Nitrogen dioxide reductions from satellite and surface observations during COVID-19 mitigation in Rome (Italy).

Lockdown restrictions were implemented in Italy from 10 March 2020 to contain the COVID-19 pandemic. Our study aims to evaluate air pollution changes, with focus on nitrogen dioxide (NO), before and during the lockdown in Rome and in the surroundings. Significant NO declines were observed during the COVID-19 pandemic with reductions of - 50%, - 34%, and - 20% at urban traffic, urban background, and rural background stations, respectively. Tropospheric NO vertical column density (VCD) from the TROPOspheric Monitoring Instrument (TROPOMI) was used to evaluate the spatial-temporal variations of the NO before and during the lockdown for the entire area where the surface stations are located. The evaluation is concerned with the pixels including one or more air quality stations to explore the capability of the unprecedented high spatial resolution to monitor urban and rural sites from space with relation to the surface measurements. Good agreement between surface concentration and TROPOMI VCD was obtained in Rome (R = 0.64 in 2019, R = 0.77 in 2020) and in rural sites (R = 0.71 in 2019). Inversely, a slight correlation (R = 0.20) was observed in rural areas during the lockdown due to very low levels of NO. Finally, the TROPOMI VCD showed a sharp decline in NO, larger in urban (- 43%) than in rural sites (- 17%) as retrieved with the concurrent surface measurements averaging all the traffic and urban background (- 44%) and all the rural background stations (- 20%). These results suggest air pollution improvement in Rome gained from implementing lockdown restrictions.