Assessing WHO2021 air quality guideline levels in Europe: Population exposure and PM2.5 reduction potential

Although anthropogenic emissions have decreased during the last two decades, air pollution is still problematic in many regions in Europe. In 2021, the World Health Organisation has released updated air quality guideline levels that base on newest medical research and have drastically decreased in comparison to the previous levels, except for SO₂. These levels reflect the minimum concentration for which evidence of adverse health effects have been found. This study analyses the air quality in Europe using simulations by EURAD-IM for the year 2016 with optimized emissions to evaluate guideline exceedances for PM_2.5, NO₂, and O₃. All air pollutant concentrations in Europe exceed the WHO2021 annual guideline levels by a large amount. High PM_2.5 and O₃ concentrations are homogeneously distributed across Europe with 99 % and 100 % of the EU38 population (European Union and eleven other European states) exposed to concentrations above the guideline levels. High NO₂ concentrations are linked to densely populated areas. However, 61% of the EU38 population is exposed to concentrations above the guideline level. For PM_2.5, the effect of different aerosol classes on the bulk mass and different PM reduction scenarios are analysed. NO₃^- shows the largest contribution to total PM_2.5 while primary anthropogenic aerosols are mainly responsible for extremely high PM_2.5 concentrations of up to 30 μg m⁻³. In the extreme case of completely removing different aerosol classes from the bulk mass, jointly removing NO₃^- and primary anthropogenic aerosols as well as removing all secondary inorganic aerosols decrease PM_2.5 annual concentrations to below 5 μg m⁻³, which is the WHO2021 annual guideline level, in most parts of Europe. However, a more realistic scenario, in which the decrease of aerosol concentrations linearly follows the targeted anthropogenic emission reductions by the European Union, still shows an exceedance of the annual WHO2021 guideline levels by a factor of 2 - 3 throughout Europe. Thus, besides implementing strong emission reduction plans, other ways of removing air pollutants from the atmosphere need to be designed and implemented.