Methane emissions from China: a high-resolution inversion of TROPOMI satellite observations

Abstract

Abstract. We quantify methane emissions in China and the contributions from different sectors by inverse analysis of 2019 TROPOMI satellite observations of atmospheric methane. The inversion uses as a prior estimate the latest 2014 national sector-resolved anthropogenic emission inventory reported by the Chinese government to the United Nations Framework Convention on Climate Change (UNFCCC) and thus serves as a direct evaluation of that inventory. Emissions are optimized with a Gaussian mixture model (GMM) at up to 0.25∘×0.3125∘ resolution. The optimization is done analytically assuming log-normally distributed errors on prior emissions. Errors and information content on the optimized estimates are obtained directly from the analytical solution and also through a 36-member inversion ensemble. Our best estimate for total anthropogenic emissions in China is 65.0 (57.7–68.4) Tg a−1, where parentheses indicate the uncertainty range determined by the inversion ensemble. Contributions from individual sectors include 16.6 (15.6–17.6) Tg a−1 for coal, 2.3 (1.8–2.5) for oil, 0.29 (0.23–0.32) for gas, 17.8 (15.1–21.0) for livestock, 9.3 (8.2–9.9) for waste, 11.9 (10.7–12.7) for rice paddies, and 6.7 (5.8–7.1) for other sources. Our estimate is 21% higher than the Chinese inventory reported to the UNFCCC (53.6 Tg a−1), reflecting upward corrections to emissions from oil (+147 %), gas (+61 %), livestock (+37 %), waste (+41 %), and rice paddies (+34 %), but downward correction for coal (−15 %). It is also higher than previous inverse studies (43–62 Tg a−1) that used the much sparser GOSAT satellite observations and were conducted at coarser resolution. We are in particular better able to separate coal and rice emissions. Our higher livestock emissions are attributed largely to northern China where GOSAT has little sensitivity. Our higher waste emissions reflect at least in part a rapid growth in wastewater treatment in China. Underestimate of oil emissions in the UNFCCC report appears to reflect unaccounted-for super-emitting facilities. Gas emissions in China are mostly from distribution, in part because of low emission factors from production and in part because 42 % of the gas is imported. Our estimate of emissions per unit of domestic gas production indicates a low life-cycle loss rate of 1.7 % (1.3 %–1.9 %), which would imply net climate benefits from the current “coal-to-gas” energy transition in China. However, this small loss rate is somewhat misleading considering China's high gas imports, including from Turkmenistan where emission per unit of gas production is very high.

Assessing Methane Emissions From the Natural Gas Industry: Reviewing the Case of China in a Comparative Framework

“…For the transmission and distribution phase, emissions can be measured directly at the component level [18]. According to final calculation results, the proportion of methane emissions by phase in China, the United States and Canada are different: the United States and Canada have the largest proportion of methane emissions in the production phase [23,24], while China has the highest proportion in the distribution phase [25]. Emissions quantification methods for each phase are summarized in Figure 1.…”

“…Figure 1 contains a detailed comparison of emissions from different phases of natural gas systems in China, the United States and Canada in recent years. These data are obtained by high-resolution inversion using atmospheric methane observations [25,34], and the standard deviation and uncertainty range of the results are in brackets. Although this improves the estimation of methane emissions to some extent, this method is vulnerable to the accuracy of emissions inventory data [34].…”

“…Although this improves the estimation of methane emissions to some extent, this method is vulnerable to the accuracy of emissions inventory data [34]. Data for China, the United States and Canada for all phases of the natural gas supply chain are taken from two studies [25,34]. These data are easily affected by the objective difference in approaches to methane quantification in each country, and the subjective factors chosen in the study.…”

“…The reason for this difference may be that the production and consumption structure of natural gas and the basic situation of pipelines are different across these countries. First, the inversion analysis of China, the United States and Canada is based on the national emissions inventories from before 2016 [25,34]. Before 2016, China's natural gas production was lower than that of the United States and Canada [11].…”

“…Third, compared to the United States and Canada, China's natural gas is more dependent on imports. 42% of China's natural gas needs to be imported, including from countries like Turkmenistan, which has very high emissions per unit of natural gas production [25]. Therefore, China's high methane emissions in the distribution phase is likely to be affected by international trade.…”