Assessment of air quality with TROPOMI during COVID-19 pandemic: NO2 over the Po valley

An analysis of the air quality over the Po valley has been performed by using both satellite and in situ observations of NO2 for the COVID-19 years, 2019-2021. To match satellite observations to those in situ, we have used a geostatistical re-gridding technique. The tools allow us to scale the satellite NO2 retrievals to a finer, spatial resolution, which helps us to perform a better spatial colocation with in situ observations. The satellite data consist of Level 2 (L2) NO2 retrievals from TROPOMI (the TROPOspheric Monitoring Instrument), whereas in situ observations are taken at eleven diverse stations, which are spread over the Po valley. The Po Valley, in winter 2019/20, has been the first region in Europe to be severely hit by the COVID-19 pandemic. The Italian government introduced severe restriction measures from March to May 2020 (lockdown). We compared TROPOMI NO2 concentration during winters 2018-19 (no-COVID-19) and the following 2 winters. The observations of TROPOMI, in agreement with the in-situ measurements, saw a significant decrease in the NO2 concentration in March 2020 after the introduction of the lockdown. But they also found a general decrease in lower tropospheric NO2 in winter 2019/2020, the warmest winter ever observed that has limited the use of power for residential and commercial heating. NO2 concentrations raise almost to the pre-COVID-19 values in 2020/21 winter.