Do the PurpleAir air quality sensors have a heater in them? I know they use the Plantower PMS5003 sensor which shows a working humidity range of 0-99% on the datasheet.
The Plantower sensor will
work width humidity range 0-99%. That does not mean the measurements are as correct as a heated professional station.
None of the consumer versions will work that good in high humidity conditions. They all over-estimate the PM.
I own 2 Luftdaten, 1 PurlpleAir and 2 Ecowitt sensors.
All have the same "problem" with high humidity, and they follow roughly the same graph-path when humidity increases.
Wim