Fig. 4: Diagram of impacts of meteorology and emission reductions on the ozone concentration in the YRD region during COVID lockdown.

The meteorology effects dominate the increase of both O3 and Ox concentrations. The emission reduction doesn’t lead to a fast chemical production of O3, but rather a reduced production. The increase of O3 is mainly driven by a greater reduction in the chemical loss of O3 via NO titration processes.