Community Spotlight: West Coast Calibration Factor Study

Community Spotlight: West Coast Calibration Study

Each time a PurpleAir sensor is installed, and air quality data is uploaded to the PurpleAir Map, the quality and reach of our community increases. In this post, we highlight the results from a scientific study and discuss what it means in the bigger picture. The network improves with each additional sensor, and studies like the one in this post are also part of the PurpleAir story.

A Tri-State Calibration Study

Again showing that PurpleAir sensors provide useful air quality data from sensors of accuracy and longevity, a team of investigators recently published the research paper, Calibration of PurpleAir PA-I and PA-II Monitors Using Daily Mean PM2.5 Concentrations Measured in California, Washington, and Oregon from 2017 to 2021.

The researchers noted that "Large quantities of real-time particle data are becoming available from low-cost particle monitors," which recognizes PurpleAir's contribution. To validate data from "the largest network of monitors in the United States," the researchers evaluated PurpleAir sensor data and presented a "calibration factor" for PurpleAir's sensors.

The researchers analyzed 83 million hourly average PM2.5 values collected from PurpleAir sensors in Washington, Oregon, and California between 1 January 2017 and 8 September 2021. 

Comparisons were made between the average daily outdoor PM2.5 readings from PurpleAir sensors and the daily averages from 47 air quality sensors federally validated, referred to as standardized Federal Reference Methods.

Through this comparison, the researchers determined a new calibration factor of 3.4, an update from the currently available CF=3.

The study authors wrote that the PurpleAir PM2.5 measurements "can agree well with regulatory monitors when an optimum calibration factor is found."

Why are Conversion Factors Important?

When looking at the PurpleAir Map, visitors get a view of air quality data for any address, neighborhood, city, state, or even country in the PurpleAir network. PurpleAir sensors measure particulate matter, or PM, to monitor the size of particles - solid and liquid - in the air we inhale.

The PM number refers to the size, such as PM2.5 and PM10. The number shows the diameter of the average particle in microns, or 0.00003937 inches (0.001 millimeters). These particles can come from dust, smoke, and other sources.

Conversion factors are ways to align different measurement standards so that everything from particle density, composition, and size can be compared across measurement approaches. On the PurpleAir Map, each sensor’s map marker shows real-time data with the option to select the conversion factor of interest. By default, this is set to display no conversion factor. These various conversion factors, averaging times, and alternative data layers can be changed with the graphing options on the sensor's graph window. Different sources use slightly different factors for their data. To read more about conversion factors, click here.

What ALT CF=3 Means For You

These studies are important because they show how the data layers and options on the PurpleAir Map can be an important part of your air quality strategy. For example, when you look closer at the map's tools, there's a lot of information for you to discover.

When selecting ALT CF=3 on the PurpleAir Map, this applies a conversion to the real-time data to more closely align with federal reference sensors.

This study evaluated this performance (throughout the predominantly wildfire-smoke-affected western states of Washington, Oregon, and California) found a factor of 3.4 to be 12% more closely aligned with the reference sensors. We will continue evaluating this conversion to determine if it should be added as a new option on the map. 

The examples below show how the Map Tools lets you access other ways to use the PurpleAir Map. As the Map Tools settings change, so do the values and information from each sensor.

The more information you have, the better your ability to plan ahead and make decisions. Be sure to check out the PurpleAir Map!

The PurpleAir Map and where to access the Map Tools menu bar.

Main View of the PurpleAir Map. Arrow shows where to access the Map Tools menu bar.

Map Tools menu bar with Conversion Factor options.

Close-up View of the Map Tools menu bar. Arrow shows the Conversion Factor options. 

Picking a conversion factor of cf=3 and then clicking the sensor shows the PM 2.5 AQI of cf=3 as a value of 38.

Picking a conversion factor of cf=3 and then clicking the sensor 'Trego'. A pop-up window shows the PM 2.5 AQI of cf=3 is now a value of 32.


To read the full study article, visit: https://bit.ly/3zlORBt

To learn about our other PurpleAir Community projects, visit: https://bit.ly/3OhVJEp

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