  Air pollution is among the highest contributors to mortality worldwide,
especially in urban areas. During spring 2020, many countries enacted social
distancing measures in order to slow down the ongoing COVID-19 pandemic. A
particularly drastic measure, the 'lockdown', urged people to stay at home and
thereby prevent new COVID-19 infections during the first (2020) and second wave
(2021) of the pandemic. In turn, it also reduced traffic and industrial
activities. But how much did these lockdown measures improve air quality in
large cities, and are there differences in how air quality was affected? Here,
we analyse data from two megacities: London as an example for Europe and Delhi
as an example for Asia. We consider data during first and second wave lockdowns
and compare them to 2019 values. Overall, we find a reduction in almost all air
pollutants with intriguing differences between the two cities except Delhi in
2021. In London, despite smaller average concentrations, we still observe
high-pollutant states and an increased tendency towards extreme events (a
higher kurtosis of the probability density during lockdown) during 2020 and low
pollution levels during 2021. For Delhi, we observe a much stronger decrease of
pollution concentrations, including high pollution states during 2020 and
higher pollution levels in 2021. These results could help to design policies to
improve long-term air quality in megacities.
