The United States Environmental Protection Agency (US EPA) has developed
correction for Purple Air sensors that performs better than
either Purple Air's uncorrected PM2.5 concentrations (which report high
in wood fire situations) and LRAPA corrected concentrations (which report
low when AQI exceeds 150).
PurpleAir devices with the US EPA correction report the correct AQI category
92% of the time and within one category 100% of the time. This performance
holds true even for smoke only events.
As of October 4, 2020 at 2:00 PM PDT, PaloAltoWeather.com reports
PM2.5 and PM2.5 AQI with the US EPA correction. The uncorrected and
LRAPA values are no longer reported.
All historical graphs have been updated to reflect the US EPA correction.