How good is good enough? -- chasing the dream of accurate and reliable low cost sensors in air quality

a few years now, there are a number of projects around consumer level (as opposed to research grade) air quality sensors. (Air Quality Egg, Speck, Dustduino, Air Beam and my own PACMAN). The goal of most of these projects has been to empower the general public about air quality and bring the problem closer to home so that people don't see themselves as passive receptors in this but relevant actors who can have an impact through their individual actions.

The success of these initiatives has been varied with some developing a relatively large community (AQEgg), others with more institutional support (SPECK, Air Beam) and some with not so great results yet (TZOA). However, all of them eventually face the data quality monster and things can get very ugly very quickly if not taken seriously!

Good data ... but good for what?

Why is Particulate Matter so special?

Air pollution is one of those things, like weather and climate, that because it affects everyone we all have an opinion about it. This makes it an ideal area for "citizen" and "traditional" scientists to collaborate and work towards improving our lives. However, air pollution is not easy (I've been at it for 15 years and I still can't claim to get it) and clarity on the concepts used is key to maintain the communication and collaboration. 

One of those concepts that is very easy to get confused about is particulate matter (PM).