Hi guys, thanks a lot for sharing your excellent project; I have a question related to the Dustduino code (i posted this question earlier on the 'sketches' page). I understand how you get to countP1 and countP2, but then you relate PM10count to P2 (number > 2.5micron) - i don't understand why it is not more accurate to say that PM10count relates to P1 (number > 1micron) because PM10 should include all the particles up to 10 micron. Is there a reason you use P2 for PM10? For PM25count i understand why you take P1 - P2. Would appreciate your comment- thanks- Tom

You first need to remember that PM10 and PM2.5 are measures of "mass" of particles in the air (micrograms/m3) while P1 and P2 give "counts" per volume (number of particles/m3). Now, the MASS of a particle is proportional to the cube of its diametre (for simplicity, let's assume that all the particles have the same density) so a particle of size 5microns would actually weigh 125 times more than a 1micron particle. In the air, the size distribution is such that there are (typically) more smaller particles but that is not enough to compensate for the size difference. So, the mass of particles smaller than 10microns is actually controlled by the number of larger particles in the population which is why PM10 should correlate better with the number of particles larger than 2.5microns than with the number of particles larger than 1micron.

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