The graph is wonderful and would be very informative if one could be confident of the story it tells. In all cases of use of particle counters one has to ask how the process gets from counts to concentrations. The methodology used here is quite questionable and no attempt has been made to validate it. In addition there is the question of the appropriateness of comparing 10 minute counts (translated into concentrations) to epa standards that are 24 hour averages and/or yearly averages. (Actually they are more complex than even this.) I applaud this citizen attempt and say this as hopefully constructive and informative criticism.

Did you see the article AmySoyka posted about the Dylos? http://publiclab.org/notes/Schroyer/06-26-2014/broadcast-dust-readings-from-a-smartphone#c9306

@donblair-- the author doesn't seem to understand the terminology in use. The conclusions are based on a misunderstanding of what PM2.5 actually means I posted this comment:

PM2.5 is not the same as 2.5µ. it means particles with a mean aerodynamic diameter of 2.5µ. So a PM2.5 measurement is 50% above 2.5µ and 50% below. it includes 0.5µ particles. The same goes for PM10, it includes 2.5µ particles. https://en.wikipedia.org/wiki/Aerosol http://publiclab.org/wiki/particle-sensing

maintenance is an issue-- look at these dylos monitors in China building up particulates http://aqicn.org/faq/2013-10-21/dylos-air-particule-conter-experiment-maintenance/

As I understand it PM2.5 implies an aerodynamic diameter of 2.5 microns, straight forward with no "mean" involved. The "mean" enters with the epa approved monitoring equipment which use a "cyclone" with a 50% cut point of 2.5 microns. This is the point at which 50% of the particulate mass captured will be below 2.5 microns and 50% above. Similar statements would apply to PM10.

I've been operating on the assumption that the EPA/ISO standard = PM2.5. I guess this is an etymology question. which came first: PM2.5 or the EPA/ISO standard PM2.5? the standard defines PM2.5 particulates by the 50% cutoff point. If we're measuring PM2.5 I've been assuming that we're getting an equivalent to this filtration standard, not a measurement of just 2.5µ particles.

Let's try again. Gravimetric monitors capture particles and allow a weight of those captured to be determined. The distribution of aerodynamic diameters of the particles captured will depend upon the particulate in the air being sampled and the design of the monitor. I've seen an explanation of a 50% cutoff point of 2.5 microns as a particulate with aerodynamic diameter of 2.5 microns has a 50% probability of being captured. The probability of capture decreases as the aerodynamic diameter increases and the probability of capture increases as the aerodynamic diameter decreases. So a epa approved PM2.5 monitor output will be capable of including some particulate with aerodynamic diameter greater than 2.5 microns as well as less than 2.5 microns. The actual distribution will depend on what is in the air sampled. How any of this is actually determined and put into the design is beyond me, but it has to do with the design of the cyclone I believe.