Dynamics of urban carbon emissions: A comparative analysis of pre- and post-lockdown through eddy covariance measurements.

Urban areas are the major anthropogenic source of atmospheric CO2, thus making long-term and continuous observations of their carbon emission dynamics extremely important. The COVID-19 lockdown served as a natural experiment that provided a unique opportunity to analyse the contribution of human activities to CO2 emissions from urban areas. In 2020, Beijing experienced COVID-19 confinement with different levels of restrictions on social mobility and economic activity, resulting in reductions in CO2 emissions. To investigate the response mechanisms of CO2 flux to restriction measures, we analysed CO2 flux data obtained using the eddy covariance technique from 2015 to 2020, and compared CO2 flux during the COVID-19 confinement period in 2020 with the preceding years (2015-2019) and across various levels of confinement. The results showed that: (1) the annual CO2 flux was 2.1 ± 0.2 kg C/(m2·yr) in 2020, which showed a significant reduction of 31.8 % compared to the adjacent 2019; (2) the reduction in CO2 flux was closely related to the level of restrictions on human activities; (3) most reductions occurred during the morning (85.7 %) and evening (32.7 %) peak traffic times, indicating that commuting-related transportation is a primary contributor to urban CO2 emissions. It is suggested that measures that reduce transportation-related CO2 sources should be considered as priorities for reducing urban CO2 emissions. The dynamic variation of urban CO2 flux captured by the eddy covariance technology is conductive to strengthening the supervision of the implementation of urban carbon emission reduction policies, promoting the achievement of dual carbon goals.