Observed changes in the temperature dependence response of surface ozone under NOx reductions.

Due to the strong temperature dependence of surface ozone concentrations (O₃), future warmer conditions may worsen ozone pollution levels despite continued efforts on emission controls of ozone precursors. Using long-term measurements of hourly O₃ concentrations co-located with NO_x concentrations in stations distributed throughout Germany, we assess changes in the climate penalty, defined as the slope of ozone-temperature relationship during the period 1999-2018. We find a stronger temperature sensitivity in the urban stations over the southwestern regions, especially in the first period of the study (1999-2008).We show a decrease in the climate penalty in most of stations during the second period of the study (2009-2018), with some exceptions (e.g. Berlin) where the climate penalty did not show significant changes. To examine the impacts of NO_x reductions on the O₃ sensitivity to temperature, we propose a statistical approach based on generalized additive models (GAMs) to describe ozone production rates, inferred from hourly observations, as a function of NO_x and temperature, among other variables relevant during the O₃ production. We find lower O₃ production rates during the second period (2009-2018) at most stations and a decreasing sensitivity to temperature, pointing out that lowering NO_x concentrations resulted in decreasing O₃ production rates. However, we also observe changes in the shape of the function representing the O₃-temperature relationship, which indicate that NO_x reductions alone can not explain the changes in the temperature dependence of O₃. Our analysis would suggest that decreasing NO_x concentrations are not the only factor causing the observed changes in the climate penalty factor. [ABSTRACT FROM AUTHOR]