This article deals with concentrations of particulate matter (PM_2.5 and PM₁₀), NO₂ and O₃ in Europe and their evolution in the period 2003-2019. It is based on models that provide information with a detailed spatial resolution of 0.1 degrees. The authors highlight the evolution of both pollution days and the population living in clean areas. In addition, a geographical division of Europe is made to contrast the results in the different areas and, finally, the evolution of episodic situations, even composite ones, which may have a particular impact on the health of the population, is studied. It is therefore a very complete analysis, both in terms of the spatial scale and the period analysed, which determines its publication in a leading journal. 

Although the authors of this analysis have emphasised the most striking results, it should not be forgotten that, without ignoring the usefulness of the models, there may be notable discrepancies with the measured data. For example, the sometimes significant dispersion between predicted and observed values is shown. In addition, concentrations or trends are presented following the administrative boundaries of provinces, whose size and population can change significantly from one country to another. These schemes may impact both the population and air quality managers, but they depart from the distribution of concentrations, as urban centres will be sources or sinks of concentrations, which will contrast with rural sites. O₃ concentrations are also analysed, but only one of the precursors is considered. Volatile organic compounds are not presented, so it is risky to target control measures without this information.   

Finally, the study of the evolution of episodic situations is correct. However, no distinction is made between cases of pollution of natural or anthropogenic origin, which limits decision-making on control measures.

Is the research of good quality? 

"As the article mentions, it seems to be a robust method based on hundreds of data collected across Europe over a considerable period of time". 

What would you highlight from their results?  

"The decrease in NO_x [nitrogen oxides] and particulate emissions, which has been observed in other pollution studies on the use of biofuels and so on. And the difficulty of reducing the concentration of tropospheric ozone given its 'multipathway' formation". 

Are there any limitations to take into account?  

"Not in terms of data processing, but the explanation, especially with regard to fine particles and ozone, is complex given the sources of formation of both. The chemistry of these processes needs to be further investigated. 

How can PM_2.5 and O₃ levels be reduced, and do large fires play a role?

"Of course fires have an influence, and in recent years we have had major fires in Europe in particular, but other natural phenomena such as volcanoes (Iceland, La Palma, etc.) also play a role. 

It is a complex issue and, as the article shows, we need to continue our research".