We quantified the COVID-19 transmissibility by the basic reproductive number.
The COVID-19 transmissibility could be negatively associated with ambient ozone.
The daily 1-h maximum ozone might cover 7.6% of the transmissibility variability.
It echoes a previous study of negative effects of ozone on the flu transmission.
The ground-level ozone may be a “double-edged sword” to public health.
Keywords: Air pollution; COVID-19; China; Ozone; Reproduction number.