Does the EPA Correction Algorithm for Wildfire Smoke PM2.5 Developed for PurpleAir Work for AirGradient Monitors?

AirGradient Blog
#1

Low-cost air pollution monitors have many advantages, such as size and price. On the other hand, these monitors provide estimates for PM2.5 with higher uncertainties compared to expensive reference monitors. We are aware of these uncertainties, and hence, we have initiated a global co-location project, where we sent our Open Air outdoor monitors to more than 20 sites around the globe. Our invaluable local partners have set up these monitors next to their reference stations and made their reference data accessible to us. The comparison between the reference and Open Air data enables us to evaluate the accuracy of our monitors. Moreover, it allows us to implement correction algorithms to increase the accuracy of our raw readings. Although the co-location project is still ongoing, we now have several months of data that we can use to implement an algorithm.

This is a companion discussion topic for the original entry at https://www.airgradient.com/blog/epa-correction-and-airgradient/
PM correction algorithms refer to RH, not clear if calibrated or raw
#2

Just want to add that we are planning to present these results in our air quality forum in October. In case you have any questions on above, please post them here.

#3
#4

The current PM₂.₅ correction algorithms read as such (both in this blog post and the referenced documentation page):

PM2.5 = [0.524 x AGraw] – [0.0862 x RH] + 5.75

referencing the RH variable. Meanwhile the RH correction from the documentation page reads as such:

RH(calib) = RH(raw) * 1.259 + 7.34

So it isn’t super clear without context if the PM correction ought to be using the calibrated or raw RH readings (EDIT: though I see that the blog post does clarify this in text: “The variable ‘RH’ stands for raw relative humidity readings from the low-cost monitor.”). Could the formulas be adjusted to make this clearer?

Thank you.

#5

Hello, thank you for the very informative blog post! I was wondering whether it would be possible to share the data used for the colocated analysis? This could be either direct download links and / or instructions on how to obtain the data (and run the analysis). I think it would be quite interesting to allow readers to reproduce and even try to contribute further by extending the comparisons.

#6

Hi Nagisa,

thanks for the feedback!

It is indeed the raw RH value that is used in the PM correction formula.

The raw RH value is measured by the PM sensor exactly at the point of sensing. Therefore, this value best represents the humidity that the measured particles were exposed to at the time of measurement.
The corrected RH represents the ambient relative humidity, or the humidity found outside the monitor box. The RH difference between the inside and outside of the box might be due to the insulation effect of the monitor case or slight heating from the components.

I will get this changed on the website.

Thanks!
Anika

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#7

Hi Guilherme,

Good point, we will publish the data! We just need to double check with some of our partners if they are happy to have their reference data made public. (If not we will simply exclude these data and still publish the rest).

We will let you know here in the forum as soon as the data were uploaded!

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#8

Theoretically, this correction algorithm comes to the indoor variant?

#9

Yes. It will also come to the indoor monitor. We did not roll it out there because of the display we have on the indoor monitor, the implementation takes a bit more time.

#10

Ah okay, because I had posted on the website where airgradient published its corecorrection algorithms and explained how far the progress is from these, read the following at the airgradient one display:
The correction can be switched on with local PUT.
What does that mean?

#11

This is in preparation of applying the EPA algorithm for indoor monitors. In that case the display of the monitor also needs to display the corrected values. The PUT requests allows other integrations like for Home Assistant to already prepare for within their integrations and to switch the PM2.5 value on the display to EPA corrected.

#12

Meanwhile, a link has been added at the end of the blog post for data sharing.

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