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<p>Tropopause folds are the key process underlying stratosphere-to-troposphere transport (STT) of ozone, and thus they affect tropospheric ozone levels and variability. In the present study we perform a process-oriented evaluation of Copernicus Atmosphere Monitoring Service (CAMS) reanalysis (CAMSRA) O<span class="inline-formula">₃</span> during folding events over Europe and for the time period from 2003 to 2018. A 3-D labeling algorithm is applied to detect tropopause folds in CAMSRA, while ozonesonde data from WOUDC (World Ozone and Ultraviolet Radiation Data Centre) and aircraft measurements from IAGOS (In-service Aircraft for a Global Observing System) are used for CAMSRA O<span class="inline-formula">₃</span> evaluation. The profiles of observed and CAMSRA O<span class="inline-formula">₃</span> concentrations indicate that CAMSRA reproduces the observed O<span class="inline-formula">₃</span> increases in the troposphere during the examined folding events. Nevertheless, at most of the examined sites, CAMSRA overestimates the observed O<span class="inline-formula">₃</span> concentrations, mostly at the upper portion of the observed increases, with a median fractional gross error (FGE) among the examined sites <span class="inline-formula">&gt;0.2</span> above 400 hPa. The use of a control run without data assimilation reveals that the aforementioned overestimation of CAMSRA O<span class="inline-formula">₃</span> arises from the data assimilation implementation. Overall, although data assimilation assists CAMSRA O<span class="inline-formula">₃</span> to follow the observed O<span class="inline-formula">₃</span> enhancements in the troposphere during the STT events, it introduces biases in the upper troposphere resulting in no clear quantitative improvement compared to the control run without data assimilation. Less biased assimilated O<span class="inline-formula">₃</span> products, with finer vertical resolution in the troposphere, in addition to higher IFS (Integrated Forecasting System) vertical resolution, are expected to provide a better representation of O<span class="inline-formula">₃</span> variability during tropopause folds.</p>