Surface ozone trends reversal for June and December in an Atlantic natural coastal environment

Surface ozone and temperature trends were investigated using records from 2000 to 2021 in Southwestern Europe, at El Arenosillo observatory, focusing on June and December. The ozone trends for daily percentiles were increasing in June for lower percentiles (2.5 ± 1.2 ppb decade^-1 for the 5th percentile) and decreasing for higher (- 2.2 ± 1.4 ppb decade^-1 for the 95th percentile); in December, the trends were growing in the entire range of percentiles, with a peak of 2.2 ± 0.8 ppb decade^-1. A declining trend was obtained for the geopotential height at the pressure level of 850 hPa (Z850) in June while highlighting the upward trend in December (26.3 ± 6.5 m decade^-1). The hourly trends for ozone and temperature were also explored in these months. In June, the nocturnal ozone trends were growing (4.0 ± 1.2 ppb decade^-1 or 10% decade^-1 at 8:00 UTC) associated with temperature rises while in the daytime, a decrease in temperature was observed along with an ozone decreasing trend (- 2.6 ± 1.6 ppb decade^-1 or - 5% decade^-1 at 18:00 UTC). Hourly ozone and temperature trends in December were increasing with peaks of 3.0 ± 0.9 ppb decade^-1 (~ 8% decade^-1) at 12:00 UTC and 1.6 ± 0.3 °C decade^-1 at 19:00 UTC. Two representative scenarios of these months were studied. The ozone decreases in June could be associated with several factors, decreasing in temperatures and a possible weakening of the anticyclonic conditions leading to changes in the mesoscale processes' development. The strengthening of the Azores anticyclone in December could be enhancing the upward ozone trend observed. It is unknown whether the reversal ozone pattern trends found in this region are a local phenomenon; although we suggest that it could be happening on a larger scale as well, future studies should be carried out.