A decade of CO flux measured by the eddy covariance method including the COVID-19 pandemic period in an urban center in Sakai, Japan.

Cities constitute an important source of greenhouse gases, but few results originating from long-term, direct CO emission monitoring efforts have been reported. In this study, CO emissions were quasi-continuously measured in an urban center in Sakai, Osaka, Japan by the eddy covariance method from 2010 to 2021. Long-term CO emissions reached 22.2 ± 2.0 kg CO m yr from 2010 to 2019 (± denotes the standard deviation) in the western sector from the tower representing the densely built-up area. Throughout the decade, the annual CO emissions remained stable. According to an emission inventory, traffic emissions represented the major source of CO emissions within the flux footprint. The interannual variations in the annual CO flux were positively correlated with the mean annual traffic counts at two highway entrances and exits. The CO emissions decreased suddenly, by 32% ± 3.1%, in April and May 2020 during the period in which the first state of emergency associated with COVID-19 was declared. The annual CO emissions also decreased by 25% ± 3.1% in 2020. Direct long-term observations of CO emissions comprise a useful tool to monitor future emission reductions and sudden disruptions in emissions, such as those beginning in 2020 during the COVID-19 pandemic.