Project SPOTlight (Sensing Pollution Outdoors Together)
Monthly PurpleAir Monitor Data Report
Time Period: May 1 - May 31, 2025
Number of Monitors: 24
Questions addressed in this report:
- How do this month’s PM2.5 levels compare to previous months?
- How do the PurpleAir monitor readings compare to regulatory monitors?
Outstanding Questions:
- How have weather and temperature inversions impacted PM2.5 readings from PurpleAir?
- What are the primary sources of elevated PM2.5 in the region?
Use this link to ask any questions you have after reviewing this report and our team will respond! The link is managed by our partner organization, Beaver County Marcellus Awareness Community (BCMAC).
Figure 1: Average PM2.5 readings across all Project SPOTlight PurpleAir monitors
The red dashed line indicates the primary National Ambient Air Quality Standard (NAAQS) for annual PM2.5 levels. If a region averages higher than 9 µg/m³ over the course of three years, the region is considered out of attainment.
In May 2025, the average PM2.5 level recorded by Project SPOTlight’s PurpleAir monitors was 4.57 µg/m³. This figure includes an EPA adjustment applied to raw PurpleAir data. The adjustment is a correction developed by the EPA to improve accuracy, aligning PurpleAir readings more closely with regulatory-grade monitors. Figure 1 above compares averages between months going back to September 2024.
Comparing PurpleAir Monitors to Regulatory Monitors
We are in the early stages of understanding the data we’re receiving from the PurpleAir monitors, so it’s important to examine how our readings compare to regulatory monitors. The regulatory monitors that are closest to our PurpleAir monitors are at Fort McIntosh Park and Avalon. Both are managed by the Pennsylvania Department of Environmental Protection (DEP). Figure 2 shows where the regulatory monitors are located.
Figure 2: Map showing where regulatory monitors are located. The smaller green dots indicate PurpleAir monitors and other low-cost sensors located throughout the region.
Figure 3 shows that throughout the month of May, the PurpleAir monitor readings closely aligned with both the Fort McIntosh and Avalon monitor readings. You can see that when there were spikes in PM2.5, the PurpleAir monitors tended to read higher than the Fort McIntosh monitor, though they followed the same pattern. This is a good indication that the PurpleAir monitors are generally accurate, meaning we can trust what they report as long as:
Figure 3: Daily average PM2.5 readings on PurpleAir monitors and regulatory monitors. Note that this PurpleAir data does not have the EPA correction factor applied to it.
This is the second monthly report we've released with PurpleAir data. We plan to release a similar report every month and would like to expand on what we're sharing.
Project SPOTlight is an EPA-funded program aimed at increasing community air quality monitoring capacity in and around Beaver County. The EPA grant is managed by the Breathe Project. Questions? Email spotlight@breatheproject.org.
