A paper published today (4/29/2023) in Sensors (Open Access) looks at spatial variation of all 10,000 outdoor and 4,500 indoor PurpleAir monitors that were active in Washington, Oregon, and California between 2017 and Sept. 8, 2021.
My co-author Tongke Zhao and I found that
- The PurpleAir data using the ALT-CF3 (or pm2.5_alt) algorithm was just as good as the Federal Equivalent Methods (FEM) in estimating PM2.5 throughout Washington, Oregon, and California
- The updated calibration factor of 3.4 for the PMS 5003 sensor matched the “gold standard” Federal Reference Methods (FRM) to within 2%.
- Outdoor PM2.5 measurements by PurpleAir monitors are highly correlated to about 10 km
- Air quality improves with distance from central site regulatory monitors. The improvement reached about 12% at a distance of 10-20 km.
- Indoor PM2.5 cannot be estimated quantitatively using outdoor measurements alone no matter how close the indoor-outdoor monitors are. This casts doubt on epidemiological studies that do not monitor indoor air.