Yes, just shared this with my air quality colleagues.
The original paper is here: https://www.sciencedirect.com/science/article/pii/S0048969720321215
I haven't read and digested it properly yet but the most polluted places are generally the most densely populated so it may not be causal?
Didnt read it either but I'm thinking maybe there is a few regions with pretty light population density but with a lot of industry that would be up there?
Guess the easy way to find out would be to read it but I'm quite busy sunbathing in my French balcony...