What are R2 values in Air Quality?

R2 values PurpleAir

South Coast AQMD and Air Pollution

The South Coast Air Quality Management District (AQMD) is conducting efforts that will help improve air quality and public health in environmental justice communities in South California. Specifically, the counties and areas the agency covers include:

  • Los Angeles – 4,753 mi² (12,310 km²)
  • Orange – 948 mi² (2455 km²)
  • Riverside – 7,303 mi² (18,915 km²)
  • San Bernardino – 20,105 mi² (52,072 km²)
  • The Coachella Valley – 777 mi² (2012 km²)

Together, they account for half of the state's population of almost 40 million people. The agency regulates emission standards, mainly from stationary sources like power plants, refineries, and gas stations. In total, the agency oversees almost 30,000 businesses. The South Coast AQMD ensures organizations follow federal and state clean air standards through its Air Quality Management Plan.

Not All Sensors Are Made the Same

The South Coast AQMD also evaluates the accuracy of various air quality sensors, such as the outdoor PurpleAir PA-II. Recognizing the benefits and challenges of having low-cost sensors widely available, South Coast AQMD is helping to assess the wide range of performance from these sensors through their AQ-SPEC program.

Accurate sensors "significantly augment and improve current ambient air monitoring capabilities that now predominantly rely on more sophisticated and expensive fixed-site federal-reference monitoring devices and methods." For the consumer, the good news is that the South Coast AQMD has a program for testing consumer-targeted sensors.

The goal is to provide a guide that details each sensor's accuracy in the lab under controlled settings and in the field where conditions are dynamic. The standard measurements or baselines in these tests are set by Federal Reference Methods (FRM) and Federal Equivalent Methods (FEM).

In their assessment of more than 50 low-cost consumer sensors, PurpleAir sensors perform best-in-class. At the heart of our achievement is how closely our sensors matched the results of the smallest particulate matter: PM1.0 and PM2.5. This impressive performance is neatly captured by a "coefficient of determination," or R².

Introducing R² Values

An R² value shows how closely a sensor's data correlates with FRM/FEM concentration measurements. In other words, R² is a statistical term representing how well a sensor performs compared to reference monitoring technologies. For example, a value of =1 would mean the sensor has a perfect correlation to the reference monitor, and =0 would mean no correlation to a reference monitor. The table below shows PurpleAir R² scores.


   Pollutant size   



PurpleAir PA-I-Indoor   




PurpleAir (PA-II)


   0.93 to 0.97   


As noted by the South Coast AQMD testing results, PurpleAir sensors are leading the industry, with the PA-II testing up to 97% correlation with FRM/FEM concentration measurements for small particulate matter, or PM2.5.

Now that you know more about the R² value, feel free to read the various sensor evaluations from the SCAQMD. They have a complete list of particulate matter and performance evaluations, as well as gaseous sensors.

Worried about air quality?

Monitor the Particulate Matter levels around the world with our free, real-time PurpleAir Map. or join PurpleAir's mission to make air quality data accessible to everyone by investing in an air quality monitor for your home. 

Together, we can be informed and make changes in our daily habits and the community to improve air quality.



AQ-SPEC. PM sensor evaluations. Air quality sensor summary reports. Retrieved August 9, 2022, from http://www.aqmd.gov/aq-spec/evaluations/summary-pm