NOTES AND CORRESPONDENCE Did Atmospheric CO2 and CH4 Observation at Yonagunijima Detect Fossil-Fuel CO2 Reduction due to COVID-19 Lockdown?

Synoptic-scale variabilities of atmospheric CO2 and CH4 observed at Yonagunijima (Yonaguni Island, YON, 24.47 degrees N, 123.01 degrees E) during winter (from January to March) in 1998 - 2020 were examined. The monthly mean variability ratios (Delta CO2 /Delta CH4) based on correlation slopes within 24 h time windows showed a clear increasing trend, which is mainly attributed to the unprecedented increase in the fossil fuel-derived CO2 (FFCO2) emissions from China. A similar increasing trend of the Delta CO2 /Delta CH4 ratio had been reported for the observation at Hateruma Island (HAT, 24.06 degrees N, 123.81 degrees E), located at approximately 100 km east of YON. Nevertheless, the absolute values for YON were 34 % larger than those for HAT. Additionally, the monthly average in February 2020 for YON showed no marked change, whereas that for HAT showed an abrupt considerable decrease associated with the FFCO2 emission decrease in China presumably caused by the COVID-19 lockdown. Investigating the diurnal variations, we found that the local influences were larger at YON, especially during daytime, than at HAT. Using nighttime data (20-6 LST) and a longer time window (84 h), we succeeded in reducing the local influences and the resulting monthly mean Delta CO2 /Delta CH4 ratio showed considerable similarity to that observed at HAT including the abrupt decrease in February 2020. These results convinced us that the Delta CO2 /Delta CH4 ratio could be successfully used to investigate the relative emission strength in the upwind region.