Correction: The “underestimate” refers to the 4% reduction in RH vs. the 15% reduction in RH compared to ambient. I believe a better description is 15 “percentage points” below ambient. For example, if the ambient RH is 90%, the PurpleAir monitor will read 75%. If the ambient RH is 50%, the PurpleAir monitor will read 35%. Of course, at very low ambient RH levels, the difference will be reduced by fewer than 15 percentage points.
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