New EPA correction?

Data
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The latest update contains “An updated US EPA conversion.”

Can you let us know what it is, or point us to a document?

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Hi @dclaar

Thanks for the question. We’ve updated the map descriptions to more clearly define the difference. To keep it short, the new conversion uses new CF_1 equations for indoor sensors posted here: Correction and Accuracy of PurpleAir PM2.5 Measurements for Extreme Wildfire Smoke.

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So this is not relevant for external sensors, and may only be useful for wildfire smoke?

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CF1 data (and thereby the EPA conversion for CF1 data) can be used outdoors. It’s just not the manufacturer’s (Plantower’s) recommendation. Thus, on the PurpleAir map, outdoor sensors display ATM data, and indoor sensors display CF1 data.

However, if you wanted to download the raw CF1 data and apply the EPA conversion, you could do so using the CF1 formula. The EPA created this conversion for that very purpose.

PurpleAir sensors tend to overestimate pollution levels when heavy wildfire smoke is present––the EPA conversion solves this issue.

I hope that answers your questions!

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Thanks, that makes sense!

For reference: The original EPA correction was:
PM2.5 corrected= 0.52*[PA_cf1(avgAB)] - 0.085*RH +5.71

from Slide 8, “PurpleAir PM2.5 U.S. Correction and
Performance During Smoke
Events”, International Smoke Symposium, April 21, 2020

I only have a screenshot of (and code for) the next correction, which was some time in 2021,

New EPA equation
New EPA equation1544×881 107 KB
.

   if self.interface.pm2_5_cf_1 <= 343:
      aqi = 0.52 * pm2_5_cf_1 - 0.086 * humidity + 5.75
    else:
      aqi = 0.46 * pm2_5_cf_1 + 3.93 * 10**-4 * pm2_5_cf_1**2 + 2.97

The latest is similar to that screenshot, but not the same, as they now have 3 ranges instead of 2:

(1) PAcf1 < 570 (corrected = 300 µg/m3 at 50% RH)
PM2.5 = PAcf1 × 0.524 − 0.0862 × RH + 5.75

(2) 570 ≤ PAcf1 < 611
PM2.5 = (0.0244 × PAcf1 − 13.9) × [Equation (3)] + (1 − (0.0244 × PAcf1 − 13.9)) × [Equation (1)]

(3) PAcf1 ≥ 611 (corrected 400 µg/m3)
PM2.5 = PAcf1**2 × 4.21 × 10−4 + PAcf1 × 0.392 +3.44

In the equations, RH represents the relative humidity, as measured by the PurpleAir.

Equation 1 can be used for concentrations up to 300 µg/m3, while Equation (3) can be used for concentrations of 300 µg/m3 and above.

The equations for the lower concentrations are the same: The equation for the higher concentrations has slightly different adjustments: I’ve rearranged the equations so that they can more easily be compared.

old = PAcf1**2 x 3.93 x 10**-4 + PAcf1 x 0.46 + 2.97
new = PAcf1**2 × 4.21 × 10**−4 + PAcf1 × 0.392 +3.44

I haven’t graphed them to see how much difference there is.

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