It's paywalled. https://www.sciencedirect.com/science/article/abs/pii/S00950...

If I had access, I'd try to dig into more depth into how they approached decoupling this from temperature, because that seems like a really strong confounder. I'm also a bit curious about the underlying physical cause of the change in pollution levels, (weekends? summertime?) because that seems like it could also be a confounding common cause.

Alas, I'm not interested in paying $35 for the paper.