PurpleAir's $250 Air Pollution Monitor Gives Government Equipment Run for Money

July 17, 2017, 2:40 AM EDT

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Thinking about a personal air pollution sensor? Based on cost and on laboratory and field evaluations by air pollution experts, we recommend the PA-II by PurpleAir (purpleair.com) as the best outdoor air pollution monitor for personal use.

Advances in technology have led to the development of relatively cheap air pollution sensors suitable for personal use. These Personal Air Pollution Sensors (PAPS) often have measurement capabilities competitive with commercial sensors used by the EPA to make official air pollution measurements for their AirNow network, which cost upwards of $20,000.

There are six air pollutants deemed dangerous enough to human health that the EPA issues official standards for them. By far the most dangerous pollutant is PM2.5—fine-sized particles less than 2.5 micrometers in diameter. These fine particles are blamed for about 90% of the approximately 100,000 premature yearly deaths due to outdoor air pollution in the U.S., and about 85% of the estimated 3 million global outdoor air pollution deaths each year.

The PA-II is purpleair.com’s second-generation low-cost PM2.5 sensor, and employs a dual laser beam that counts particles based on their reflectivity. As of July 16, 2017, the PA-II retailed for $229, plus $20 for shipping. The PA-II was evaluated by the South Coast Air Quality Management District (SCAQMD), the air pollution control agency for all of California’s Orange County and the urban portions of Los Angeles, Riverside and San Bernardino counties. They have an Air Quality Sensor Performance Evaluation Center that compares low-cost air monitoring sensors alongside one or more of the high-grade operational air monitoring sensors used by EPA. (Note that EPA also has a web page with intercomparison results for Personal Air Pollution Sensors, but their page is three years old, and does not include information for the PA-II from purpleair.com.)

According to SCAQMD’s intercomparison tests of nineteen low-cost PM2.5 sensors made by different companies, the PA-II had the highest correlation with measurements made in side-by-side tests in the field with the expensive commercial-grade sensors used by EPA. Thus, when a change in PM2.5 levels occurred, the PA-II did the best job of tracking the change. However, the PA-II tended to have a high bias in these field tests, typically overestimating the PM2.5 levels by 36 - 48%. It is important to note that on their website, the following disclaimer is posted: The field evaluations reports included on this website contain data collected at our monitoring station during a specific 30- to 60-day period and may not be duplicated at the same or different location, season, time period, or weather conditions (e.g. temperature, relative humidity, pressure, wind speed/direction). As the overall sensor performance may be affected by the specific environmental conditions experienced at our location during the time of testing, replication and/or duplication may not possible to achieve.” SCAQMD tested three different PA-II units in Riverside, California between 12/08/2016 and 01/26/2017. They stated that all three units agreed with each other very well, and did “track well the diurnal PM2.5 variations” (1-hour mean) observed by the two commercial-grade sensors, and “well” or “very well” for the 24-hour mean.

Southern California experienced a wide range of dry and rainy weather conditions during the 49-day test period, with temperatures ranging from 33 to 78 degrees. We can expect that PA-II units installed in other climates will likely achieve similar accuracies in those temperature ranges. However, the tests were not done in below-freezing temperatures, or in very hot weather, so it is possible that PA-II units experiencing those conditions will show different behavior. Laboratory tests of the PA-II from the SCAQMD were not yet available at the time of this writing, but were in preparation.

I installed a Purple Air PA-II sensor at my office in Hartland, MI in early June, and have been impressed with its performance. Configuring the sensor to send data over my wireless network to purpleair.com was pretty straightforward, and took me less than 10 minutes (though I have heard from one person who had found the process more challenging.) The PA-II requires a 2.4 GHz wireless network to send data to, and an electrical outlet. I simply threaded my wire through the top of the door to the outside, and plugged it in inside. The PA-II comes with 33 feet of wire. The unit does not have a display panel—you have to go on the Internet to a web page to see the data. Note that the PA-II also comes with a temperature sensor; the readings from this sensor tend to run at least 10 degrees Fahrenheit too high, due to the heat generated by the PA-II.

It's been very interesting to watch the PM2.5 numbers from my sensor, which is situated just 150 feet from a busy four-lane 55-mph highway, M-59. I see elevated levels of PM2.5 when the wind is blowing from the road towards my sensor, and when mixing is poor (like occurs in the evening when winds die down and an atmospheric inversion forms.) I was surprised to see that a neighborhood fireworks display on July 4 caused unhealthy levels of PM2.5 of 60 micrograms per cubic meter (which I discussed in a July 5 post.) The federal standard for 24-hour exposure to PM2.5 is 35 micrograms per cubic meter.

Weather Underground co-founder Dr. Perry Samson, an air pollution scientist with the University of Michigan, has also installed a PA-II unit, in Ann Arbor, Michigan. His has been up since April 2017, and he has thus far been impressed with its performance—so much so that he requested all the PurpleAir data from the July 4 weekend, and is considering presenting a paper at the annual meeting of the American Meteorological Society on the link between fireworks and PM2.5 levels measured by the PA-II sensors (I’ll be a co-author!)

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

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Dr. Jeff Masters

Dr. Jeff Masters co-founded Weather Underground in 1995 while working on his Ph.D. in air pollution meteorology at the University of Michigan. He worked for the NOAA Hurricane Hunters from 1986-1990 as a flight meteorologist.


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