Detection of fossil-fuel CO2 plummet in China due to COVID-19 by observation at Hateruma

The COVID-19 pandemic caused drastic reductions in carbon dioxide (CO2) emissions, but due to its large atmospheric reservoir and long lifetime, no detectable signal has been observed in the atmospheric CO2 growth rate. Using the variabilities in CO2 (Delta CO2) and methane (Delta CH4) observed at Hateruma Island, Japan during 1997-2020, we show a traceable CO2 emission reduction in China during February-March 2020. The monitoring station at Hateruma Island observes the outflow of Chinese emissions during winter and spring. A systematic increase in the Delta CO2/Delta CH4 ratio, governed by synoptic wind variability, well corroborated the increase in China's fossil-fuel CO2 (FFCO2) emissions during 1997-2019. However, the Delta CO2/Delta CH4 ratios showed significant decreases of 29 +/- 11 and 16 +/- 11 mol mol(-1) in February and March 2020, respectively, relative to the 2011-2019 average of 131 +/- 11 mol mol(-1). By projecting these observed Delta CO2/Delta CH4 ratios on transport model simulations, we estimated reductions of 32 +/- 12% and 19 +/- 15% in the FFCO2 emissions in China for February and March 2020, respectively, compared to the expected emissions. Our data are consistent with the abrupt decrease in the economic activity in February, a slight recovery in March, and return to normal in April, which was calculated based on the COVID-19 lockdowns and mobility restriction datasets.