The calibration discussed above is only for the PA-II monitors with PMS 5003 sensors. Also, it is only for outdoor air in California. This leaves us with some unfinished business:
- Do we need a separate calibration for outdoor air in other areas of the country?
- Do we need a separate calibration for wildfire smoke?
- Do we need a calibration for indoor air?
- Do we need a calibration for the PA-I monitor with the PMS 1003 sensor.
My colleagues at Stanford and I actually made a start on 3) above-- the calibration for indoor air. We published a study of the source strength of vaping marijuana in 2020
We found that a single puff of marijuana vapor had about 2-6 times as much PM2.5 as a single puff of a tobacco cigarette. We also found, by comparing the PurpleAir monitors used in the study to co-located SidePak research monitors, (and then to gravimetric pump-filter-microbalance methods) that the best calibration factor for the marijuana vapor (mixed with outdoor air penetrating the exposure chamber (a room in my house)) was again 3.0. So we have one estimate of the calibration factor for a single indoor source. Once we go through another thousand or so sources we might have a useful indoor air calibration factor. This study required 124 6-hour experiments over the course of one year. We have our work cut out for us and our descendants!
REgarding other unfinished business, for problem 1) above the EPA studied about 6 geographci areas in the US and came up with a single “nationwide” calibration,with a correction for RH. However, their correction factor depends ultimately on the CF_1 system and therefore has the same faults as the CF_1 system.
A recent study by Liang found a new calibration factor for wildfires, but again it depended on the CF_1 method.
https://www.pnas.org/doi/10.1073/pnas.2106478118
However, in this case, the concentrations were so high that we need not worry about zeros appearing very often, so their calibration factor (0.52) is probably pretty good. It suggests that the normal overestimate of 50% or so by the CF_1 method increases to nearly a factor of 2 overestimate for wildfire smoke.