# The "Apply Conversion" Field

The “Apply conversion“ field determines which conversion factor to apply to the raw sensor data. The default field-selection is “no“. Conversions help accommodate different types of pollution with different particle densities. Conversion may be needed when inferring the mass of any combination of particulates derived from particle counts.

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}. Atmospheric Environment, Volume 256, 2021, 118432. ISSN 1352-2310. View their scientific, peer-reviewed paper.

The “ALT cf=3” mass concentration is given by:

\rho_{\text{ALT}} = \frac{\pi}{800} \left( \frac{3 \sqrt{15}}{50} n_{0.3} + \frac{50 \sqrt{2} - 3 \sqrt{15}}{50} n_{0.5} + \left(5 \sqrt{10} - \sqrt{2} \right) n_{1.0} - 5 \sqrt{10} \cdot n_{2.5} \right)

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}.

If you’d like to delve into the derivation of this formula, continue to the PurpleAir Implementation of the Alternative (ALT) Method.

US EPA: This field selection is courtesy of the United States Environmental Protection Agency Office of Research and Development, correction equation from their US wide study (updated version from Sept 16 2020) validated for wildfire and woodsmoke.

For \text{PM}_{2.5} mass concentrations below 250 \mu \text{g/m}^3 (greater than 250 may underestimate true PM_{2.5}), the US EPA mass concentration is given by:

\rho_{\text{PM}_{2.5}} = 0.52 \rho _{\text{cf_1}} - 0.085 \phi + 5.71

where \text{PM}_{2.5} of \text{cf_1} is \rho _{\text{cf_1}}, and the relative humidity is \phi.

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.

The Woodsmoke mass concentration is given by:

\rho_{\text{woodsmoke}} = 0.55 \rho _{\text{cf_1}} + 0.53

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.

The AQandU mass concentration is given by:

\rho_{\text{AQandU}} = 0.778 \rho _{\text{cf_1}} + 2.65

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.

For \text{PM}_{2.5} mass concentrations below 65 \mu \text{g/m}^3, the LRAPA mass concentration is given by:

\rho_{\text{LRAPA}} = 0.5 \rho _{\text{cf_1}} - 0.66

where \text{PM}_{2.5} of \text{cf_1} is \rho _{\text{cf_1}}.