The “Apply conversion“ field determines which conversion factor is applied to sensor data. The default field-selection is “no“.
PurpleAir sensors use laser counters to detect particulate matter (PM) suspended in the air. To convert particle counts to mass concentration values (μg/m3), the laser counters must assume an average particle density. They use an average because not all PM is the same density. For instance, PM2.5 from wildfire smoke has a different density than PM2.5 from dust blowing off of 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.
Conversions help accommodate different types of pollution with different particle densities.
Conversion Options
No: This default field-selection ensures no conversion is applied to the sensor data.
ALT cf=3: This field selection is a transparent and reproducible alternative method (ALT) of calculating \text{PM}_{2.5} from the particle numbers in three size categories. The authors present it as a calibration of the low-cost PurpleAir outdoor monitors using an improved method of calculating \text{PM}_{2.5}. Here is a link to their peer-reviewed paper.
The “ALT-CF3” mass concentration is given by:
where the particle count per dL of air of particles sized by at least 0.3 \mu \text m is n_{0.3}, by at least 0.5 \mu \text m is n_{0.5}, by at least 1.0 \mu \text m is n_{1.0}, and by at least 2.5 \mu \text m is n_{2.5}.
US EPA: This field selection is courtesy of the United States Environmental Protection Agency Office of Research and Development. This conversion from their US wide study (updated version from Barkjohn et al., 2021) is designed for use during wildfire smoke conditions.
- y={0 ≤ x <30: 0.524x - 0.0862RH + 5.75}
- y={30≤ x <50: (0.786*(x/20 - 3/2) + 0.524*(1 - (x/20 - 3/2)))x -0.0862RH + 5.75}
- y={50 ≤ x <210: 0.786x - 0.0862RH + 5.75}
- y={210 ≤ x <260: (0.69*(x/50 – 21/5) + 0.786*(1 - (x/50 – 21/5)))x - 0.0862RH*(1 - (x/50 – 21/5)) + 2.966*(x/50 – 21/5) + 5.75*(1 - (x/50 – 21/5)) + 8.84*(10^{-4})x^{2}(x/50 – 21/5)}
- y={260 ≤ x: 2.966 + 0.69x + 8.8410^{-4}*x^2}
y = corrected PM2.5 μg/m3
x = PM2.5 cf_atm (lower)
RH = Relative humidity as measured by the PurpleAir
Note that the formula above applies only to ATM data. For CF=1 data, you should use the formula from this 2022 EPA study. If you’d like to learn more about CF=1 and ATM, you can check out this article: What is the Difference Between CF=1, ATM, and ALT?
Woodsmoke: This field selection is from a study in Australia comparing Purple Air with NSW Government TEOM PM_{2.5} and Armidale Regional Council’s DustTrak measurements. View their published, peer-reviewed study. This conversion is meant for woodsmoke conditions.
The Woodsmoke mass concentration is given by:
where \text{PM}_{2.5} of \text{cf_1} is \rho _{\text{cf_1}}.
AQandU: This field selection is courtesy of the University of Utah, conversion factors from their study of the PA sensors during a winter in Salt Lake City. Visit their website. This conversion is meant for those who live in the Salt Lake City valley.
The AQandU mass concentration is given by:
LRAPA: This field selection is courtesy of the Lane Regional Air Protection Agency, conversion factors from their study of the PA sensors. Visit their website. This conversion is meant for those who live in Lane County, Oregon.
For \text{PM}_{2.5} mass concentrations below 65 \mu \text{g/m}^3, the LRAPA mass concentration is given by:
where \text{PM}_{2.5} of \text{cf_1} is \rho _{\text{cf_1}}.
Learn More
PurpleAir Map Guide
Map Troubleshooting Guide
Which AQI Data Layer to Choose