Surface Ozone-Temperature Relationship: The Meridional Gradient Ratio Approximation

The daily variation of ground-level ozone (O-3), a harmful pollutant, is positively correlated with air temperature (T) in many midlatitude land regions in the summer. The observed temporal regression slope between O-3 and T (dO(3)/dT) is referred to as the "ozone-climate change penalty" and has been proposed as a way to predict the impact of future climate warming on O-3 from observations. Here, we use two chemical transport models to show that the spatial variation of dO(3)/dT is primarily determined by simultaneous meridional advection of O-3 and T. Furthermore, the sign and magnitude of dO(3)/dT can be approximated by their climatological meridional gradient ratio (O-3 gradient divided by T gradient). Consideration of expected changes in the meridional gradients of T and O-3 due to climate change indicates that dO(3)/dT will likely change. Caution is needed when using the observed climate penalty to predict O-3 changes.