Surface ozone trends over a 21-year period at El Arenosillo observatory (Southwestern Europe)

The near-surface ozone measured at El Arenosillo observatory (in Southwestern Europe) over 21 years (2000-2020) was analyzed to explore its trends. Using monthly averages and their anomalies, we obtained trends of 0.9 +/-& nbsp;0.7 ppb decade-1 (p = 0.211) and 1.2 +/- 0.4 ppb decade(-1) (p = 0.001), respectively. Daytime and nighttime trends were 0.6 +/-& nbsp;0.5 ppb decade(-1) (p = 0.198) and 1.4 +/-& nbsp;0.4 ppb decade -1 (p < 0.001), respectively. The seasonal trends showed upward values for winter, spring and autumn between 1.6 and 1.8 ppb decade-1, with downward values for summer. The surface temperature and geopotential height at the 500 hPa level (Z500) showed upward trends, which were higher in winter and autumn, while NOx and VOC emissions presented downward trends. The surface ozone trend could be attributed mostly to the cold months and occurred mainly in the nocturnal period. The higher temperatures and anticyclonic conditions, i.e., a warmer climate, along with the reduction of the precursor emissions, could be leading to less destruction by NO titration in cold seasons and nighttime, whereas the reduction of VOC would not favor the ozone formation and accumulation processes in warm seasons and diurnal time. In the near future, new studies will be necessary to explore other processes involved in this upward ozone trend.