© 2019 American Chemical Society. Understanding quantitative relationships between ambient ozone concentrations and precursor emissions is important to policymakers and stakeholders. Such relationships are often captured as ozone isopleth diagrams developed using air quality models. Model-based approaches have limitations, including errors stemming from uncertainties in inputs and modeled processes, and can be computationally burdensome. We develop and apply an empirical method based on ozone design values calculated in the South Coast Air Basin, California, for 1975-2016 to construct ozone isopleths. The study domain is the area with the highest ozone levels in the United States that has experienced high levels of emissions control. Quadratic and log-quadratic models were constructed, and both capture the actual observations very well (R2 ∼0.98) and re-create the general characteristics of traditional air quality model-generated isopleths. The empirical approach benefits from being based on observations that are highly accurate (but have a low spatial coverage). Analysis shows that uncertainties are textless30% near likely future control levels. Furthermore, the method shows that the nitrogen oxide (NOx)-volatile organic compound (VOC)-ozone system in the 1970s was in the region where VOC controls were most effective and then more recently moved to the region where both VOC and NOx controls are effective.