Report finds Spain must significantly reduce nitrogen oxide emissions to lower tropospheric ozone levels before 2030
The conclusions of the report "Scientific Bases for a National Ozone Plan," published by the Sub-Directorate General for Pollution Prevention of the Ministry for the Ecological Transition and the Demographic Challenge, indicate that it is essential, before 2030, to achieve objectives such as a 60% reduction in nitrogen oxide (NOx) emissions from road traffic compared to 2019; a 20% decrease in the same emissions from maritime transport; and the frequent monitoring of industrial facilities that account for a high percentage of total ozone precursor emissions.
Tropospheric ozone is found in the lower layers of the atmosphere and can have harmful effects on both human health and ecosystems, especially forests and agricultural yields. It is a secondary pollutant generated through photochemical reactions (in the presence of sunlight) from other primary pollutants, known as "precursors," which are mainly nitrogen oxides and volatile organic compounds (VOCs). Its main sources include road, maritime, and air traffic, as well as industrial activities.
In the preliminary study that served as the basis for the report, the scientific staff who prepared it stressed that during the period prior to the pandemic there was systematic noncompliance across almost all Spanish regions with the guideline values of the World Health Organization and the standards of the 2008 European Directive, an issue made even more critical by the publication in 2024 of a new directive with stricter targets. The decline in levels during the pandemic suggests that there is room for action to improve ozone levels in Spain.
The multidisciplinary group in charge of drafting the report consisted of around forty scientists, coordinated by Xavier Querol of the Institute of Environmental Assessment and Water Research (IDAEA-CSIC), and included contributions from research teams at the Universitat Jaume I of Castelló, the Barcelona Supercomputing Center (BSC), the Mediterranean Center for Environmental Studies (CEAM), the University of the Basque Country (UPV/EHU), and the collaboration of the universities of Zaragoza, Huelva and Aveiro (Portugal).
At the UJI, the Environmental and Energy Engineering Applied to Industrial Processes Group (GAIA) of the Agustín Escardino Institute of Ceramic Technology, led by Eliseo Monfort, contributed to campaigns measuring volatile organic compounds (VOCs) and to the analysis of inventories and corrective measures regarding industrial emissions in areas with high ozone concentrations.
To ensure that the information collected was realistic, 50 of the 80 largest industrial companies emitting ozone precursors, across eleven industrial sectors located in Madrid, Catalonia, Andalusia, and the Valencian Community, were visited.
Based on ozone levels and local precursor contributions, the study identified four types of atmospheric basins in Spain:
- R1, the lowest-level basins with mainly external contributions, including the Canary Islands, the north, and the northwest of the Iberian Peninsula.
- R2, basins requiring moderate preventive measures due to contributions from neighboring areas, including the Balearic Islands, the Ebro Valley, and the central plateaus.
- R3, where regulatory limits are exceeded and local generation of ozone precursors is significant, including Madrid, northern Barcelona, the Guadalquivir Valley, and the Valencian Community, where intensive precursor reduction policies are essential.
- R4, which included Puertollano, characterized by high local production of pollutants and geographic conditions that limit dispersion, requiring the implementation of strict emission controls.
Main recommendations
The most relevant measures should be applied as a priority in the R3 and R4 basins (Community of Madrid, Catalonia, Andalusia, Valencian Community, and Puertollano), although many may also be effective in R2 and R1. In recent years, with some variations, an overall improvement has been observed, driven by the increasing adoption of renewable energies, which have reduced precursor emissions nationwide. However, in the Madrid basin a sustained increase in ozone levels has been recorded over the last 15 years, making it advisable to investigate the causes of this trend in greater depth.
In general, the report recommends maximizing the use of ozone-precursor-free energy sources (solar, wind, hydroelectric, and nuclear) between April and September to reduce emissions during the most critical period.
Regarding road traffic, the scientific staff considers it essential to continue driving technological transition in the transport sector: fleet renewal, electrification, or reduced activity (fewer vehicles), with public administrations contributing, for example, through low-emission vehicle procurement.
For maritime traffic, the report suggests promoting the designation of the Mediterranean Sea as a Nitrogen Emission Control Area (NECA), which, in the opinion of the scientific team, could make a significant contribution to lowering ozone concentrations in Mediterranean areas.
Data on the distribution of emissions by industrial facilities indicate that a relatively small number of facilities account for a large share of ozone precursor emissions. Therefore, it is recommended to prioritize control measures in these facilities to achieve significant reductions. Furthermore, continuous monitoring of emissions should be promoted to improve management and surveillance systems, as well as predictive models, and to allow for the activation of protocols in episodes of high pollution.
Regarding volatile organic compound emissions, in addition to measures applicable to traffic and industry, the report recommends controlling certain solvents in domestic use, paints, and construction, as well as introducing appropriate protocols for biomass and agricultural residue burning, livestock waste management, and fuel supply stations.
Provided by Jaume I University
