| Summary: | The study analyzes the long-term trends (1998–2019) of concentrations of the air pollutants ozone (O<sub>3</sub>) and nitrogen oxides (NO<sub>x</sub>) as well as meteorological conditions at forest sites in German midrange mountains to evaluate changes in O<sub>3</sub> uptake conditions for trees over time at a plot scale. O<sub>3</sub> concentrations did not show significant trends over the course of 22 years, unlike NO<sub>2</sub> and NO, whose concentrations decreased significantly since the end of the 1990s. Temporal analyses of meteorological parameters found increasing global radiation at all sites and decreasing precipitation, vapor pressure deficit (VPD), and wind speed at most sites (temperature did not show any trend). A principal component analysis revealed strong correlations between O<sub>3</sub> concentrations and global radiation, VPD, and temperature. Examination of the atmospheric water balance, a key parameter for O<sub>3</sub> uptake, identified some unusually hot and dry years (2003, 2011, 2018, and 2019). With the help of a soil water model, periods of plant water stress were detected. These periods were often in synchrony with periods of elevated daytime O<sub>3</sub> concentrations and usually occurred in mid and late summer, but occasionally also in spring and early summer. This suggests that drought protects forests against O<sub>3</sub> uptake and that, in humid years with moderate O<sub>3</sub> concentrations, the O<sub>3</sub> flux was higher than in dry years with higher O<sub>3</sub> concentrations.
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