Tropospheric ozone is a unique pollutant. Unlike nitrogen dioxide or particulate matter, it is not directly emitted by vehicles or industries, but rather forms in the atmosphere through photochemical reactions between various precursor pollutants in the presence of solar radiation. Its concentrations rise particularly during the warmer months, when higher radiation levels coincide with elevated temperatures, and can affect human health as well as crops and ecosystems.
To analyze its behavior in Navarre, we studied hourly records from nine stations of the Air Quality network t between 2021 and 2025, focusing on April through September. This period features the most favorable conditions for ozone training , and it also coincides with increased exhibition of the population due to outdoor activities and with high physiological activity in vegetation, which can take up ozone through its stomata. We analyzed seasonal o average concentrations, the daily maximum of the eight-hour moving averages (MDA8)—which is relevant to health—and the AOT40 index, used to assess the cumulative o exhibition in vegetation.
The results do not show a clear upward or downward trend over these five years: the regional average from April to September remained roughly between 61 and 65 µg/m³, depending on the year. However, there is a very marked and persistent spatial h patron saint . Funes and Tudela I recorded the highest seasonal concentrations, with averages of 80.3 and 76.7 µg/m³, respectively, while Felisa Munárriz had the lowest value, at 53.5 µg/m³. This regional heterogeneity is clearly evident in the distribution of the stations (Figure 1).
Figure 1. Spatial distribution of average ozone concentrations in Navarre during April–September 2021–2025. The points represent stations that measure O₃, and the color scale indicates the concentration average for the period analyzed (µg/m³).
A quantitative comparison between monitoring stations confirms this patron saint. The fact that some stations located away from traffic have higher ozone levels than those in urban areas may seem contradictory, but it is characteristic of this pollutant. On streets with heavy traffic, there is an abundance of nitrogen monoxide (NO), which reacts rapidly with O₃ and consumes it locally—a process known as ozone “ degree program .” As air masses move away from these sources, this consumption decreases and the photochemical reactions capable of producing ozone continue, leading to higher concentrations in certain suburban or rural locations. Meteorological conditions, regional transport, and the availability of precursors also play a role.
Spatial differences are also reflected in the health indicators from exhibition (Figure 2). Funes and Tudela I recorded numerous days with an MDA8 exceeding 100 µg/m³, the World Health Organization’s reference letter . The current European rules and regulations uses 120 µg/m³ and allows for up to 25 exceedances per year on a three-year average. Starting in 2030, that maximum will be reduced to 18 days, while the long-term European goal for 2050 will come much closer to the WHO recommendation, based on 100 µg/m³ and a very low issue of days exceeding the limit. This comparison sample shows that current legal compliance does not necessarily equate to the absence of exhibition that is potentially relevant to health.
Figure 2. issue Annual average of days with elevated ozone concentrations during April–September 2021–2025. Days with MDA8 >100 and >120 µg/m³ are shown. The figures for 18 and 25 days specifically correspond to the 120 µg/m³ threshold set by the European Environment Agency ( rules and regulations ), while the World Health Organization ( reference letter ) corresponds to the 100 µg/m³ threshold.
Vegetation largely contributes to this patron saint. The AOT40, which quantifies the exhibition accumulated by ozone during the period of peak vegetation activity, was particularly high at Tudela I and Funes (Figure 3). Although the average remained below the current European value goal of 18,000 µg/m³·h, both stations were clearly above the European long-term goal of 6,000 µg/m³·h. In a region with a significant amount of agricultural land, this result underscores that ozone must be considered not only as a public health issue but also as a stressor on crops and ecosystems.
Figure 3. exhibition Cumulative vegetation ozone during 2021–2025, expressed as AOT40. The vertical lines indicate the European long-term goal (6,000 µg/m³·h) and the “ goal ” value (18,000 µg/m³·h). The average was calculated using only those years that met the minimum coverage criterion; Tudela I and Sangüesa have four valid years for this indicator, and the remaining stations have five.
Overall, the data indicate that the dominant feature of ozone in Navarre during 2021–2025 is not a uniform temporal trend, but rather a persistently uneven spatial distribution. The higher concentrations observed in certain areas, particularly in the Ribera region, justify maintaining continuous monitoring and considering both health and ecological criteria together. Ozone is not merely an indicator of air quality: it can exacerbate respiratory diseases, increase the risk of hospitalization, and particularly affect children, the elderly, and people with cardiorespiratory conditions. Furthermore, climate change may intensify the meteorological conditions that favor its training —higher temperatures, increased solar radiation, and episodes of atmospheric stability—meaning this problem could become even more significant in the coming decades. Reducing the population’s exhibition and protecting crops and ecosystems will therefore become increasingly important in light of this scenario and the progressively stricter European air quality targets.
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Jesús Miguel Santamaría
Director researcher , from . Biodiversity and Environment Institute
01.10.2026
