Q: Should I use the conversion factors on the PurpleAir map?

A: The default setting on the PurpleAir map is the US EPA PM2.5 AQI calculated from the raw PM2.5 concentration reported by PurpleAir sensors. You have the option to apply LRAPA or AQandU conversion to PM2.5 data on the PurpleAir map.

Purple Air uses a laser particle counter to count particulate matter (PM) suspended in the air. This counting process can detect particle sizes from 0.3 μm up to 10 μm. To convert the particle count to a mass concentration (μg/m3), laser particle counters must assume an average particle density. They must use an average because not all PM of a particular size consists of the same stuff. For instance, PM2.5 from wildfire smoke will have a different density than PM2.5 from dust blowing off a gravel pit. This means the mass concentration reported by a PurpleAir sensor varies depending on the specific composition of PM for a given area. The AQ&U conversion factor was developed by the University of Utah to convert the mass concentration data measured by PurpleAir sensors to best fit the PM composition of wintertime air in Salt Lake City.

Salt Lake City is a large urban area with a variety of industrial operations within a mountain valley. Individuals living in comparable environments could consider using the AQ&U conversion factor. Those living in a climate similar to that of Lane County, Oregon, in the Pacific Northwest, could consider using the LRAPA conversion factor. As more research becomes available for different areas, PurpleAir will post additional conversion factors.