That is a cool site. I had not known there was such an extensive sensor network out there. One thing to remember about the kind of location the parent comment

That is a cool site. I had not known there was such an extensive sensor network out there. One thing to remember about the kind of location the parent comment wrote about: particulates are very local. Trace gases like O3 or CH4 can have very sharp spatial gradients - e.g., near highways, major streets, or dairy farms. You can observe sharp decreases from emission levels to background levels within 10s of meters. And PM2.5 mixes even more poorly than these trace gases, meaning PM2.5 shows sharper gradients. So, if you live in a built-up area and you really want to find out about PM2.5 due to your open window, or on your patio or backyard, you may need a very close-by measurement. You will still observe correlations between health outcomes and mean PM2.5 within, say, one 1km^2 monitoring footprint versus an adjacent footprint. So the background level is definitely telling you something.