict implementation and enforcement of the China VI emission standard are vital for mitigating these emissions and achieving climate benefits from NGVs. Further research is needed to refine CH4 emission factors under diverse driving conditions and to improve the accuracy of emission inventories. The methodology developed in this study is readily transferable to other regions with large NGV fleets, providing a valuable tool for assessing the true climate impact of this technology.

The study acknowledges several limitations. The on-road CH4 measurements, while extensive, may underestimate emissions due to the exclusion of cold-start emissions and sporadic venting events from onboard fuel tanks. The estimation of CH4 EFs for heavy-duty NG trucks relied on data from NG buses due to the difficulty of directly measuring truck emissions. The accuracy of scenario projections depends on the accuracy of future vehicle population estimates and the degree of China VI standard enforcement. Despite these limitations, the study's findings provide a robust and much-needed assessment of the climate implications of NGVs in China.