Surface Ozone‐Temperature Relationship: The Meridional Gradient Ratio Approximation.

The daily variation of ground‐level ozone (O3), a harmful pollutant, is positively correlated with air temperature (T) in many midlatitude land regions in the summer. The observed temporal regression slope between O3 and T (dO3/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 O3 from observations. Here, we use two chemical transport models to show that the spatial variation of dO3/dT is primarily determined by simultaneous meridional advection of O3 and T. Furthermore, the sign and magnitude of dO3/dT can be approximated by their climatological meridional gradient ratio (O3 gradient divided by T gradient). Consideration of expected changes in the meridional gradients of T and O3 due to climate change indicates that dO3/dT will likely change. Caution is needed when using the observed climate penalty to predict O3 changes. Plain Language Summary: At Earth's surface, ozone is a harmful pollutant. In the summer, we observe higher ozone concentrations on hotter days over land in the midlatitudes. This leads researchers to expect higher ozone concentrations as a result of global warming, based on chemistry that associates higher ozone concentrations with higher temperatures. Here, we show that the relationship between ozone and temperature is largely controlled by the north‐south movement of air which simultaneously transports ozone and heat. Therefore, the background spatial distributions of ozone and temperature determine how ozone and temperature covary. The ozone‐temperature relationship in the future may look different from today, because global warming is not spatially uniform. We advise caution in using observed ozone‐temperature relationship to estimate future ozone changes. Key Points: Transport by meridional advection of O3 and T explains the spatial variation in the daily O3‐T relationshipDaily regression slope dO3/dT can be approximated by the ratio of O3 and T mean meridional gradients when temperature gradients are strongGradient ratio suggests dO3/dT will change with warming, making it questionable to use observed dO3/dT in O3 projections [ABSTRACT FROM AUTHOR]