I've been meaning to do some data mining on how much PM2.5 differs with even a few hundred feet of elevation. I have a theory that even around Silicon Valley, living even up a little bit in the foothills would have you breathing massively cleaner air most of your life vs living in the pits of the valley. I see it all the time when I'm out there for recreation or just driving, the smoggy haze all over that just sits in the valley and the Bay. It's probably not too tough of a data processing challenge to compare the stats on Purple Air AQI sensors in reasonably close geo proximity vs elevation to get some statistically-significant data on this.