(2020) estimated a 30% drop in urban CO 2 emissions from the San Francisco Bay Area (California, USA), mostly related to changes in traffic. An ICOS report ( Papale et al., 2020), where monitoring net emissions of CO 2 at neighborhood scale over several cities throughout Europe were described, showed that the reduction in urban CO 2 release to the atmosphere during lockdown ranged from 8 to 75%, as a function of the underlying urban activities and extension of urban green spaces. It is well established that cities are responsible for ~70% of energy-related GHG emissions ( Hoornweg et al., 2011), mostly related to housing and traffic sectors ( Pichler et al., 2017), and their relevance in terms of CO 2 emissions driving global climate change is widely recognized ( Duren and Miller, 2012 Baur et al., 2014). While at the global scale the COVID-related emission reductions were relatively small and the impact on GHG concentrations was hardly detectable, the pandemic-related restrictions had a disproportionate effect on atmospheric CO 2 at urban scale, where emission reductions were expected to be large and the impact on CO 2 concentrations detectable. Accordingly, the atmospheric CO 2 concentrations measured at the Mauna Loa Observatory (Hawaii, USA) increased by 0.7% (up to 416.21 ppm) in April 2020 compared with April 2019 ( Tans and Keeling, 2020). On the other hand, the COVID-19-related CO 2 emission drop in May 2020 was estimated to account only for 0.4 ppm on the expected concentration of CO 2 in the atmosphere at a global scale ( Betts et al., 2020), equivalent to a reduction of about 0.1%. Similarly, a clear reduction in greenhouse gas (GHG) emissions due to COVID-related restrictions was observed, with a decrease in daily global CO 2 emissions at national scale ranging from 11 to 25% by April 2020 compared to 2019 levels ( Le Quéré et al., 2020) and an overall drop in global CO 2 emissions of 6.4% (corresponding to 2.3 billion tons) in 2020 ( Tollefson, 2021). As a collateral effect, these restrictions produced a suite of indirect positive impacts on the environment, largely improving both water and air quality, as documented by recent literature (e.g. Consequently, worldwide governments adopted strategic restrictive measures suppressing industrial and commercial activities, limiting human movements (including social distancing), in order to contain the diffusion of confirmed positive cases and number of deaths ( Brauner et al., 2020 Haug et al., 2020). The alarming spread and gravity of the Severe Acute Respiratory Syndrome Coronavirus-2 (SARS-CoV-2) infection led, on March 11, 2020, the World Health Organization (WHO) to declare the coronavirus (COVID-19) outbreak a global pandemic.
Finally, the investigation of the shape of the frequency distribution of the analyzed variables revealed interesting aspects concerning the dynamics of the systems. Although the direct impact of restriction measurements on CO 2 concentrations was less clear due to seasonal trends and background fluctuations, an in-depth analysis of the daily local CO 2 enhancement with respect to the background values revealed a progressive decrease throughout the lockdown phase at the end of the heating season (>10 ppm), followed by a net increase (ca.
Similarly, the δ 13C-CO 2 values recorded the change in the prevailing sources contributing to urban atmospheric CO 2, confirming the effectiveness of carbon isotopic data as geochemical tracers for identifying and quantifying the relative contributions of emitting sources. The temporal trends of the analyzed parameters, combined with the variations in emitting source categories (from inventory data), evidenced a fast response of flux measurements to variations in the strength of the emitting sources. In this study, continuous measurements of CO 2 fluxes, atmospheric CO 2 concentrations and δ 13C-CO 2 values were performed in the historical center of Florence (Italy) before, during and after the almost two-month long national lockdown. Accordingly, cities experienced a significant decrease of CO 2 emissions.
The outbreak of COVID-19 pandemic was accompanied by global mobility restrictions and slowdown in manufacturing activities.
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