Low-cost sensors (LCS) are increasingly being used to measure fine
particulate matter (PM2.5) concentrations in cities around the world. One of
the most commonly deployed LCS is the PurpleAir with about 15,000 sensors
deployed in the United States. However, the change in sensor performance over
time has not been well studied. It is important to understand the lifespan of
these sensors to determine when they should be replaced, and when measurements
from these devices should or should not be used for various applications. This
paper fills in this gap by leveraging the fact that: 1) Each PurpleAir sensor
is comprised of two identical sensors and the divergence between their
measurements can be observed, and 2) There are numerous PurpleAir sensors
within 50 meters of regulatory monitors allowing for the comparison of
measurements between these two instruments. We propose empirically-derived
degradation outcomes for the PurpleAir sensors and evaluate how these outcomes
change over time. On average, we find that the number of 'flagged'
measurements, where the two sensors within each PurpleAir disagree, increases
in time to 4 percent after 4 years of operation. Approximately, 2 percent of
all PurpleAir sensors were permanently degraded. The largest fraction of
permanently degraded PurpleAir sensors appeared to be in the hot and humid
climate zone, suggesting that the sensors in this zone may need to be replaced
sooner. We also find that the bias of PurpleAir sensors, or the difference
between corrected PM2.5 levels and the corresponding reference measurements,
changed over time by -0.12 ug/m3 (95% CI: -0.13 ug/m3, -0.11 ug/m3) per year.
The average bias increases dramatically after 3.5 years. Climate zone is a
significant modifier of the association between degradation outcomes and time.Comment: 28 pages, 5 figures, 4 table