Real Life Review: Purple Air PA-II Air Quality Sensor

Living in the high desert of Southern California and within distant sight of Cajon Pass,there have been many days when I can see the wall of haze / smog / general yuck in the air rising up the pass from the LA basin and Inland Empire and overflowing the pass, spreading out in the Victor Valley. It is not unusual for days to start out clear and the San Gabriel Mountains are clearly visible, yet by mid-evening the pollution has obscured them entirely.   When I turn and look the other way, however, the air is usually perfectly clear.  It’s prompted me to wonder how much of the pollution I see actually makes it to my location, and if there are elements I can’t see which are making it up here.

The local air quality district does have monitoring stations, but they are few and far between and do not have nearly the spatial resolution to be of use at an individual level. I looked into renting a similar sensor for a few weeks, but the cost far outweighed my curiosity.  I basically gave up on being able to lean more about the air in my specific environment.

A week or so ago I found out about a novel, web enabled, air quality sensor for measuring particulates that was starting to gain a following in the “citizen science” community which is being produced and distributed by “Purple Air”, which also hosts a real-time map of the installed sensors which have had data sharing activated.   It uses the same type of laser particle detection and quantifying system as some of the higher end (and much higher cost) certified systems, but instead of being aimed at the regulatory and compliance side of the spectrum it is targeted more at regular people.  Rather then being a calibrated and NIST traceable instrument the approach of the organization involved seems to be to go for sensor-to-sensor consistency rather than absolute accuracy, however they have had the sensors evaluated by the Air Quality Sensor Performance Evaluation Center, a branch of California’s South Coast Air Quality Management District, and a technical result of that evaluation is available on their website.   At slightly over $200 it seemed a reasonable way to satisfy my curiosity, if still a bit on the expensive side, so I ordered it.

Delivery was faster than I expected, which was a good thing as in the period between ordering and receiving I began looking for additional information about installation, setup, and use, and found basically nothing, which launched off a major research activity that could well have spun on for weeks.  When I did receive it, it was deceptively easy to get going – plug it in, connect to it’s standalone wifi network in order to configure it to your wifi network, and then go on the PurpleAir website to register it as a monitoring station.  All told it took under 10 minutes from pulling it out of the box to streaming data. I was interested in making sure it worked prior to doing the final install, so for that initial test I simply hung it on an existing nail on the front porch and let it run for a few hours while I looked into other installation options.

I ended up deciding that the front porch was probably the best spot overall as it offered a good combination of protection from the elements, separation from “domestic” pollution (e.g. my smoker and grill on the back porch) and good access to ambient air.  I had no objection to where I had first put it, but it comes with a 33 foot power cord and that location was only a few feet from the plug and left a mess of extra cable, so I basically neatly ran the cable up and around the porch structure and located the sensor at the point where I ran out of power cable.  The sensor itself is basically the size of a 3″ PVC pipe cap (which is what the outer cover actually is) and has an aluminum mounting strip attached to it.

The initial data showed what I had been suspecting – my local air quality was much better both on a relative and absolute scale than the official data, which was basically an extrapolation of the reading from one site about 15 miles away from me.  Areas “down the hill” in the Ontario / San Bernardino area were showing air quality index values of low 60’s, whereas I was in the low 20’s (on the AQI scale 0 is best).  A few hours later though I had a test case in the other direction – the smoke plume from a distant forest fire settled over my location for a few hours, moved elsewhere, then came back, …  which resulted in the PM2.5 values shown below.  At the peak of the smoke, my local AQI value was up to 130.  By sheer coincidence, I had data for both high and low ranges within my first 24 hours of usage, which has increaed my initial confidence in the values the unit is putting out.

In summary, so far I really like my PA-II air quality sensor, and it is nice to have data to back up the observation that the air quality where I am is as good as I think it is.  There are a few issues with some details of their site registration process when you want to share data with other sites such as Weather Underground and link it to an existing weather station, but I’m certain that will get smoothed out over the next few months if not days.  At the moment it appears that slightly over 500 of these sensors are installed worldwide, so time will tell how they do in terms of drift and longevity… but assuming that remains good, at under $250 including shipping and no direct operating costs, I can easily see a day when every public building and many private ones incorporate something like this.  That would allow a huge improvement in our ability to monitor and mitigate particulate pollution, and is something I would much rather see my tax dollars flow into than building a wall across a desert.

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2 Responses to Real Life Review: Purple Air PA-II Air Quality Sensor

  1. Noah Cain says:

    This was great. I’m actually an Apple Valley resident and 7th grade student who built my own PM10 sensor last year but used my California State Science Fair (middle school environmental science 1st place), AQMD, and now national science fair (Society for Science and the Public top 30 finalist in Washington D.C. in October) prize money to purchase two Purple Air sensors and other materials.

    My previous findings were that the highest exposure for kids each day is in the parent pick-up lane as they stand by all the idling cars.

    I am doing my next research right now on childhood PM exposure around their schools and using spatial interpolation and creating 3D renderings of exposure zones for public understanding of risk areas and finding ways to limit exposure to PM.

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