Early spring near-surface ozone in Europe during the COVID-19 shutdown: Meteorological effects outweigh emission changes.

This paper analyses the impact of the control measures during the COVID-19 lockdown in Europe (15 March-30 April 2020) on 1-h daily maximum nitrogen dioxide (NO) and maximum daily 8-h running average ozone (MDA8 O) observations obtained from the European Environment Agency's air quality database (AirBase). Daily maximum NO decreased consistently over the whole continent, with relative reductions ranging from 5% to 55% with respect to the same period in 2015-2019 for 80% of the sites considered (10th - 90th percentiles). However, MDA8 O concentrations showed a different pattern, decreasing over Iberia and increasing elsewhere. In particular, a large region from northwestern to central Europe experienced increases of 10-22% at urban background stations, reaching typical values of the summer season. The analysis of the expected NO and O concentrations in the absence of the lockdown, using generalised additive models fed by reanalysis meteorological data, shows that the low NO concentrations were mostly attributed to the emission reductions while O anomalies were dominated by the meteorology. The relevance of each meteorological variable depends on the location. The positive O anomalies in northwestern and central Europe were mostly associated with elevated temperatures, low specific humidity and enhanced solar radiation. This pattern could be an analogue to study the limits of pollution control policies under climate change scenarios. On the other hand, the O reduction in Iberia is mostly attributable to the low solar radiation and high specific humidity, although the reduced zonal wind also played a role in the proximity of the Iberian Mediterranean coast.