This dataset contains monthly air chemistry observations within the European monitoring network ICP-IM (“International Co-operative Programme on Integrated Monitoring of Air Pollution Effects on Ecosystems”) from 2008 to 2023. In this program, Navarra is an important monitoring location since the sample-plot situated at "Señorio de Bertiz Natural Park" is the only monitoring point in Spain (ES02). Specifically, this plot is a European beech forest (Fagus sylvatica) located in an oceanic climate zone, characterised by mild temperatures and constant rainfall throughout the year. These collaborative programs have been developed to address issues such as air pollution and climate change. In recent decades, ozone has become one of the most significant pollutants due to its harmful effects on human health and vegetation. Therefore, the aim of this dataset is to provide long-term data on ozone exposure, enabling scientists to detect and analyze trends related to climate change over extended periods. Thus, this dataset collects information on ozone (O3, µg/m³), a mandatory parameter of the ICP-IM program. The monitoring process is explained in the guide provided by the programme, resulting in a dataset with monthly resolution for two different forest plots. In each plot, samples are collected at two points, which are then analyzed in the laboratory according to the 'Passam' protocol. This protocol explains the procedure for quantifying ozone collected by passive dosimeters, the ones used in this monitoring and based on UV-Visible spectrophotometry. It is important to note that, to ensure high quality data, rigorous quality processes (QA/QC) are included both in the field and in the laboratory. Also, after the ozone values are collected, the data go through an automated process to check if they exceed the detection limit. Consequently, there may be months where no data is available.