Abstract. PurpleAir sensors, which measure particulate matter (PM), are
widely used by individuals, community groups, and other organizations
including state and local air monitoring agencies. PurpleAir sensors
comprise a massive global network of more than 10 000 sensors. Previous
performance evaluations have typically studied a limited number of PurpleAir
sensors in small geographic areas or laboratory environments. While useful
for determining sensor behavior and data normalization for these geographic
areas, little work has been done to understand the broad applicability of
these results outside these regions and conditions. Here, PurpleAir sensors
operated by air quality monitoring agencies are evaluated in comparison to
collocated ambient air quality regulatory instruments. In total, almost
12 000 24 h averaged PM2.5 measurements from collocated PurpleAir
sensors and Federal Reference Method (FRM) or Federal Equivalent Method
(FEM) PM2.5 measurements were collected across diverse regions of the
United States (US), including 16 states. Consistent with previous
evaluations, under typical ambient and smoke-impacted conditions, the raw
data from PurpleAir sensors overestimate PM2.5 concentrations by about
40 % in most parts of the US. A simple linear regression reduces much of
this bias across most US regions, but adding a relative humidity term
further reduces the bias and improves consistency in the biases between
different regions. More complex multiplicative models did not substantially
improve results when tested on an independent dataset. The final PurpleAir
correction reduces the root mean square error (RMSE) of the raw data from
8 to 3 µg m−3, with an average FRM or FEM
concentration of 9 µg m−3. This correction equation, along with
proposed data cleaning criteria, has been applied to PurpleAir PM2.5
measurements across the US on the AirNow Fire and Smoke Map
(https://fire.airnow.gov/, last access: 14 May 2021) and has the potential to be successfully used in other air
quality and public health applications.