Meteorology-Normalized Impact of COVID-19 Lockdown upon NO2 Pollution in Spain

Abstract:

The spread of the new coronavirus (COVID-19) forced the Spanish Government to implement extensive lockdown measures in March 2020. Strong reductions of nitrogen dioxide (NO2) were then observed in many regions of the country. This study aims at quantifying to what extent the lockdown is responsible for the reduction of NO2 concentrations observed during that period. While many studies simply consider a baseline NO2 concentration obtained from past observed concentrations, the confounding effect of meteorological variability can lead to strong errors in the quantification of the NO2 reduction. In order to properly take this into account, we designed a methodology relying on the use of machine learning (ML) models (gradient boosting machine) fed with meteorological data (from ERA5 reanalysis) and other time features, to estimate more accurately the so-called business-as-usual NO2 concentrations that would have been observed in the absence of lockdown. The meteorology-normalised change of NO2 was finally obtained by comparing the business-as-usual NO2 concentrations to the observed ones. We applied this analysis for a selection of urban background and traffic stations covering the more than 50 Spanish provinces and islands. We quantified the meteorology-normalised NO2 changes over the first 41 days of lockdown (from March 14^th to April 23^rd 2020).

Results show that the ML models perform remarkably well in most locations. The lockdown measures were found to be responsible for a 50% reduction of NO2 concentrations on average over all Spanish provinces and islands. Stronger NO2 reductions were highlighted during the phases II (the most stringent one) and III than during phase I. In largest cities, a stronger meteorology-normalised NO2 change is highlighted at traffic stations compared to urban background ones. We also show the importance of taking into account meteorological variability for accurately assessing the impact of the lockdown on NO2 levels, in particular at fine spatial and temporal scales.

Petetin, H., Bowdalo, D., Soret, A., Guevara, M., Jorba, O., Serradell, K., and Pérez García-Pando, C.: Meteorology-normalized impact of COVID-19 lockdown upon NO2 pollution in Spain, Atmos. Chem. Phys. Discuss., https://doi.org/10.5194/acp-2020-446, in review, 2020.