Abstract
The COVID-19 lockdowns led to major reductions in air pollutant emissions. Here, we quantitatively evaluate changes in ambient NO
, O
, and PM
concentrations arising from these emission changes in 11 cities globally by applying a deweathering machine learning technique. Sudden decreases in deweathered NO
concentrations and increases in O
were observed in almost all cities. However, the decline in NO
concentrations attributable to the lockdowns was not as large as expected, at reductions of 10 to 50%. Accordingly, O
increased by 2 to 30% (except for London), the total gaseous oxidant (O
= NO
+ O
) showed limited change, and PM
concentrations decreased in most cities studied but increased in London and Paris. Our results demonstrate the need for a sophisticated analysis to quantify air quality impacts of interventions and indicate that true air quality improvements were notably more limited than some earlier reports or observational data suggested.