Air Quality Sensors
What are low-cost air quality sensors?
Low-cost air quality sensors are a relatively new technology that measure specific air pollutants, typically particulate matter and some gaseous pollutants, and cost much less than traditional air quality monitors. In the United States, air quality has traditionally been measured according to standards established by the United States Environmental Protection Agency (EPA) using regulatory monitors that are designated as federal reference method (FRM) or federal equivalent method (FEM). These monitors cost tens of thousands of dollars and require significant infrastructure and trained personnel to operate, whereas, low-cost sensors may only cost a few hundred dollars and can be simple to operate.
These low-cost air quality sensors are becoming increasingly popular with citizen scientists and community groups that are trying to measure air quality conditions near their homes and in their communities. The major challenge, with these low-cost sensors, is the interpretation of data. Since these sensors are compact and inexpensively built, the data can vary significantly from the regulatory FRM and FEM monitors.
The Lane Regional Air Protection Agency (LRAPA) maintains a network of ambient air quality monitors that utilize FRM or FEM monitors. These monitors are approved by the EPA to help determine if the National Ambient Air Quality Standards (NAAQS) for criteria pollutants, such as particulate matter, are being achieved or exceeded. These monitors are regularly maintained, independently audited, and have high standards for quality assurance and quality control. Data from these monitors are held to rigorous review through multiple levels of checks and validation. Low-cost sensor data, on the other hand, do not have to meet these same standards and are not approved by the EPA for comparison to the NAAQS due to the nature of the low-cost manufacturing and varying sensor responses.
Low-cost air quality sensors cannot replace traditional regulatory monitors, but they do create new opportunities to increase and expand access to air quality monitoring and can play a part in tracking air quality. LRAPA has deployed a network of low-cost air quality Purple Air sensors throughout Lane County. The Purple Air sensors provide particulate matter (PM) data in communities without regulatory monitors. The data can be particularly useful during summer wildfire smoke intrusion episodes and the winter time home wood heating season.
How does the Purple Air sensor data compare to regulatory data?
Most studies comparing Purple Air sensors to regulatory monitors have found Purple Air sensors report higher levels than regulatory monitors. Recognizing this discrepancy, Purple Air has added the ability to apply "Conversion" factors to their website map. This option is in the lower left corner of the Purple Air map. By applying a conversion factor, the Purple Air map can show PM values closer to those of regulatory monitors.
Raw Purple Air data in our area is about two times higher than the real values. We know this from collocating Purple Air sensors with our regulatory monitors and comparing measurements. The data from these measurements is then used to develop a conversion or calibration factor.
To improve the accuracy of the Purple Air sensor data for our area, follow these instructions:
- Go to www.purpleair.com, select View Map, and zoom in on your location.
- On the Purple Air website map, in the bottom left corner, click on the Map Data Layer box and select the Conversion drop down and select LRAPA (highlighted in red below). This conversion, also called correction or calibration, adjusts the raw Purple Air values so that they match regulatory air monitors more closely. For information on how this correction factor was developed this document gives a short explanation
The LRAPA conversion equation was developed by Lane Regional Air Pollution Agency in Eugene, Oregon, and is best suited for our area.
Purple Air Data Summary
The color scheme on the Purple Air website – which corresponds to the EPA’s ‘air quality index’ or AQI – relates to the most current data point collected by the Purple Air sensor. If you want to use Purple Air’s data for informational purposes, click on a monitor and look at the other averages to see what has been happening over the last few hours. The pop-up data box will show different average levels, from the most current to the last week. Even though EPA health standards are based on a 24-hour average, looking at the data trend over the last few hours can help you see trends.
Purple Air Data Graph
When you click on a monitor, a time series graph will pop up. The time series graph uses uncorrected data, even if you select the LRAPA conversion. We have let Purple Air know about this issue and hope they fix it soon. The graph can still be useful to see trends and check on the health of the sensor itself.
Even though the values in the time series graph are raw uncorrected data and therefore are not accurate, the graph is still useful to look at general patterns over time and to compare one internal Purple Air sensor to the other.
The Purple Air unit has two internal sensors, which are both graphed on the time series. If the sensors are reading as designed, they will track each other closely (as in the below graph).
If the sensors are reading differently (as in the example graph below) then one, or both, of the sensors may be malfunctioning. This can usually be remedied by cleaning the sensors with clean air from a handheld bulb type duster or a small can of compressed air. If you see a graph like below then the data should be considered suspect, especially if the levels are much higher or lower than other sensors in the area.
LRAPA sponsored Purple Air sensors
As mentioned above, LRAPA has multiple Purple Air sensors deployed around Lane County. These sensors are monitored, quality controlled, and maintained by LRAPA. These sensors can be displayed on this map developed for LRAPA by the Lane Council of Governments (LCOG). LRAPA had this map developed so that we could display the Purple Air monitors that LRAPA has deployed, with the conversion factors built in for better accuracy and ease of use. The map only displays the Purple Air sensors that LRAPA has stewardship over.