Abstract. We have constructed two data sets of hourly resolution reanalyzed near-surface ozone (O 3 ) concentrations for the period 1990-2013 for Sweden. Long-term simulations from a chemistry-transport model (CTM) covering Europe were combined with hourly ozone concentration observations at Swedish and Norwegian background measurement sites using retrospective variational data analysis. The reanalysis data sets show improved performance over the original CTM when compared to independent observations. In one of the reanalyses, we included all available hourly near-surface O 3 observations, whilst in the other we carefully selected time-consistent observations. Based on the second reanalysis we investigated statistical aspects of the distribution of the near-surface O 3 concentrations, focusing on the linear trend over the 24-year period. We show that high nearsurface O 3 concentrations are decreasing and low O 3 concentrations are increasing, which is reflected in observed improvement of many health and vegetation indices (apart from those with a low threshold).Using the CTM we also conducted sensitivity simulations to quantify the causes of the observed change, focusing on three factors: change in hemispheric background concentrations, meteorology and anthropogenic emissions. The rising low concentrations of near-surface O 3 in Sweden are caused by a combination of all three factors, whilst the decrease in the highest O 3 concentrations is caused by European O 3 precursor emissions reductions.While studying the impact of anthropogenic emissions changes, we identified systematic differences in the modeled trend compared to observations that must be caused by incorrect trends in the utilized emissions inventory or by too high sensitivity of our model to emissions changes.

<p><strong>Abstract.</strong> We have constructed two data sets of hourly resolution reanalyzed near-surface ozone (O<sub>3</sub>) concentrations for the period 1990–2013 for Sweden. Long-term simulations from a chemistry-transport model (CTM) covering Europe were combined with hourly ozone concentration observations at Swedish and Norwegian background measurement sites using data assimilation. The reanalysis data sets show improved performance than the original CTM when compared to independent observations. <br><br> In one of the reanalyzes we included all available hourly near-surface O<sub>3</sub> observations, whilst in the other we carefully selected time-consistent observations in order to avoid introducing artificial trends. Based on the second reanalysis we investigated statistical aspects of the near-surface O<sub>3</sub> concentration, focusing on the linear trend over the 24 year period. We show that high near-surface O<sub>3</sub> concentrations are decreasing and low O<sub>3</sub> concentrations are increasing, which is mirrored by observed improvement of many health and vegetation indices (apart from those with a low threshold). <br><br> Using the chemistry-transport model we also conducted sensitivity simulations to quantify the causes of the observed change, focusing on three processes: change in hemispheric background, meteorology and anthropogenic emissions (Swedish and other European). The rising low concentrations of near-surface O<sub>3</sub> in Sweden are caused by a combination of all three processes, whilst the decrease in the highest O<sub>3</sub> concentrations is caused by O<sub>3</sub> precursor emissions reductions. <br><br> While studying the relative impact of anthropogenic emissions changes, we identified systematic differences in the modelled trend compared to observations that must be caused by incorrect trends in the utilised emissions inventory or by too high sensitivity of our model to emissions changes.</p>

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