Traffic, and especially the noise caused by it would definitely be a suspect. However, the paper did take into account the precipitation, with the same amount o

Traffic, and especially the noise caused by it would definitely be a suspect. However, the paper did take into account the precipitation, with the same amount of traffic causing less PM 2.5 when it rains. This doesn't mean it is impossible, as rain might e.g. still be correlated with less traffic. The nature of a hidden variable is exactly that its indirect correlations would be not pinpoint-able in the data. But IMHO this paper does a decent job of trying to account for lots of dimensions, and trying identify the existence of dominant hidden variables from the data at hand. This is far from a rush job or a 'spurious correlation' piece of work, and its results most certainly are interesting and a very good basis for further investigation into an important observation. I'd even go further and say that these results warrant societal value systems that subscribe to the precautionary principle to take this on board in further decisions about pollution in populated areas.