So methinks that a CO2 sensor would have to be based off of multiple LEDs + Photodiodes, to measure the absorbed light at _different_ frequencies. (LEDs and Photodiodes typically are only active at one frequency: such as 900nm or 700nm).
If "all" frequencies are deflected, its PM2.5 (some particle got in the way of the measurement).
If "some" frequencies are deflected, then its CO2.
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The question after that comes down to calibration and test data. Is there an easy way to create CO2 density and/or PM2.5 particles of a certain density?
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There's a big difference.
1. Photodiodes and LEDs are extremely common parts manufactured by a wide variety of companies around the world. By tying yourself to Photodiodes/LEDs, you are less tied to any particular company.
2. Photodiodes and LEDs are well under 10-cents each. Though some more carefully calibrated ones / specialty ones may reach as high as $1 to $2.