In the US we went thru this years ago when NY and Mass began blaming the Ohio valley states for their pollution problems.
Transport of ground level ozone extends out relatively short distances before it disperses into background noise. But there are certain times or conditions when exchanges between the higher and lower levels of the atmosphere occur. Common sense tells you that it is a wash because if China is polluting San Francisco then San Francisco is polluting Denver, or Amarillo, or Atlanta.
We regulate this in the US using attainment areas, which are densely populated areas and we ignore rural areas which are non-attainment areas. At the same time we measure a national average of ozone in parts per million, which can become an issue. Bush lowered it to 75 ppm then Obama lowered it to 70 ppm.
Ozone is a function of hours of sunlight per day because ozone is produced in a photo-chemical reaction between NOx and VOCs, plus temperature and pressure because chemical reactions take place faster as temperature and pressure increase: for each 10 degrees centigrade you raise the temp, the rate of the reaction doubles. Because of this ozone is always worse in the summer when the days are longer and the air temp is higher.