Methane Emissions from Natural Gas Vehicles in Beijing, Baoding, and Shijiazhuang, China during CAREBEIJING Field Campaign

Abstract:

From 2002-2012, number of Natural Gas Vehicles (NGVs) increased from 36,000 to 1.6 million (44 times) and is projected to reach 4.0 million by 2020. Deploying NGVs is an effective way to relieve Chinese air pollution, since NGVs can reduce emissions of particulate matter (PM) and NOx by 91% and 59% compared to gasoline vehicles (GVs). However, previous studies in US showed that NGVs might emit more methane (CH₄) than GVs, which is an important greenhouse gas and a precursor to air pollution. Despite the tremendous growth of NGVs and importance of CH₄, CH₄ emissions from NGVs in China haven’t been carefully studied yet. During CAREBEIJING field campaign in 2013 and 2014, we used a mobile platform to conduct on-road measurement of CH₄, carbon monoxide (CO), carbon dioxide (CO₂), and NH₃ (ammonia) in Beijing, Baoding, and Shijiazhuang, China. We found that the mean on-road CH₄:CO₂ ratio from NGVs is (0.004±0.0003) g CH4/g CO2, which is 20 times higher than previous report on CH₄:CO₂ emission ratio from US cars. Although NGVs can reduce 25% CO₂ compared to GVs, CH₄ emitted from NGVs would diminish this number to 19%. Using this emission ratio, we estimate that NGVs in China emitted 37 Gg CH₄ in 2010. CH₄ emissions from NGVs would increase to 207 Gg in 2020 (by a factor of 560%) if no regulation of CH₄ emissions from NGVs were implemented. Most of these emissions occur in urban areas, requiring analyses of their impacts on air quality in such regions. Therefore, a mass balance model is used to study the proportion of CH₄ emissions from NGVs in total urban CH₄ emissions for Baoding city. Finally, in order to better inform policy decisions, a life-cycle analysis is made to estimate potential CH4 emissions from NGV related industrials. Overall, NGVs are promising alternative fuel vehicles in China, but more attention should be paid to quantify their CH₄ emissions.