At high NOx concentrations, other compounds limit ozone production, e.g. hydrogen oxide radicals (HOx)
Additional ozone destruction by reactions with NOx becomes increasingly important at the highest NOx concentrations
Eventually it even leads to negative net ozone production rates Ozone is an important greenhouse gas
Hence NOx related ozone changes have an impact on climate that is more complex than climate effects of CO2 increases. Significant increase in surface ozone has been observed at northern hemisphere mid-latitudes in the 1970s and 1980s