For the last few years, I’ve been analyzing the performance of the PurpleAir monitors. Last week the Sensors journal published my detailed comparison of the Plantower CF_1 algorithm with an alternative algorithm called ALT-CF3.
This is Open Access. There is also a link within the pdf that takes you to the Supplemental Information (pictures of the PurpleAir and SidePak research-grade monitors, for example.)
ALT-CF3 is a based on a method that has been used for decades to calculate PM2.5 from monitors providing estimates of particle numbers in several size categories. 33 PurpleAir monitors within 500 m of 27 Federal Reference monitors in California supplied the data over 18 months resulting in the choice of 3.0 as the calibration factor (CF) (Wallace et al., 2021).
Some good news from the new paper in Sensors is that the ALT-CF3 algorithm produces extremely good precision, averaging 4-8%, compared to about 10-15% for the CRF_1 algorithm.
More good news is that the Limit of Detection (LOD) of the ALT-CF3 algorithm is below 1 ug/m3, compared to the CF_1 LOD of 3-5 ug/m3. Since outdoor PM2.5 is often below 3-5 ug/m3, this means that a rather high percentage (sometimes more than half) of outdoor measurements using the CF_1 algorithm are below the LOD.
Even more good news is that the monitors showed sustained high performance for the entire 3 years of the study.
A great advantage of the PurpleAir monitors is that they can be used indoors to provide the first actually measured long-term exposures to PM2.5 that we have. However, indoor concentrations are usually lower than outdoors, meaning that an even higher percentage of CF_1 measurements will fall below the LOD.
A drawback to using the CF_1 or CF_ATM algorithms is that concentrations falling below a certain value are all given a value of zero. Statisticians do not agree with this approach, preferring that the reported concentrations not be altered. At low concentrations, zeros often predominate. The ALT-CF3 algorithm NEVER reports zeros, since there are always some particles in the 0.3-0.5 um size category.
In the Map page, there are five alternative conversions offered, such as ALT-CF3, LRAPA, EPA, etc. However, four of these depend on the CF_1 algorithm, and thus have the same problems with the poor precision, prevalence of zeros at low concentrations, etc. due to the CF_1 algorithm. The only conversion that does not depend on CF_1 is the ALT-CF3 version.
Fortunately, the ALT-CF3 version is available for downloading on the PurpleAir API website. There it is named PM2.5 alt.