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 (ΔCO₂) and methane (ΔCH₄) 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 ΔCO₂/ΔCH₄ ratio, governed by synoptic wind variability, well corroborated the increase in China's fossil-fuel CO₂ (FFCO₂) emissions during 1997-2019. However, the ΔCO₂/ΔCH₄ 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 ΔCO₂/ΔCH₄ 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.