An analysis of the 22-yr ozone (O<sub>3</sub>) series (1988–2009) at the subtropical high mountain Izaña station (IZO; 2373 m a.s.l.), representative of free troposphere (FT) conditions, is presented. Diurnal and seasonal O<sub>3</sub> variations as well as the O<sub>3</sub> trend (0.19 ± 0.05% yr<sup>−1</sup> or 0.09 ppbv yr<sup>−</sup>), are assessed. A climatology of O<sub>3</sub> transport pathways using backward trajectories shows that higher O<sub>3</sub> values are associated with air masses traveling above 4 km altitude from North America and North Atlantic Ocean, while low O<sub>3</sub> is transported from the Saharan continental boundary layer (CBL). O<sub>3</sub> data have been compared with PM<sub>10</sub>, <sup>210</sup>Pb, <sup>7</sup>Be, potential vorticity (PV) and carbon monoxide (CO). A clear negative logarithmic relationship was observed between PM<sub>10</sub> and surface O<sub>3</sub> for all seasons. A similar relationship was found between O<sub>3</sub> and <sup>210</sup>Pb, but only for the summer-time. The highest daily O<sub>3</sub> values (90th percentile) are observed in spring and in the first half of summer-time. A positive correlation between O<sub>3</sub> and PV, and between O<sub>3</sub> and <sup>7</sup>Be is found throughout the year, indicating that relatively high surface O<sub>3</sub> values at IZO originate from the middle and upper troposphere. At IZO we find a~good correlation between O<sub>3</sub> and CO in winter, supporting the hypothesis of long-range transport of photochemically generated O<sub>3</sub> from North America. Aged air masses, in combination with sporadic inputs from the upper troposphere, are observed in spring, summer and autumn. In summer-time high O<sub>3</sub> values seem to be the result of stratosphere-to-troposphere (STT) exchange processes in regions neighbouring the Canary Islands. Since 1995–1996, the North Atlantic Oscillation has changed from a predominantly high positive phase to alternating between negative, neutral or positive phases. This change results in an increased flow of the westerlies in the mid-latitude and subtropical North Atlantic, thus favouring the transport of O<sub>3</sub> and its precursors from North America, and a~higher frequency of storms over North Atlantic, with a~likely higher incidence of STT processes in mid latitudes. These processes lead to an increase of tropospheric O<sub>3</sub> in the subtropical North Atlantic region after 1996 that has been reflected in surface O<sub>3</sub> records at IZO.