The impact of COVID-19 lockdown on atmospheric CO in Xi'an, China.

Lockdown measures to control the spread of the novel coronavirus disease (COVID-19) sharply limited energy consumption and carbon emissions. The lockdown effect on carbon emissions has been studied by many researchers using statistical approaches. However, the lockdown effect on atmospheric carbon dioxide (CO) on an urban scale remains unclear. Here we present CO concentration and carbon isotopic (δC) measurements to assess the impact of COVID-19 control measures on atmospheric CO in Xi'an, China. We find that CO concentrations during the lockdown period were 7.5% lower than during the normal period (prior to the Spring Festival, Jan 25 to Feb 4, 2020). The observed CO (total CO minus background CO) during the lockdown period was 52.3% lower than that during the normal period, and 35.7% lower than the estimated CO with the effect of weather removed. A Keeling plot shows that in contrast CO concentrations and δC were weakly correlated (R = 0.18) during the lockdown period, reflecting a change in CO sources imposed by the curtailment of traffic and industrial emissions. Our study also show that the sharp reduction in atmospheric CO during lockdown were short-lived, and returned to normal levels within months after lockdown measures were lifted.