Yes, environmental conditions do affect the ability of an optical sensor, not too much to "see" a particle but rather to "size" it.
All of these optical sensors work by measuring the intensity of the light scattering from the particles and that intensity is related to their size which is how they can be "assigned" to PM10 or PM2.5 or any other size bin they have. So, if you have hygroscopic particles (they absorb water), then they will grow in high humidity environments. That's not a problem as they will grow for any sensor that measures them but the issue is that all "regulatory" measurement has a preconditioning of their sample to comply with whatever standard they are supposed to follow. In New Zealand the guidance is to have a heated inlet that guarantees that you're always sampling particles in the same conditions, even if that means that you may be losing volatiles compounds in the process (water being one of the volatiles).
I have been working with these sensors for a while now. Here is a presentation I have last year on what we've learned so far:
[EDIT: I saw that the link didn't work so here is a shorter one that does work]
It can be summarised as "After using the Sharp sensors now we've moved to the Plantower PMS series and are quite happy".
A more involved answer could be that, even though the Sharp sensors served us well and we got a lot of really good information from them, their response to temperature (much more than to humidity .... see at the bottom the PDF of a paper we submitted to AMT but didn't follow up to publication) is annoying so when we tested the Plantower PMS3003 and found no significant response to temperature we moved all our low-cost sensors to that technology.
You can check our project's blog here:
Our "calibration" exercise for PM2.5 and PM10 (showing a much better agreement with regulatory PM2.5 than PM10) here:
Wow. These are awesome resources!! Thanks for clarifying the seeing/sizing distinction.
My quick perusal of the attached pdf didn't find anything about effects of humidity. What you say about humidity effecting size makes sense but I would suggest the question for people using optical sensors who are not part of some organized scientific project is what effect within a bin, 2.5µg/m3 for example, would high humidity have? My experience is that counts increase; which I would speculate is due to smaller particles which might not be counted under low humidity become visually large enough to be counted.
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