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I'd be careful w. a blanket '25% of truth' declaration, esp. for indoor environments where larger particles (dia > 1 um) may be important. Boils down to physics and light scattering. Nephelometers vs Optical Particle Counters. Quick 101 w. neat pics! if you want to nerd out.. 1/n https://twitter.com/ShellyMBoulder/status/1358260106773868544
At its heart the plantower is a nephelometer. This means it doesn't detect particles one-by-one. Instead it converts the total scattered light signal (from all the particles passing through the sensor) into a 'count'. 2/n
This approach is advantageous for tracking conc. changes in accumulation mode aerosol (particles < 0.5 um, typ. w. mode ~0.1-0.25 um) bc the neph gives you way more bang for your buck at these sizes (scattered light signal from lots of small particles all at once). 3/n
vs Optical Particle Counters (or OPCs) which ARE resolving individual particles one-by-one and thus run out of signal-to-noise by the time PM ~ 0.3-0.4 um in diameter (the size detection limit of most low-cost OPCs). OPCs are GREAT for larger particles though. 4/n
The reason we shouldn't put all our eggs in a neph-only PM sensor basket has to do w. the bigger particles (PM diameter > 1 um). Larger particle detection w. plantower has two enemies: (1) INERTIA and (2) the DETECTOR. 5/n
Let's do INERTIA first. Here's the inside of that PMS1003. Basic rule of thumb small (0.1 um) particles follow gas flow, big particles (1 um) have a much harder time turning corners. Think aircraft carrier - tough to turn on a dime. 6/n
image source: https://aqicn.org/sensor/pms1003/
image source: https://aqicn.org/sensor/pms1003/
Plantower's rollercoaster flow path: 1x 180 and 2x 90 deg turns 'A,B,C,D' means that majority of large particles (dia > 1 um) will crash into the walls before reaching the detector '
'. 7/n
image source (6-mo cont. use): https://aqicn.org/sensor/pms1003/
'. 7/nimage source (6-mo cont. use): https://aqicn.org/sensor/pms1003/
The position of the detector is also not doing you any favors when it comes to detecting larger particles (if they survive the rollercoaster) bc it's is positioned 90 degrees rel. to where the particle and laser beams intersect. 8/n
this isn't big deal for the smaller particles bc they scatter light equally in all directions (nerds: isotropically). But for PM>1um, this pos. is bad, bc larger particles will scatter light in the forward and backward dir rel. to laser beam (where there is no detector). 9/n
Bottom line: Be careful w. plantower-based PM sensors if you care about PM > 1 um. @SouthCoastAQMD has done some work. Lab (Arizona Road Dust): 10/n
Image source: http://www.aqmd.gov/docs/default-source/aq-spec/laboratory-evaluations/purple-air-pa-ii---lab-evaluation.pdf?sfvrsn=28
Image source: http://www.aqmd.gov/docs/default-source/aq-spec/laboratory-evaluations/purple-air-pa-ii---lab-evaluation.pdf?sfvrsn=28
And Field: 11/n
Image Source: http://www.aqmd.gov/docs/default-source/aq-spec/field-evaluations/purple-air-pa-ii---field-evaluation.pdf?sfvrsn=11
Image Source: http://www.aqmd.gov/docs/default-source/aq-spec/field-evaluations/purple-air-pa-ii---field-evaluation.pdf?sfvrsn=11
Tons of people have done quality work on plantower side of things - I just wanted to reiterate some of the basics here, esp. in the context of > 1 um detection fundamentals and the potential widespread adoption of PA-II's or PA-I's in classrooms. 12/n
At @quant_aq we hope to help. Embracing the limits of low-cost pollution sensor tech is (in our opinion) an excellent starting point to designing sensors that can adequately address indoor and outdoor air pollution problems. 13/ 'fin'
