Correction Factor Question


We are a community organization in Rochester, MN, focused on establishing an air quality network. We have completed a two-week data collection phase using 10 sensors, collocated at the regional reference sensor. We are now preparing to analyze this data to assess if a correction factor is needed. A specific question we have is regarding the choice of variable for measuring outdoor PM2.5. Our current understanding suggests that PM2.5_alt might be preferable, as it does not default to zero at very low levels. We would appreciate any further insights or recommendations on selecting variables from PurpleAir sensors.


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This is well tread ground.

You are warned now that a correction factor for PA sensors is complicated due to interference from the internal adjustment algorithms; its not like developing CFs for a lab-grade nephelometer.

You should start by contacting EPA and reaching out to their PurpleAir researchers, who have spent a very long time creating correction factors for PA sensors. Some of their more recent research is here:

Next, there is a thread here with a list of scholarly articles written about PA sensors. I would scour those and understand the methodologies used and the challenges of working around the Plantower adjustment factors.

Ultimately, it is up to you if you need to apply local CFs, but in most cases, the EPA AirNow adjustment is likely sufficient for most monitoring needs.

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Hi @Matthew_Spiten Doug provides some really good insights here. If you want to learn more about conversions and similar factors affecting PurpleAir monitors, you might also want to check out this community article: What is the Difference Between CF=1, ATM, and ALT?

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Hello Matthew Spiten,
The Wisconsin Department of Natural Resoures (DNR) uses (PA x 0.514) + 1.8304 to convert PurpleAir Data so that PurpleAir data can be put side by side with $100,000 EPA PM2.5 monitors on United States EPA AirNow Maps of Smoke and Fire. Our organization Residents Against Wood Smoke Emission Particulates is using this formula to demonstrate that residents can simply download 3 days of data, average it and convert that average to EPA regulatory numbers and tell if PM2.5 in ambient air in the yards of near neighbors of indoor residential wood burners rises in average levels in a 3 day period above EPA NAAQS (currently 12 micrograms per cubic meter annual and 35 micrograms per cubic meter in a 24 hour period). NAAQS is National Ambient Ari Quality Standards, which are set by the EPA. Our website is https: // and from the website you can click to over 600 videos and podcasts as well as downloadable excel templates for calculating percent above NAAQS for 12, 25 and 35 micrograms per cubic meter in a 3 day period. I am a scientist and do not appreciate the mystification of simple data downloads. Calculations showing hyperlocalized exceedences of government created PM2.5 limits are simple. If it exceed the limits, it is a harmful pollution. A Ring camera pointed at the roof of an indoor residential wood burner so as not to invade privacy backs up the identification of indoor residential wood burning as the source of PM2.5 pollution. At a NAJEC meeting last week one government employee said using $100,000 PM2.5 regulatory monitors to detect pollution above government set limits was “overkill”, and endorsed use of residential PM2.5 monitors.