IMO, the single biggest problem is that even five-figure reference instruments disagree considerably in the 0-20 ug/m3 range, and if you look at the figures of the SCAQMD tests, you won't see much more than noise in the scatter plots in that range, with increasing consistency at larger concentrations. Some of this is due to mismatches in response times and synchronization, but a lot of it is due to different disturbances, noise processes, and varying sensitivities to different particle size distributions in different sensor types.
With inexpensive optical scattering sensors, the situation is even worse. While it is easy enough enough to "count" individual 2.5um particles, the scattering equations work out to an order of 10^6 reduction in scattering amplitude, per particle, going down to 0.3um (when measured with red or infra-red light), and different particle compositions will scatter differently. On top of that, the number of particles increases significantly per unit mass concentration, making the signal processing a lot harder once one can't just threshold individual "blips" in the signal.
Whether the sensors are particle counting or nephelometric in principle, the basic trade-off is that to see smaller particles, they need higher amplification factors, which in turn increases thermal noise, also amplifies stray light, and makes the device more sensitive to EM interference. Many signal processing pipelines do simplistic noise filtering, throwing out much of the baby with the bathwater.
On top of these principal difficulties, the optical scattering type sensors are quite sensitive to temperature variation, and aging of the photoelectronic components, which is why field tests under varied conditions often depress their accuracy even further.
Long story short, it is very easy to build a sensor with qualitatively good correlations to actual PM concentration, as long as the PM concentration is sufficiently high, but the health effects have no threshold, and every added bit of pollution counts, starting at zero. Unfortunately, commodity PM sensors are quite bad at quantifying these low, yet meaningful, ambient pollutant levels, which is probably why IKEA chose their traffic light thresholds the way the did: not because of how it relates to health, but because this is what they could do with a $12 device.
With inexpensive optical scattering sensors, the situation is even worse. While it is easy enough enough to "count" individual 2.5um particles, the scattering equations work out to an order of 10^6 reduction in scattering amplitude, per particle, going down to 0.3um (when measured with red or infra-red light), and different particle compositions will scatter differently. On top of that, the number of particles increases significantly per unit mass concentration, making the signal processing a lot harder once one can't just threshold individual "blips" in the signal.
Whether the sensors are particle counting or nephelometric in principle, the basic trade-off is that to see smaller particles, they need higher amplification factors, which in turn increases thermal noise, also amplifies stray light, and makes the device more sensitive to EM interference. Many signal processing pipelines do simplistic noise filtering, throwing out much of the baby with the bathwater.
On top of these principal difficulties, the optical scattering type sensors are quite sensitive to temperature variation, and aging of the photoelectronic components, which is why field tests under varied conditions often depress their accuracy even further.
Long story short, it is very easy to build a sensor with qualitatively good correlations to actual PM concentration, as long as the PM concentration is sufficiently high, but the health effects have no threshold, and every added bit of pollution counts, starting at zero. Unfortunately, commodity PM sensors are quite bad at quantifying these low, yet meaningful, ambient pollutant levels, which is probably why IKEA chose their traffic light thresholds the way the did: not because of how it relates to health, but because this is what they could do with a $12 device.