PM_2.5 emissions, concentrations and air quality index during the COVID-19 lockdown

We have read with great interest the paper “Air quality during the COVID-19: PM_2.5 analysis in the 50 most polluted capital cities in the world”, published in Environmental Pollution (Rodríguez-Urrego and Rodríguez-Urrego, 2020). We found the paper useful and resourceful, having used available open data at the World Air Quality Index website (World Air Quality Index project, n. d.). However, when we saw the data that the authors used to analyze the impact of the COVID-19 lockdown on air quality, we realized that they have made fundamental mistakes. We illustrate this as follows.

1. The authors used PM_2.5 air quality index (AQI) data but the authors presented them as PM_2.5 concentrations

In section 2 (Methodology), the authors mention that they “collected the PM_2.5 data from an online platform”, but the authors did not mention any conversion of these data into PM_2.5 concentrations. The methodology flow diagram (Fig. 1) does not show a conversion of extracted AQI data into concentrations before the graphics and analysis box. From the methodology description and scheme, therefore, a reader would expect to see results in PM_2.5 AQI. However, all references to the PM_2.5 behavior in section 4 (PM_2.5 assessment before-during quarantine), appear as changes in PM_2.5 concentrations (in μg/m³), not changes in PM_2.5 AQI, as expected from the methodology.

Presenting PM_2.5 AQI data as PM_2.5 concentrations leads the authors to misinterpretation. As an example, the authors wrote “London, Vienna, Brussels and Prague, are capitals that generally have a good AQI level, with an average record of 31 μg/m³”. The corresponding AQI for this PM_2.5 concentration is 109, which is considered as “Unhealthy for sensitive groups” (USEPA, n.d.). It is not a good AQI level, as the authors wrote. An AQI level of 31 (no units) is, in fact, Good air quality, but corresponds to a PM_2.5 concentration of 9.1 μg/m³. The conclusions also have this kind of mistake. In summary, it is necessary that the authors remove the concentration units (μg/m³) throughout section 4, whenever the authors are referring to an AQI, which has no units. Or else, that the authors convert all AQI data into PM_2.5 concentrations if they want to present your results in μg/m³ units.

2. The authors refer to PM_2.5 emissions, when they should actually refer to PM_2.5 concentrations or AQI

In the abstract, the authors wrote “this paper describes the behavior of PM_2.5 emissions particulate matter (sic) from the 50 most polluted capital cities …“, but they actually showed the behavior of PM_2.5 AQI, as recorded by air quality monitoring stations. The sentence “Likewise, the impact at the local and global level of this emissions behavior, which averaged 12% of PM_2.5 decrease in these cities” has the same conceptual mistake. The same occurs in section 4, where the authors wrote “Likewise, in the analyzed countries presented below, decreases, increases or a constant level of PM_2.5 emissions are observed during confinement”, but such reductions, increases or constant levels should refer to PM_2.5 concentrations or PM_2.5 AQI, not to PM_2.5 emissions. Reductions in local emissions, for example, do not necessarily imply reductions in PM_2.5, since there can be long-range transport and formation of secondary PM_2.5 affecting concentrations in a given city. In summary, the authors should change the word “emissions” by “concentrations” in the abstract and in section 4.

3. The authors did not show the correct Fig. 1

Finally, a less fundamental mistake. The title in Fig. 1 is wrong, since it shows a methodology chart flow but the title appears as “Fig. 1. Capital cities Ankara, Astana, Bangkok, Beijing, Colombo, Delhi, Dhaka, Kabul and Hanoi, PM_2.5 levels before and during quarantine”. In fact, there is no figure showing results for these cities, but Fig. 1 is mentioned in the text as showing such information in section 4: “From Figs. 1 and 2, the most polluting capitals of Asia are presented”. This should also be corrected.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.