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

Abstract. 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.